Water Density Calculator

Enter a water temperature to get its density in kg/m³, g/cm³, and lb/ft³ using the Kell equation, plus the mass of an optional volume.

Quick Facts

Density formula
Kell equation (0-100°C)
A 5th-order polynomial ratio fit to lab measurements, accurate to about 0.001%.
Maximum density
999.97 kg/m³ at 3.98°C
Water is densest just above freezing, not at 0°C — its famous anomaly.
Reference point
1,000 kg/m³ = 1 g/cm³
The historical basis for defining the kilogram from one liter of water at 4°C.

Your Results

Calculated
Density
-
ρ from the Kell equation, in kg/m³
Density (g/cm³)
-
= kg/m³ ÷ 1,000
Density (lb/ft³)
-
US customary units
Mass of Volume
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density × your volume input

Ready

Enter a water temperature, then press Calculate.

Formula and Method for Water Density

Water's density is not a fixed constant — it changes with temperature (and, to a smaller degree, pressure and dissolved solids) because thermal expansion and the hydrogen-bonded structure of the liquid reshape how tightly molecules pack together. This calculator uses the Kell equation, an empirical formula fitted to precise laboratory measurements of pure water at standard atmospheric pressure, to compute density accurately from 0°C to 100°C. It then converts that density into common units and, if you provide a volume, calculates the mass of that water.

How the calculation works

Enter a temperature and its unit; the calculator converts it to Celsius (t), then evaluates the Kell equation:

ρ(t) = (999.8395 + 16.945176t − 0.0079870401t² − 0.000046170461t³ + 0.00000010556302t⁴ − 0.00000000028054253t⁵) / (1 + 0.01687985t)

where t is the temperature in °C and ρ is the resulting density in kg/m³. This fifth-order polynomial ratio matches measured water density to within about 0.001% across its valid range. The result is then converted to g/cm³ (divide by 1,000) and lb/ft³ (multiply by 0.0624280). If you also enter a volume, the calculator converts it to cubic meters and multiplies by the density to get the mass of water in that volume.

Why water's density is not constant

Unlike most liquids, water reaches its maximum density (about 999.97 kg/m³) near 3.98°C, not at its freezing point. Below that temperature, hydrogen bonds begin arranging molecules into the more open, lower-density structure that becomes ice at 0°C; above 4°C, ordinary thermal expansion takes over and density falls steadily as temperature rises. This anomaly is why ice floats, why lakes freeze from the top down, and why the coldest, densest water in a winter lake settles near the bottom at about 4°C rather than at the surface.

Where water density calculations matter

  • Fluid mechanics: pipe flow, pump sizing, and open-channel calculations convert between volume and mass flow rates using density.
  • HVAC and boiler systems: metering heated or chilled water accurately requires correcting for density changes with temperature.
  • Buoyancy and mixing: density differences between water layers drive convection currents in lakes, oceans, and cooling towers.
  • Metrology: the kilogram was historically defined from the mass of one liter of water at its point of maximum density, 4°C.

Frequently Asked Questions

Why does water's density change with temperature?
Water molecules are held together by hydrogen bonds that respond to temperature in an unusual way. Above about 4°C, ordinary thermal expansion spreads molecules apart and density falls as temperature rises. Below 4°C, the hydrogen-bond network starts arranging into the more open lattice structure of ice, which also lowers density — so density rises on cooling only until it peaks near 4°C, then falls again on the way to freezing.
What temperature gives water its maximum density?
Pure water is densest at about 3.98°C (39.2°F), where it reaches roughly 999.97 kg/m³. This is why deep lakes stabilize near 4°C at the bottom in winter, with colder, less dense water and ice staying near the surface.
Is seawater denser than fresh water?
Yes. Dissolved salts add mass without adding much volume, so typical seawater (around 35 g/kg salinity) has a density near 1,020-1,029 kg/m³ at room temperature — roughly 2-3% higher than the fresh water this calculator computes.
What temperature range is this calculator valid for?
The Kell equation used here is fitted to measurements of liquid water at standard atmospheric pressure between 0°C and 100°C (32°F-212°F). Outside that range — ice, steam, or water under very different pressure — a different equation of state is required.

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