What is the density of water?
The density of water is the ratio of its mass m to its volume V. Density is usually denoted by the symbol ρ (rho):
ρ = m / V
For pure water at 4 °C (39.2 °F), density reaches its maximum of approximately 999.97 kg/m³ — often rounded to 1 000 kg/m³ (1 g/cm³) for practical purposes. At any other temperature, and especially when dissolved salts are added, water becomes slightly less or more dense.
Water density units
Density can be expressed in many combinations of mass and volume units. The most common ones for water are:
- kg/m³ — the SI unit; pure water ≈ 1 000 kg/m³
- g/cm³ (= g/mL) — convenient for lab use; pure water ≈ 1.000 g/cm³
- lb/ft³ — US customary; pure water ≈ 62.43 lb/ft³
- lb/gal (US) — common in the US; pure water ≈ 8.345 lb/gal
The density of water in kg/m³ is 1 000× greater than in g/cm³ and roughly 16× smaller than in lb/ft³.
Temperature dependence of water density
Most liquids become steadily denser as they cool, but water behaves differently — it reaches its maximum density at 3.98 °C (≈ 39.2 °F), then expands as it freezes. This is why ice floats: solid water is about 9 % less dense than liquid water at the same temperature.
Our calculator uses a precise 5th-order polynomial (valid from 0 °C to 100 °C):
ρ(T) = ρ₀ + a₁T − a₂T² + a₃T³ − a₄T⁴ + a₅T⁵
with coefficients:
- ρ₀ = 999.83311 kg/m³
- a₁ = 0.0752 a₂ = 0.0089 a₃ = 7.36413 × 10⁻⁵
- a₄ = 4.74639 × 10⁻⁷ a₅ = 1.34888 × 10⁻⁹
The density of salt water
Adding dissolved salts (salinity) increases water density. Ocean salinity is typically 35 PSU (Practical Salinity Units, ≈ 35 g of salt per kg of water), raising density to about 1 025 kg/m³. The Dead Sea reaches ~280 PSU — dense enough that humans float effortlessly.
The calculator applies a simplified UNESCO-80 salinity correction:
Δρ ≈ (0.8246 − 0.0041 × T) × S [kg/m³]
where T is temperature in °C and S is salinity in PSU.
Speed of sound in water
The speed of sound depends on water density and compressibility. In pure water at 25 °C it is about 1 497 m/s — nearly 4.4× faster than in air. Adding salt and/or raising temperature further increases this speed. Our calculator uses the simplified Mackenzie formula:
c ≈ 1 402.7 + 4.88T − 0.048T² + 0.000165T³ + 1.35S (m/s)
Should eggs float or sink?
A fresh hen's egg has a density of roughly 1 070–1 080 kg/m³. In pure fresh water (density ≈ 1 000 kg/m³) an egg sinks because water is less dense. In typical seawater (≈ 1 025 kg/m³) it still sinks — but barely. Once salinity exceeds about ~90 PSU (and density exceeds ~1 070 kg/m³), the egg will float. This is why the egg-float test works: an egg that floats in tap water is old — its air cell has grown, reducing its average density below 1 000 kg/m³.
How to use this water density calculator
- Choose your unit system: Metric (°C, kg/m³) or American (°F, lb/ft³).
- Enter the water temperature (0–100 °C / 32–212 °F).
- Enter the salinity in PSU (0 for pure fresh water, 35 for typical seawater). Use the preset buttons for common values.
- Click Calculate Density to see results in all major units, speed of sound, and egg-float status.