Boiling Point Calculator
ChemistryCalculate the boiling point of a pure liquid or solution using the Clausius-Clapeyron equation or boiling point elevation formula. Supports water, ethanol, and more.
100
40.7
Boiling Point (°C)
100
Boiling Point (K)
373.15
Boiling Point (°F)
212
What is a Boiling Point?
The Boiling Point Calculator computes the boiling point of a pure liquid (water, ethanol, methanol, acetone, or a custom liquid) at any atmospheric pressure using the Clausius-Clapeyron equation. Enter the pressure in mmHg and select the substance to instantly get the boiling temperature in °C, K, and °F.
The boiling point is the temperature at which a liquid's vapour pressure equals the surrounding external pressure. Since vapour pressure is a function of temperature (it increases exponentially with temperature), reducing the external pressure reduces the temperature at which boiling occurs. At 1 atm (760 mmHg), water boils at 100°C; at half that pressure (380 mmHg, approximately 5,500 m altitude), it boils at about 81°C.
This pressure-dependence is described by the Clausius-Clapeyron equation, which requires only the normal boiling point (at 760 mmHg) and the enthalpy of vaporisation (ΔHvap) to predict the boiling point at any other pressure. The calculator uses established ΔHvap values for each preset substance and applies the equation with automatic unit handling. For the Vapor Pressure Calculator, which solves the inverse problem (finding vapour pressure at a given temperature), see the related tool.
How to use this Boiling Point calculator
- Select the liquid from the Substance dropdown: Water, Ethanol, Methanol, Acetone, or Custom.
- Enter the operating pressure in mmHg in the Pressure field. Standard atmospheric pressure at sea level is 760 mmHg. Adjust lower for altitude or vacuum conditions, higher for pressurised systems.
- If using Custom, enter the normal boiling point in °C in the Normal Boiling Point field and the enthalpy of vaporisation in kJ/mol in the ΔHvap field.
- Read the Boiling Point (°C) — compare to the normal boiling point to understand the pressure effect.
- For cooking at altitude, use the result to estimate required extra cooking time (food cooks slower below 100°C) and whether a pressure cooker is needed.
Formula & Methodology
Clausius-Clapeyron equation (solved for T₂):ln(P₂/P₁) = (ΔHvap/R) × (1/T₁ − 1/T₂) 1/T₂ = 1/T₁ − (R/ΔHvap) × ln(P₂/P₁) T₂ = 1 / [1/T₁ − (R/ΔHvap) × ln(P₂/P₁)]Where: T₁ = normal boiling point (K, at P₁ = 760 mmHg), T₂ = boiling point at P₂, ΔHvap in J/mol (× 1000 from kJ/mol), R = 8.314 J/(mol·K) Worked example — water at Shimla altitude (≈ 640 mmHg): ΔHvap(water) = 40,700 J/mol, T₁ = 373.15 K (100°C), P₁ = 760 mmHg, P₂ = 640 mmHg1/T₂ = 1/373.15 − (8.314/40700) × ln(640/760) = 0.002680 − 0.0002043 × (−0.1719) = 0.002680 + 0.0000351 = 0.002715 T₂ = 1/0.002715 = 368.3 K = 95.2°CAt Shimla's altitude, water boils at approximately 95°C — explaining why rice and pulses take 15–20% longer to cook without a pressure cooker compared to sea-level cooking.
Frequently Asked Questions
The boiling point of a liquid is the temperature at which its vapour pressure equals the surrounding atmospheric pressure. At this temperature, the liquid transitions to gas throughout its volume — not just at the surface (which is evaporation). The normal boiling point is measured at exactly 1 atm (760 mmHg); the standard boiling point is measured at 1 bar (750.1 mmHg). For water, the normal boiling point is 100°C (212°F) and the standard boiling point is 99.61°C.
Boiling point decreases as pressure decreases and increases as pressure increases. This is because boiling occurs when vapour pressure equals external pressure — at lower external pressure, the liquid needs less vapour pressure (achieved at a lower temperature) to start boiling. The Clausius-Clapeyron equation quantifies this relationship: ln(P₂/P₁) = (ΔHvap/R) × (1/T₁ − 1/T₂). This is why water boils at ~90°C in the Himalayas and why autoclaves sterilise at >100°C by raising pressure.
The Clausius-Clapeyron equation ln(P₂/P₁) = (ΔHvap/R) × (1/T₁ − 1/T₂) relates the vapour pressure ratio to the temperature ratio via the enthalpy of vaporisation (ΔHvap) and gas constant (R = 8.314 J/mol·K). It can be rearranged to find the boiling point at any pressure: 1/T₂ = 1/T₁ − (R/ΔHvap) × ln(P₂/P₁), where T₁ is the normal boiling point (at P₁ = 760 mmHg) and T₂ is the boiling point at P₂.
Boiling point is the temperature at which a liquid boils (bulk vaporisation at equilibrium vapour pressure = external pressure). Flash point is the temperature at which a flammable liquid produces enough vapour to ignite momentarily in the presence of a spark or flame — it is typically well below the boiling point. For petrol, the boiling point is about 40–200°C (range for mixture), but the flash point is about −43°C. Understanding both is important for handling flammable chemicals safely.
At 760 mmHg (sea level, 1 atm), water boils at 100°C. At 530 mmHg (≈3,000 m altitude), water boils at about 90°C. At 522 mmHg (pressure cooker at 1 bar gauge), water boils at about 120°C. At 15 psi gauge (standard autoclave, ≈2 atm absolute), water boils at 121°C. Under vacuum at 20 mmHg, water boils at about 22°C — used in vacuum evaporation processes in food and pharmaceutical manufacturing.
Select your substance from the dropdown (water, ethanol, methanol, acetone, or custom). Enter the atmospheric pressure in mmHg in the Pressure field. If selecting Custom, enter the normal boiling point in °C and the enthalpy of vaporisation in kJ/mol. The calculator applies the Clausius-Clapeyron equation and returns the boiling point in °C, K, and °F.
Ethanol (ΔHvap = 38.6 kJ/mol) has a normal boiling point of 78.4°C at 760 mmHg. At reduced pressure of 20 mmHg (vacuum distillation), the calculated boiling point drops to about 8°C — explaining why ethanol can be distilled from heat-sensitive mixtures under vacuum. At 1,520 mmHg (2 atm), ethanol boils at about 102°C. This pressure-temperature behaviour is critical in the design of vacuum distillation units used in industrial alcohol production.
No — for ideal solutions, the boiling point rises relative to the pure solvent (boiling point elevation), and the vapour composition differs from the liquid composition. For non-ideal azeotropic mixtures, the vapour and liquid have the same composition at the azeotrope — which boils at a fixed temperature (e.g., ethanol-water azeotrope boils at 78.1°C at 1 atm, lower than water's 100°C). This calculator computes the boiling point of the pure substance. Use the [Boiling Point Elevation Calculator](/boiling-point-elevation-calculator/) for solutions.
At sea level (Mumbai, Chennai, Kochi), water boils at 100°C. In Bengaluru (920 m altitude), water boils at about 97°C. In Shimla (2,200 m), it boils at about 92°C. In Leh (3,500 m), at about 87°C. In Darjeeling (2,050 m), at about 93°C. These lower boiling temperatures mean food takes longer to cook at altitude and pressure cookers are recommended for dal, rajma, and rice in high-altitude kitchens.
Ethylene glycol (antifreeze): 197°C. Glycerol: 290°C. Dimethyl sulfoxide (DMSO): 189°C. Acetic acid: 118°C. Dimethylformamide (DMF): 153°C. Most salts decompose or melt above 500°C before they boil. Mercury: 357°C. The high boiling points of these solvents make them useful as reaction media for high-temperature synthesis. Water's relatively high boiling point (100°C) for its molecular weight is due to strong hydrogen bonding.