In short: water expands as it warms above 4 °C — heating it from room temperature (20 °C) to boiling (100 °C) increases its volume by about 4.2%. A US gallon heated that way gains roughly 0.157 liters (0.042 gal). The exact figure depends on both the temperature range and how much water you start with — use the calculator above for any volume and range.
These figures are planning aids based on published density data. For expansion-tank sizing, pressure-relief or other safety-critical work, confirm with a licensed engineer.
How to calculate thermal expansion of water
Thermal expansion happens because heating water lowers its density — the same mass simply takes up more room. Since mass is conserved, the volume at a new temperature can be found directly from the ratio of densities at the two temperatures:
This calculator looks up the density of pure water at both temperatures using the Kell equation, forms the ratio, and reports the change in whichever unit you prefer, along with the resulting final volume, percent change and the average expansion coefficient over your chosen range.
Worked example. Take 1 US gallon (3.7854 L) heated from 20 °C to 60 °C. At 20 °C, water's density is 998.20 kg/m³; at 60 °C it is 983.20 kg/m³. The ratio 998.20/983.20 = 1.01526, so the final volume is 1 × 1.01526 = 1.01526 gallons — a gain of 0.01526 gallons, or 0.0578 liters, which is 1.53% more than you started with.
Which way is intuitive? It can help to think of it backwards: colder, denser water is more "packed," so a gallon of it is a slightly smaller gallon's worth of molecules than a gallon of hot water. Heat it and the same molecules need more space.
Why water's expansion isn't a straight line
Most liquids expand at a fairly steady rate as they warm. Water is unusual: its density peaks at about 4 °C (39 °F), not at freezing. Warm it from 0 °C to 4 °C and it actually shrinks slightly before the normal pattern resumes. Above 4 °C, the rate of expansion itself keeps climbing — water expands more per degree near boiling than it does near room temperature, which is why the same 20-degree jump produces a bigger change in volume at high temperatures than at low ones.
Under the hood, this calculator uses the Kell equation, the standard polynomial fit for the density of air-free water between 0 °C and 150 °C at one atmosphere, accurate to a few parts per million. You can read the full method on our methodology page.
Density and expansion coefficient by temperature
The density values this calculator reads, plus the instantaneous expansion coefficient (β) at each point — how fast volume is changing per degree, right at that temperature.
| Temperature | Density (kg/m³) | β (×10⁻⁶ per °C) |
|---|---|---|
| 0 °C / 32 °F | 999.84 | −68 |
| 4 °C / 39 °F (densest) | 999.97 | ~0 |
| 10 °C / 50 °F | 999.70 | 88 |
| 20 °C / 68 °F (room) | 998.20 | 207 |
| 25 °C / 77 °F | 997.04 | 257 |
| 40 °C / 104 °F | 992.22 | 385 |
| 60 °C / 140 °F (hot tap) | 983.20 | 523 |
| 80 °C / 176 °F | 971.80 | 641 |
| 100 °C / 212 °F (boiling) | 958.36 | 750 |
Quick reference: volume change across common ranges
How much 1,000 liters (1 m³) of water changes in volume when heated across common ranges. Scale linearly to your own volume — for 1 gallon, divide the liters figure by 264.2.
| Range | Volume change (L) | Percent |
|---|---|---|
| 0 °C → 4 °C | −0.13 | −0.013% |
| 4 °C → 20 °C | +1.77 | +0.177% |
| 15 °C → 60 °C | +16.17 | +1.617% |
| 20 °C → 60 °C | +15.26 | +1.526% |
| 10 °C → 80 °C | +28.71 | +2.871% |
| 20 °C → 100 °C | +41.57 | +4.157% |
| 0 °C → 100 °C | +43.28 | +4.328% |
Worked examples
Boiling a full kettle
+0.157 L
A 1 US gallon kettle filled at 20 °C and heated to a full boil (100 °C) expands by about 0.157 L (0.042 gal) — a 4.16% increase. It's why a "full" kettle can start to overflow just before it boils.
Sizing a water heater's expansion tank
+3.06 L
A 50-gallon tank filled with 15 °C water and heated to 60 °C (140 °F) expands by about 3.06 L (0.81 gal). In a closed plumbing system that has nowhere to go, which is exactly what a thermal expansion tank is for.
A rooftop tank warming in the sun
+1.02 L
A 200 L tank that warms from 15 °C to 35 °C on a hot day gains about 1.02 L (+0.51%) — small, but enough that tanks are typically filled a little short of the brim.
What else affects the answer
Dissolved salts. Seawater's density runs about 2–3% higher than fresh water, which shifts both ρ(T₁) and ρ(T₂) — use the "Advanced: water type" section above and set a salinity if you're working with seawater, brine or pool water.
Pressure. This calculator assumes atmospheric pressure. Water is nearly incompressible, so ordinary pressure changes have a negligible effect on the result — the exception is deep-ocean or high-pressure industrial systems, which are outside this tool's scope.
The container. Tanks, pipes and kettles expand too, just far less than the water inside them (most metals and plastics expand 5–20 times less per degree). This calculator reports the water's expansion only; a closed system's actual pressure rise depends on both the water's expansion and the container's own stiffness.
Frequently asked questions
Does water actually expand when it's heated?
Yes, above about 4 °C. As water warms, its molecules spread slightly farther apart, so a fixed mass takes up more volume. Between 20 °C and 100 °C, water's volume grows by roughly 4.2%. Below 4 °C the behavior reverses — water is one of the few substances that expands as it approaches freezing.
How much does water expand from room temperature to boiling?
About 4.2%. One US gallon at 20 °C (998.20 kg/m³) becomes roughly 1.0416 gallons at 100 °C (958.36 kg/m³) — an increase of about 0.157 liters, or 0.042 gallons, per gallon.
Why do water heaters need a thermal expansion tank?
Heating water in a closed system makes it expand with nowhere to go, which raises pressure inside the tank and pipes. A 50-gallon tank heated from 15 °C to 60 °C expands by about 0.81 gallons (3.06 L) — enough to push a closed system well past safe pressure without an expansion tank or working pressure-relief valve to absorb it.
Does water contract when heated from 0 °C to 4 °C?
Yes. Pure water reaches its maximum density at about 4 °C, so warming it from 0 °C to 4 °C actually shrinks its volume slightly — around 0.013% per 1,000 liters. Above 4 °C, the normal pattern resumes and water expands as it warms.
What is the coefficient of thermal expansion for water?
It isn't constant — it rises with temperature. Water's volumetric expansion coefficient is about −68×10⁻⁶ per °C at 0 °C, crosses zero near 4 °C, reaches about 207×10⁻⁶ per °C at 20 °C, and climbs to roughly 750×10⁻⁶ per °C near boiling. This calculator reports the average coefficient across your chosen temperature range.
Does this account for the tank or pipe expanding too?
No — this calculator covers the water only. Metal and plastic containers also expand when heated, just far less than water does, which is exactly why the mismatch causes pressure buildup in closed systems. Sizing an expansion tank or relief valve requires an engineer familiar with the specific system.
Sources & references
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems.
- International Bureau of Weights and Measures (BIPM). The International System of Units (SI).
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)