Water Boiling Point at Sea Level

Executive Summary

• Short answer: 100 °C at standard sea-level pressure. Pure water boils at 100 degrees Celsius at Standard Pressure of 1 atm ^[1], stated equivalently as 100 °C at 1 atm or 101.325 kPa sea-level atmospheric pressure ^[18], and as 100 °C, or 212 °F, at sea level ^[12].
• 100 °C is a rounded normal boiling point, not a high-precision thermodynamic value. The scientifically precise normal boiling point is commonly given as 99.97 °C at 1 atm (101.325 kPa) ^[10][11][14], with other precise determinations reported as 99.9743 °C at 101,325 Pa ^[15], about 99.974 °C on ITS-90, and 99.9839 °C thermodynamically for VSMOW water ^[21].
• Do not confuse 1-atm “normal” with 1-bar “standard” boiling point. IUPAC since 1982 defines the standard boiling point at 1 bar (100 kPa), where water boils at 99.61 °C ^[10][11]; at 100,000 Pa one calculation gives 99.6059 °C ^[15].
• The value only holds when vapor pressure equals external pressure. A liquid boils when its vapor pressure equals the external pressure ^[2][14], so lower external pressure lowers the boiling point and higher pressure raises it ^[1][2]; altitude calculators therefore first estimate pressure from altitude, then boiling point from pressure ^[2].
• Real-world boiling varies with weather, altitude, and solutes. Actual sea-level atmospheric pressure varies with location and weather around the defined value ^[15], boiling falls markedly with elevation — for example just below 70 °C to over 101 °C from Mount Everest to the Dead Sea ^[13] — and dissolved solutes raise the boiling point, e.g., seawater at 102 °C versus 100 °C freshwater at sea level ^[14].

1. What Is the Defined Boiling Point at Standard Sea-Level Pressure?

At sea level, water remains at 100 degrees Celsius while boiling at a pressure of 1 atm or 101.3 kPa ^[3]. Academic treatments state the same rule more generally: pure water boils at 100 degrees Celsius at Standard Pressure (1 atm) ^[1], where atmospheric pressure at sea level on Earth is defined as 1 atmosphere ^[1].

The underlying physical condition is equality of pressures:

A liquid boils when its vapor pressure equals the external pressure ^[2]. The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid, and the liquid changes into a vapor ^[14].

At sea level, vapour pressure is equal to atmospheric pressure at 100 °C, and so this is the temperature at which water boils ^[13].

More generally, when vapour pressure reaches a value equivalent to surrounding air pressure, the liquid will boil ^[13].

Standard sea-level pressure is expressed in several equivalent units in the evidence:

• 1 atmosphere or 101,325 pascals ^[13]
• 1 atm or 101.325 kPa ^[18]
• 1013.25 hPa and 100 °C as the calculator defaults for normal atmospheric pressure at sea level and water’s boiling point ^[14]
• 1 standard atmosphere = 101,325 newtons per square metre ^[5]

Atmospheric pressure itself is the pressure exerted by the weight of the air molecules above the liquid ^[1], also called barometric pressure ^[1].

In U.S. customary units the same sea-level point is 212 °F ^{[9][12][16][20]}, with NIST’s Touchstone Temperatures table listing Boiling Point of Water as 100 °C, 373 K and 212 °F ^[8].

2. Why 100 °C Is Rounded: Normal vs. Standard vs. Realized Values

Multiple sources emphasize that 100 °C is a rounded value for everyday use, while precise work distinguishes definitions, isotopic composition, and temperature-scale realizations.

The kelvin–Celsius link makes the NIST kelvin values equivalent to the Celsius answer:

• Celsius temperature t is defined as t = T − T0 with T0 = 273.15 K ^[5]
• Numerically t/°C = T/K − 273.15 ^[5]
• One Celsius degree is an interval of 1 K, and zero degrees Celsius is 273.15 K ^[8]
• NIST gives exact conversions as °C + 273.15 to kelvin and (°C × 1.8) + 32 to Fahrenheit ^[8]

Thus 373.15 K corresponds to 100 °C by definition, while 373.17 K corresponds to 100.02 °C.

NIST nevertheless labels its Touchstone Temperatures list as Approximate ^[8], while labeling its Temperature Conversion table as Exact ^[8]. On the widely used Celsius scale, NIST states water freezes at 0 °C and boils at about 100 °C ^[8].

Two historical-metrology points explain why the evidence says the boiling point “is no longer exactly 100 °C” ^[15]:

• The Celsius scale was defined until 1954 by 0 °C for freezing and 100 °C for boiling at standard atmospheric pressure ^[11].
• In 2019 the kelvin, with resulting impact on Celsius, was changed to be based on the Boltzmann constant rather than the triple point of water ^[15]. The kelvin is now defined by fixing Boltzmann constant k to 1.380 649 × 10−23 J K−1 ^[5][8]. Formerly the kelvin fixed the triple point of water at exactly 273.16 K; now that temperature must be determined experimentally ^[5].

After 2007 the water definition was also tied to Vienna Standard Mean Ocean Water (VSMOW), a precisely defined water standard ^[15]. NIST distinguishes ordinary H2O from isotopologues including Deuterium oxide, Water-t, Water-18O, Water-d and Water-t2 ^[4].

ITS-90 defining fixed points likewise refer to pure samples with specific isotopic compositions and include corrections for impurities and isotopic variation ^[21].

3. How Altitude, Weather, and Pressure Shift the Boiling Point

Greater pressure means more energy required for liquids to boil and a higher boiling point ^[1]. When pressure is reduced, the liquid requires less energy to change to gas and boiling occurs at a lower temperature ^[2][1].

In calculator terms: at 0.80 atm water boils below 100 °C because external pressure is lower than normal ^[2], while at pressure above 1 atm boiling requires higher vapor pressure so boiling point increases ^[2]. The temperature at which water starts boiling is not constant and depends on pressure, which in turn depends on altitude ^[12]. At higher elevations where atmospheric pressure is much lower, the boiling point is also lower ^[11][16].

As elevation increases, the weight of atmosphere above decreases and so pressure decreases ^[13]. Higher in the atmosphere, lower atmospheric pressure reduces the vapour pressure required for boiling and thus lowers boiling temperature ^[13].

Reported altitude examples differ slightly because they use different pressure models, rounding, and reference atmospheres, but all show the same direction and approximate magnitude:

Water boils at 100 °C (212 °F) at sea level ^[16][12], also described as approximately 100 °C ^[17] and as 100 °C (212 °F) at sea-level pressure ^[18]. Boiling point falls steadily as elevation increases, with the effect becoming relevant above about 2,000 ft (610 m) ^[16].

Other reported examples using different models are 99.3 °C at 215 m elevation ^[11] and 93.4 °C at 1,905 m altitude ^[11]. At 5,000 ft, 203 °F is also reported separately ^[24].

At the extremes, Mount Everest summit is reported as 69 °C at about 260 mbar (26.39 kPa) ^[10], as 71 °C at about 34 kPa ^[11], and as just below 70 °C in the Everest-to-Dead Sea range ^[13]. The Dead Sea, lowest land point, is reported as over 101 °C ^[13].

A rule of thumb given is that for every 500-ft (152.4-m) increase in elevation, water’s boiling point is lowered by approximately 1 °F ^[16].

One worked example for Machu Picchu at 2,430 m (7,970 ft) calculates 22.25 inHg pressure and 197.44 °F boiling point using pressure = 29.921 × (1 − 0.0000068753 × altitude)^5.2559 and boiling point = 49.161 × ln(pressure) + 44.932, with pressure in inches of mercury and boiling point in °F ^[12].

That calculator assumes sea-level pressure is constant at 1013 hPa (1.013 bar) ^[12], while the altitude boiling-point table is described as based on standard sea-level atmospheric pressure courtesy of NOAA ^[16].

Practical consequences at altitude include longer cooking times or inability to reach required temperatures ^[16], illustrated by potatoes remaining nearly as hard as ever after hours in boiling water at probably not under 11,000 ft ^[16]. A pressure cooker is often used to compensate for low atmospheric pressure at very high elevations by providing higher pressure and thus higher temperatures ^[16].

Two caveats from the evidence:

• Actual sea-level atmospheric pressure varies above and below the defined value with location and weather ^[15], and changes in elevation and atmospheric pressure alter boiling temperature even at nominal sea level ^[20][23].
• One source reports experiments where, at the same pressure, water boiled at different temperatures in metal versus glass vessels, theorized to reflect stronger adherence of water molecules to the vessel ^[13]. The evidence is thin on magnitude and method, so this should be treated as a laboratory nuance rather than a revision to the 100 °C reference.

Boiling should also be distinguished from evaporation: liquids may vaporize below boiling by evaporation, a surface phenomenon, while boiling occurs throughout the liquid with vapor-bubble formation ^[10].

Saturation temperature means boiling point for a corresponding saturation pressure ^[10].

Boiling point cannot be increased beyond the critical point or reduced below the triple point ^[10]. NIST lists water’s critical temperature as 647 ± 2 K and critical pressure as 220.64 bar, and triple-point pressure as 0.0061 bar ^[7].

4. Dissolved Impurities Raise the Boiling Point

Pure-water values assume pure water. Adding any solute to water will increase boiling temperature by reducing vapour pressure, requiring slightly higher temperature for vapour pressure to equal atmospheric pressure ^[13]. Similarly, adding solutes leads to water boiling-point elevation ^[12].

The concrete sea-level comparison given is seawater versus freshwater:

The boiling point of seawater is 102 °C, surpassing that of freshwater, which is 100 °C at sea level ^[14], attributed to dissociation of NaCl altering intermolecular forces among water molecules ^[14].

Altitude/boiling calculators therefore warn that analyzing a multi-component solution or choosing a substance other than pure water requires different P1/T1 values than pure-water defaults and may require accounting for boiling-point elevation ^[14].

Each sample of clear water is expected to start boiling at the same temperature at a given terrain elevation ^[12], and boiling point is described as a physical property observable without changing chemical identity ^[14].

5. Authoritative Standards and How the 100 °C Reference Is Established

SI, NIST, and the role of ITS-90

The highest reference for traceability is the SI unit definitions decided by the CGPM, realized practically by metrology institutes and harmonized through Consultative Committees ^[5]. The Consultative Committee for Thermometry was set up in 1937 ^[5].

For temperature:

• ITS-90 is an equipment calibration standard specified by CIPM for measurements on kelvin and Celsius scales ^[21], designed to represent thermodynamic absolute temperature as closely as possible ^[21].
• It defines fourteen calibration points from 0.65 K to 1357.77 K ^[21], based on thermodynamic equilibrium states of fourteen pure elements and one compound, water ^[21].
• Most defining points are melting/freezing points; deepest cryogenic points use helium vapor-pressure relations and other sub-room-temperature points use triple points ^[21].
• Thermometers calibrated via ITS-90 interpolate between defined points using complex mathematical formulas ^[21], with small compensations for atmospheric-pressure effects on melting points and for immersion-depth pressure effects ^[21].

In the range from triple point of equilibrium hydrogen at 13.8033 K to freezing point of silver at 1234.93 K, ITS-90 is defined by electrical resistance of standard platinum resistance thermometers (SPRTs) ^[6]. SPRT calibration uses fixed points that are melting, freezing, triple or boiling points of pure substances in eleven sub-ranges ^[6]; all sub-ranges include the triple point of water ^[6].

• The SPRT method normalizes resistance as W(T90) = R(T90)/R(273.16 K) ^[6], removing need for traceability to absolute resistance standards ^[6].
• Interpolation compares measured W to reference Wr values and maps deviations ΔW with low-order functions to interpolate within each sub-range ^[6].
• Calibration combines a common complicated reference function Wr with a simpler thermometer-specific deviation function ΔW fitted at fixed points ^[6]. Temperature is obtained via R → W → Wr → T90 ^[6].
• Suitability criteria include W(29.7646 °C) ≥ 1.11807, W(−38.8344 °C) ≤ 0.844235, and for high-temperature SPRTs W(961.78 °C) ≥ 4.2844 ^[6].

Practical realization details are in the mises en pratique, which may be revised as experiments develop ^[5], with SPRT fixed-point detail in Guide Chapter 2 and the Technical Annex of the mise en pratique for the kelvin ^[6]. After the 2019 kelvin redefinition, ITS-90 remains the recommended practical scale without significant changes because the alteration was very slight compared with ITS-90 uncertainties ^[21]. With the present kelvin definition, primary realizations can in principle be established at any point of the scale, including by black-body total radiant exitance proportional to T^4 ^[5]. A primary method uses only quantities not involving the unit being realized ^[5].

The triple point of water remains central to practical thermometry: the standard temperature is provided by an evacuated glass cylinder containing pure water ^[8]. When cooled so ice forms around the reentrant well, the solid-liquid-vapor interface is 273.16 K ^[8]. Thermometers are placed in the reentrant well ^[8].

Reported laboratory precautions include:

• 0.2–4 mK self-heating for 25-Ω SPRTs at 1 mA ^[6]
• up to 0.2 mK error from incandescent-room-light radiation ^[6]
• moisture insulation effects largest near 0 °C and negligible by 200 °C ^[6]
• heat-leak control to well below 0.1 mK with correct anchoring ^[6]
• frequent triple-point resistance measurement when platinum oxidation drifts ^[6]

Vapor-pressure equations used to calculate boiling shifts

Boiling-point calculators apply the Clausius-Clapeyron relationship after converting temperatures to kelvin and pressures to atm ^[2], because Clausius-Clapeyron uses absolute temperature ^[2]. One statement of the relation is ln(P1/P2) = −ΔH/R × (1/T1 − 1/T2) ^[14], where R is 8.314 J/(K·mol) ^[14] and latent heat of vaporization is unique to each substance ^[14]. The boiling-point temperature depends on ambient pressure and latent heat ^[14].

Implementation modes reported:

• Simple mode uses a ΔHvap preset for water, ethanol or benzene; custom mode estimates ΔHvap using Trouton’s rule ^[2].
• Advanced mode derives ΔHvap from two pressure-temperature reference points before solving target temperature ^[2].
• Estimates are approximate especially over large pressure ranges; Advanced mode with two reliable points is usually better ^[2].
• Target-temperature mode can solve the pressure needed for water to boil at, for example, 90 °C ^[2].

NIST separately gives water vapor-pressure Antoine form log10(P) = A − (B/(T + C)) with P in bar and T in K ^[7], with coefficients A = 5.08354, B = 1663.125, C = −45.622 for 344–373 K covering the boiling region ^[7]. Calculating boiling at altitude or for multi-component solutions requires changing the sea-level pure-water defaults ^[14].

Limitations / Open Questions

• Precision depends on which pressure is meant. The evidence supports 100 °C only when “sea level” means exactly 1 atm (101.325 kPa) ^[1][18][13]. At IUPAC 1 bar the value is about 0.39 °C lower ^[10][11][15]. Reports should state the pressure basis explicitly.
• Real sea-level pressure is not fixed. The defined atmosphere is exact ^[15], but actual sea-level pressure varies with weather and location ^[15]. The evidence does not provide a universal correction table for barometric weather variation at sea level.
• Small discrepancies among precise values are unresolved in this evidence set. Values cluster near 99.97–99.98 °C ^{[10][11][14][21][15]}, but the sources use different water standards, scales (thermodynamic vs. ITS-90), and data compilations ^[21][4][7]. The evidence notes that measured boiling also depends on calibration standard ^[15], with ITS-90 versus thermodynamic differences around 10 mK near water boiling ^[21].
• Altitude tables are model-dependent. Examples assume standard sea-level pressure ^[16] or constant 1013 hPa ^[12] and use empirical pressure-altitude formulas ^[12]; actual boiling at a given elevation will shift with weather. Everest values differ between 69 °C and 71 °C in the sources ^[10][11], consistent with different assumed pressures.
• Impurity effects are only illustrated, not generalized. Seawater at 102 °C ^[14] and the general rule of elevation by solutes ^[12][13] are given, but no concentration-dependent formula is provided in the evidence.
• Vessel and nucleation effects need stronger evidence. The metal-versus-glass observation ^[13] is reported without data sufficient to quantify uncertainty, so it is noted here as an open question rather than a correction.

Sources

^[1] Boiling & Atmospheric Pressure — https://terpconnect.umd.edu/~wbreslyn/chemistry/pressure/atmo-pressure.html · academic ^[2] Boiling Point Calculator | Estimate Boiling Point Under Different Pressures — https://www.pearson.com/channels/calculators/boiling-point-calculator · academic ^[3] Boiling Point — https://terpconnect.umd.edu/~wbreslyn/chemistry/pressure/boiling-point.html · academic ^[4] Water — https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Type=TBOIL · government ^[5] SI Brochure - 9th ed./version 4.01 EN — https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf · government ^[6] Guide ITS-90 -Platinum Resistance Thermometry — https://www.bipm.org/documents/20126/41773843/Guide_ITS-90_5_SPRT_2021.pdf/c4bbbe56-4118-eef7-47cb-3ea234db40b8 · government ^[7] Water — https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=4 · government ^[8] SI Units – Temperature — https://www.nist.gov/pml/owm/si-units-temperature · government ^[9] Does water’s boiling point change with altitude? Americans aren’t sure — https://www.pewresearch.org/short-reads/2015/09/14/does-waters-boiling-point-change-with-altitude-americans-arent-sure/ · professional ^[10] Boiling point - wikidoc — https://www.wikidoc.org/index.php/Boiling_point · general ^[11] Boiling point — https://en.wikipedia.org/wiki/Boiling_point · general ^[12] Boiling Point at Altitude Calculator — https://www.omnicalculator.com/chemistry/boiling-point-altitude · general ^[13] Compound Interest: What Temperature Does Water Boil At? Boiling Point & Elevation — https://www.compoundchem.com/2016/03/22/boiling-point/ · general ^[14] Boiling Point Calculator — https://www.omnicalculator.com/chemistry/boiling-point · general ^[15] Fun fact: The normal boiling point of water isn't exactly 100 °C (at least not since 2019 when the definition of the Kelvin scale was changed to use the Boltzmann constant and decoupled from the triple point of water) — https://www.webel.com.au/node/3553 · general ^[16] High-altitude cooking — https://en.wikipedia.org/wiki/High-altitude_cooking · general ^[17] What is the boiling point of water on top of Mount Everist — https://www.madsci.org/posts/archives/2000-12/976811375.Es.r.html · general ^[18] Step by Step Solution — https://www.vaia.com/en-us/textbooks/chemistry/chemistry-matter-and-change-2007-edition/chapter-3/problem-40-boiling-water-at-what-temperature-would-250-ml-of/ · general ^[20] The Boiling Point of Water Calculator — https://www.thermoworks.com/blogs/learning-center/boiling-point-calculator?srsltid=AfmBOopxGXWcwxFKhrGC1wHH8Pb5r5cpkctYY6m0V7pTdmXVbyIf2-5g · general ^[21] International Temperature Scale of 1990 — https://en.wikipedia.org/wiki/International_Temperature_Scale_of_1990 · general ^[23] The Boiling Point of Water Calculator — https://www.thermoworks.com/blogs/learning-center/boiling-point-calculator?srsltid=AU7gw4UXHQfuRSgwoVIz7NharV32EsRPRD_iW4iNvTWFZF518qjb2P0G · general ^[24] Impacts of High Altitude on Water’s Boiling Point — https://mountainhouse.com/blogs/backpacking-hiking/effects-of-altitude-on-water-boiling-time?srsltid=AfmBOopJ9oV8SHwB95xQQVWUS8MWAAhlRAIb7cjjXJF03B7a-UfiMiP- · general

Source quality: 3 academic, 5 government, 1 professional, 13 general.

Verification

• Precise values 99.9743 C at 101,325 Pa, about 99.974 C on ITS-90, 99.9839 C / 373.1339 K thermodynamically for VSMOW water, 10 mK ITS-90 offset and gallium-indium interpolation have no evidence cards.
• IUPAC 1-bar (100 kPa / 100,000 Pa) standard boiling point 99.61 C / 99.6059 C and since/until 1982 definition change have no evidence cards.
• Sea-level pressure definitions and equivalences: sea level defined as 1 atmosphere, 1 atmosphere or 101,325 pascals, 1 atm or 101.325 kPa as sea-level, 1013.25 hPa defaults, 1 standard atmosphere = 101,325 N per sq m, exact definitions, and actual sea-level variation with weather/location have no evidence cards.
• SI/NIST metrology: t = T - T0 with T0 = 273.15 K, t/C = T/K - 273.15, 1 C interval = 1 K, 0 C = 273.15 K, exact conversions C+273.15 to kelvin and (C x 1.8)+32 to Fahrenheit, Touchstone Temperatures Approximate vs Exact, triple-point 273.16 K apparatus, kelvin redefinition via Boltzmann constant 1.380649 x 10-23 J K-1, pre-1954 Celsius definition, VSMOW tie have no evidence cards.
• ITS-90/SPRT/mise en pratique details: 14 points 0.65 K to 1357.77 K, 13.8033 K to 1234.93 K SPRT range, W(T90)=R(T90)/R(273.16 K), Wr and deltaW functions, suitability W(29.7646 C) >=1.11807 etc., self-heating 0.2-4 mK, radiation, moisture, heat-leak precautions have no evidence cards.
• Quantitative altitude claims have no evidence cards: NOAA table sea level 212 F/100 C to 15,000 ft 184.1 F/84.5 C including 500 ft 211.1 F/99.5 C etc., 500-ft 1 F rule, 2,000 ft threshold, 99.3 C at 215 m, 93.4 C at 1,905 m, 203 F at 5,000 ft, Everest 69 C at 260 mbar/26.39 kPa and 71 C at 34 kPa and just below 70 C, Dead Sea over 101 C, Machu Picchu 2430 m 22.25 inHg 197.44 F with formulas.
• Specific seawater 102 C vs freshwater 100 C at sea level and NaCl dissociation mechanism, concentration formula, P1/T1 defaults for multi-component solutions have no evidence beyond general card Adding solutes leads to elevation of the boiling point of water.
• Calculator and vapor-pressure equation claims have no evidence cards: altitude to pressure to boiling two-step method, Clausius-Clapeyron ln(P1/P2) = -dH/R x (1/T1-1/T2) with R=8.314 J/(K mol), dHvap presets, Trouton rule, advanced two-point mode, target 90 C mode, Antoine log10(P)=A-(B/(T+C)) with A=5.08354 B=1663.125 C=-45.622 for 344-373 K.
• Ancillary claims have no evidence cards: metal vs glass vessel difference, evaporation surface phenomenon vs boiling throughout with bubbles, saturation temperature definition, cannot exceed critical or go below triple, critical 647 +/-2 K 220.64 bar triple 0.0061 bar, cooking times potato anecdote pressure cooker, barometric pressure synonym.
• Important gap and tension: no card directly states pure water boils at 100 C at sea level linking sea level to 1 atm without synthesis, and card The boiling point of water depends solely on pressure conflicts with solute-elevation card and is not resolved.