What this calculator does
The density of water is not a fixed number: it changes with temperature, and not always in the direction people expect. Water is at its densest at about 4°C, around 999.97 kg/m³, and becomes slightly less dense both as it warms toward boiling and as it cools toward freezing, which is why ice floats and why deep lakes stay a stable few degrees above zero at the bottom in winter.
This calculator uses a standard reference polynomial fitted to measured water density across the liquid range, from 0°C to 100°C at normal atmospheric pressure. The commonly quoted figure of 1,000 kg/m³ (or 1 g/mL) is a rounded approximation that only holds close to 4°C; at 25°C, room-temperature water is measurably lighter, around 997 kg/m³.
The formula
The calculator evaluates a polynomial fit to the standard water density reference table across 0°C to 100°C, at normal atmospheric pressure. The result is returned in kg/m³ and the equivalent g/mL, alongside how it compares to water's maximum density at 4°C.
| Term | Meaning |
|---|---|
| Density (ρ) | Mass per unit volume, here in kilograms per cubic metre (kg/m³), equivalent to grams per millilitre when divided by 1,000. |
| 4°C density maximum | Pure water reaches its highest density at approximately 3.98°C (rounded to 4°C), at about 999.97 kg/m³. |
| Thermal expansion | The tendency of water to expand, and so become less dense, moving away from 4°C in either direction within its liquid range. |
The inputs explained
| Field | What to enter |
|---|---|
| Water temperature (°C) | The temperature of the water, in degrees Celsius, anywhere from 0°C (freezing point) to 100°C (boiling point at sea level). |
When to use it
Converting a measured volume to mass accurately
For work needing more precision than the rounded "1 litre = 1 kg" rule of thumb, such as lab measurements or precise dosing, using the actual density at the water's real temperature avoids a small but real error.
Understanding why ice floats
Ice is less dense than liquid water at any temperature, which is unusual among common substances; this calculator shows how liquid water's own density varies with temperature, the related but separate phenomenon behind why lakes freeze from the top down.
Checking buoyancy or hydrostatic calculations at a non-standard temperature
Buoyancy, pressure and flow calculations often assume water is exactly 1,000 kg/m³; substituting the actual density at the working temperature refines those results, particularly for warmer water such as a heated tank or pool.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the density of water changes from freezing to boiling
Water density at a spread of temperatures from just above its 4°C peak through to boiling.
| Temperature | Density |
|---|---|
| 4°C | 999.972 kg/m³ |
| 10°C | 999.700 kg/m³ |
| 20°C | 998.204 kg/m³ |
| 25°C | 997.045 kg/m³ |
| 40°C | 992.216 kg/m³ |
| 60°C | 983.199 kg/m³ |
| 80°C | 971.798 kg/m³ |
| 100°C | 958.364 kg/m³ |
Questions
What is the density of water?
It depends on temperature: water is densest at about 999.97 kg/m³ near 4°C, and becomes progressively less dense moving away from that point in either direction, down to roughly 958.4 kg/m³ at 100°C. The commonly quoted 1,000 kg/m³ is a convenient rounding valid only close to 4°C.
Why is water densest at 4C rather than at freezing point?
Hydrogen bonding in water forms an increasingly open, cage-like structure as it cools toward 0°C, which actually increases its volume (and lowers its density) below about 4°C, right up until it freezes into ice, which is even less dense still. Above 4°C, ordinary thermal expansion takes over and density falls as temperature rises.
Does the density of water change with pressure or salinity?
Yes, but this calculator only covers pure water at standard atmospheric pressure. Seawater is measurably denser than fresh water at the same temperature because of dissolved salts, and density also rises slightly with pressure at greater depth.
How is this different from a general density unit converter?
A unit converter only rescales a density value you already have between units such as kg/m³ and g/mL. This calculator instead supplies the density value itself, worked out from water's known physical behaviour at a given temperature, with no density figure needed as an input.
To convert a density value you already have between units, use the density unit converter. For the buoyant force water exerts on a submerged object, see the buoyancy calculator.