In short: fully vaporizing one US gallon of water at 100 °C (212 °F) takes about 8,528 kJ (2.37 kWh, 8,083 BTU). That energy is on top of whatever it already took to heat the water to boiling — the phase change itself is the expensive part.
These figures are planning aids based on published steam-table data. For process design or safety-critical equipment sizing, confirm with a licensed engineer.
How to calculate the heat of vaporization
Turning liquid water into vapor takes energy even when the temperature does not rise — that energy goes into pulling molecules apart against the hydrogen bonds holding them together as a liquid. This is called latent heat, because the thermometer doesn't show it. The calculation is a single multiplication once you know the latent heat at your chosen temperature:
This calculator converts your entered volume to mass using the density of water at about room temperature (998.2 kg/m³) — the sample's mass doesn't change as it heats, only its volume expands slightly, so the exact reference temperature makes well under 1% difference here. It then looks up the latent heat of vaporization at your chosen temperature from steam-table data and multiplies the two.
Worked example. Take 1 US gallon (3.785 L) of water at 100 °C. Using a density of 998.2 kg/m³, that's 3.779 kg. The latent heat of vaporization at 100 °C is 2,257 kJ/kg. Multiply: 3.779 × 2,257 = 8,528 kJ, which is 2.37 kWh or 8,083 BTU. That single multiplication is all the calculator does — the work is in getting the latent heat right for the temperature.
This is not the same as boiling. Bringing water up to 100 °C uses sensible heat (mass × specific heat × temperature change) — see the specific heat calculator. This tool covers only the latent heat of the phase change itself, once the water is already at temperature. For both steps combined, use the energy to boil water calculator.
Why temperature changes the heat of vaporization
It's easy to assume vaporization only happens at 100 °C, but water can evaporate at any temperature below boiling — laundry dries, puddles disappear, and sweat cools skin, all without anything reaching a boil. What changes with temperature is how much energy each kilogram of vapor carries away. Counterintuitively, the latent heat of vaporization is highest near freezing and lowest at the normal boiling point: about 2,501 kJ/kg at 0 °C versus 2,257 kJ/kg at 100 °C, a difference of roughly 10%.
The reason is that near the boiling point, the liquid and vapor phases are already closer in molecular energy and density, so less additional energy is needed to complete the transition. This calculator reads the latent heat from a standard saturated-steam data set spanning 0–100 °C, the same reference used in mechanical and chemical engineering practice, rather than a single fixed constant. You can read the full method on our methodology page.
Heat of vaporization by temperature
The latent heat this calculator uses, from evaporation temperatures up through boiling, along with the energy that amount implies for one US gallon of water.
| Temperature | Lv (kJ/kg) | Lv (BTU/lb) | Energy per US gallon |
|---|---|---|---|
| 0 °C / 32 °F | 2,501.3 | 1,075.4 | 9,451 kJ |
| 20 °C / 68 °F (evaporation) | 2,454.1 | 1,055.1 | 9,273 kJ |
| 25 °C / 77 °F | 2,442.3 | 1,050.0 | 9,228 kJ |
| 37 °C / 99 °F (body) | 2,413.8 | 1,037.8 | 9,121 kJ |
| 50 °C / 122 °F | 2,382.7 | 1,024.4 | 9,003 kJ |
| 60 °C / 140 °F | 2,358.5 | 1,014.0 | 8,912 kJ |
| 80 °C / 176 °F | 2,308.8 | 992.6 | 8,724 kJ |
| 100 °C / 212 °F (boiling) | 2,257.0 | 970.3 | 8,528 kJ |
Quick reference: energy to vaporize common volumes
Fresh water at 100 °C (212 °F), the normal boiling point. Handy when you just need a number without opening the calculator.
| Volume | Energy (kJ) | Energy (kWh) | Energy (BTU) |
|---|---|---|---|
| 1 US cup | 533 | 0.148 | 505 |
| 1 liter | 2,253 | 0.626 | 2,135 |
| 1 US gallon | 8,528 | 2.37 | 8,083 |
| 5 US gallons | 42,641 | 11.84 | 40,416 |
| 1 cubic foot | 63,796 | 17.72 | 60,467 |
| 55-gallon drum | 469,056 | 130.29 | 444,580 |
Worked examples
Boiling a kettle dry
8,528 kJ
Fully vaporizing 1 US gallon of water already at 100 °C takes about 8,528 kJ (2.37 kWh) — separate from, and much larger than, the energy needed just to bring it to a boil.
Sweat cooling the body
288 kcal
Evaporating 500 mL of sweat at body temperature (37 °C) removes about 1,205 kJ, or 288 kcal, of heat — which is why evaporative cooling is so effective at regulating body temperature.
Boiling a pot dry on a 2 kW burner
~38 min
Boiling away 2 liters of water on a 2 kW element takes roughly 38 minutes, assuming no heat loss — far longer than the minute or two it takes just to reach boiling.
What else affects the result
Altitude and pressure. Water boils at a lower temperature at altitude, because atmospheric pressure is lower. The latent heat at that lower boiling point is slightly higher than at 100 °C, following the same trend shown in the table above. See the boiling point at altitude calculator to find the boiling point for your elevation, then use that temperature here.
Dissolved salts. Seawater and brine boil at a slightly higher temperature than fresh water (boiling point elevation), and their latent heat differs marginally from pure water's. This calculator adjusts the mass for seawater's higher density but assumes the latent heat of pure water, which is accurate to within a percent or two for typical seawater salinity.
Real-world efficiency. The time estimate assumes every watt of the heat source goes directly into vaporizing the water. Open pots, kettles and industrial boilers all lose some heat to their surroundings and container, so actual times run longer than this lower-bound estimate.
Frequently asked questions
What is the heat of vaporization of water?
The heat of vaporization is the energy needed to turn liquid water into vapor without changing its temperature. At the normal boiling point, 100 °C (212 °F), it takes about 2,257 kJ per kilogram (970 BTU per pound). This is over five times the energy needed to heat that same water from freezing to boiling.
How much energy does it take to boil away a gallon of water?
Fully vaporizing one US gallon of water at 100 °C takes about 8,528 kJ (2.37 kWh, or 8,083 BTU). That is the energy for the phase change alone, on top of whatever energy already brought the water up to boiling.
Does the heat of vaporization change with temperature?
Yes. It is highest near freezing, about 2,501 kJ/kg at 0 °C, and lowest at the normal boiling point, about 2,257 kJ/kg at 100 °C. Water evaporating at room temperature actually absorbs slightly more energy per kilogram than water boiling at 100 °C.
Is this the same as the energy needed to bring water to a boil?
No. Bringing water to a boil is sensible heat, calculated with the water's specific heat capacity. This calculator covers only the latent heat of the phase change itself, once the water is already at its vaporization temperature. Use the energy to boil water calculator to combine both steps.
How much energy does sweat evaporation remove from the body?
Evaporating water at body temperature, about 37 °C, absorbs roughly 2,414 kJ per kilogram, close to 577 kcal per liter. This is why sweating is such an effective cooling mechanism: a small amount of evaporated moisture carries away a large amount of heat.
Why is water's heat of vaporization so high compared to other liquids?
Water molecules form strong hydrogen bonds with their neighbors. Breaking enough of those bonds to let a molecule escape as vapor takes far more energy than for most other common liquids, which is why water is unusually effective at storing and moving heat as it evaporates or condenses.
Sources & references
- International Association for the Properties of Water and Steam (IAPWS). Release on the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use.
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems.
- ASME. International Steam Tables for Industrial Use.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)