In short: water is densest at about 4 °C (999.97 kg/m³). Warm it toward boiling and it expands, dropping to about 958.4 kg/m³ at 100 °C — roughly 4% lighter per liter. Cool it toward freezing and density falls again below 4 °C, which is why ice floats. Enter any temperature above for the exact figure.
These figures are planning aids based on published density data. For structural, regulatory or safety-critical work, confirm with a licensed engineer.
How to calculate water density by temperature
Pure water's density isn't a single number — it's a curve. This calculator uses the Kell equation, a fifth-order polynomial fit to laboratory measurements of air-free water between 0 °C and 150 °C at one atmosphere. It's the reference method used by NIST and most engineering handbooks:
Worked example. At T = 60 °C, the numerator terms are 999.84 + 1016.71 − 28.75 − 9.97 + 1.37 − 0.22 = 1978.97. The denominator is 1 + (0.01687985 × 60) = 2.012791. Dividing gives 1978.97 ÷ 2.012791 = 983.20 kg/m³ — exactly what the calculator returns for 60 °C above.
Why a polynomial, not a simple formula? Water's density doesn't follow a straight line or a basic curve — it rises to a peak at 4 °C, then falls in both directions at slightly different rates. The Kell equation's five terms capture that real, measured behavior far more accurately than a simplified approximation.
Why 4 °C is water's density maximum
Most liquids simply get denser as they cool, right down to freezing. Water is the exception. As it cools from higher temperatures, molecules do pack closer together — but below about 4 °C, hydrogen bonds start locking water molecules into the more open, hexagonal lattice that becomes ice. That open structure takes up more space, so density starts dropping again even though the water keeps cooling. The result is a density peak at 4 °C (39.2 °F): warmer or colder than that, and a fixed mass of water takes up slightly more room.
This is why lakes freeze from the top down and never from the bottom up: 4 °C water sinks to the bottom as the densest layer, insulating whatever is below it while colder, less-dense water — and eventually ice — stays on the surface. You can read the full method on our methodology page.
Density of water by temperature
The values this calculator returns, from freezing to boiling at standard atmospheric pressure. The percentage column shows how far each point sits from the 4 °C maximum.
| Temperature | kg/m³ | g/cm³ | lb/ft³ | lb/US gal | vs. max (4 °C) |
|---|---|---|---|---|---|
| 0 °C / 32 °F | 999.84 | 0.99984 | 62.42 | 8.344 | −0.01% |
| 4 °C / 39.2 °F (densest) | 999.97 | 0.99997 | 62.43 | 8.345 | 0.00% |
| 10 °C / 50 °F | 999.70 | 0.99970 | 62.41 | 8.343 | −0.03% |
| 15 °C / 59 °F | 999.10 | 0.99910 | 62.37 | 8.338 | −0.09% |
| 20 °C / 68 °F (room) | 998.20 | 0.99820 | 62.32 | 8.330 | −0.18% |
| 25 °C / 77 °F | 997.04 | 0.99704 | 62.24 | 8.321 | −0.29% |
| 30 °C / 86 °F | 995.65 | 0.99565 | 62.16 | 8.309 | −0.43% |
| 37 °C / 98.6 °F (body) | 993.33 | 0.99333 | 62.01 | 8.290 | −0.66% |
| 40 °C / 104 °F | 992.22 | 0.99222 | 61.94 | 8.280 | −0.78% |
| 50 °C / 122 °F | 988.04 | 0.98804 | 61.68 | 8.246 | −1.19% |
| 60 °C / 140 °F | 983.20 | 0.98320 | 61.38 | 8.205 | −1.68% |
| 70 °C / 158 °F | 977.77 | 0.97777 | 61.04 | 8.160 | −2.22% |
| 80 °C / 176 °F | 971.80 | 0.97180 | 60.67 | 8.110 | −2.82% |
| 90 °C / 194 °F | 965.32 | 0.96532 | 60.26 | 8.056 | −3.47% |
| 100 °C / 212 °F (boiling) | 958.36 | 0.95836 | 59.83 | 7.998 | −4.16% |
Worked examples
Warming an aquarium
−0.20%
Raising a 20 °C tank to a tropical 28 °C drops density from 998.20 to 996.23 kg/m³. Too small to matter for fish keeping, but the same principle causes larger tanks to expand slightly as heaters run.
Sizing a water heater
−1.65%
Heating incoming water from 10 °C to a 60 °C set-point takes density from 999.70 to 983.20 kg/m³. That expansion is exactly why closed water-heating systems need an expansion tank.
Boiler feed water
−4.16%
Compared with its coldest, densest state at 4 °C, water at a rolling boil is about 4.16% less dense — a figure boiler and steam-system engineers build into feed-water and make-up calculations.
What else affects the result
Dissolved salts. Seawater at typical ocean salinity (35 g/kg) is about 2–3% denser than fresh water at the same temperature — roughly +26.5 kg/m³. If you need salinity or brine figures, use the Water Density Calculator.
Pressure. Water is nearly incompressible, so pressure has a negligible effect at household or building scale. The Kell equation used here holds for liquid water up to 150 °C, which in practice means pressurized systems like boilers rather than open, atmospheric ones.
Dissolved air and impurities. Tap water carries small amounts of dissolved gas and minerals. Their effect on density is well under a tenth of a percent, which is why this calculator — like the reference literature — models pure, air-free water.
Frequently asked questions
What is the density of water at room temperature?
At 20 °C (68 °F), pure water has a density of about 998.2 kg/m³ (0.9982 g/cm³, 62.32 lb/ft³, 8.33 lb/US gal). This is the figure most often used as a general-purpose reference.
At what temperature is water most dense?
Water reaches its maximum density at about 4 °C (39.2 °F), where it measures 999.97 kg/m³. Below and above that point water expands and becomes slightly less dense — an unusual property most other liquids don't share.
Why does water's density change with temperature?
Heating water gives its molecules more energy, so they spread further apart and take up more space, which lowers density. Below 4 °C a different effect dominates: hydrogen bonds start forming the more open lattice structure of ice, which also increases volume. The result is a density peak at 4 °C rather than at the freezing point.
Does pressure affect water's density?
Only slightly under normal conditions — water is nearly incompressible, so pressure changes at household or building scale have a negligible effect. This calculator assumes standard atmospheric pressure (1 atm); the Kell equation it uses stays accurate for liquid water up to 150 °C under pressure.
How much does water's density change between freezing and boiling?
From its maximum at 4 °C (999.97 kg/m³) to boiling at 100 °C (958.36 kg/m³), density drops by about 4.16 percent — small enough to overlook day to day, but significant in boiler and tank-sizing calculations.
Is seawater denser than fresh water?
Yes. Dissolved salt adds roughly 26 to 27 kg/m³ at typical ocean salinity (35 g/kg), making seawater about 2 to 3 percent denser than fresh water at the same temperature. For seawater or brine figures, use our Water Density Calculator.
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)