Water Density Calculator

Per the Kell (1975) correlation at 1 atm pressure, the density of water at 20 degrees Celsius is 998.2041 kg/m³ — equivalent to 0.9982041 g/cm³ or 62.3158 lb/ft³. Is water density exactly 1000 kg/m³? Not exactly — 1000 kg/m³ is just a practical rounding, differing by only 0.18%. Enter 20 °C in “Density vs temperature” mode to see the calculation.

Pure water reaches maximum density at 3.9832 °C (water density at 4 degrees Celsius ≈ 999.9720 kg/m³; commonly rounded to 4 °C), i.e. 999.9720 kg/m³. This value is computed from the dρ/dt = 0 root of the Kell (1975) correlation — “Maximum point” mode shows it as a reference and can compare your chosen temperature against this point.

G.S. Kell’s pure-water density formula (J. Chem. Eng. Data 20(1):97–105, 1975, Equation 16): ρ = [999.83952 + 16.945176·t − 7.9870401×10⁻³·t² − 4.6170461×10⁻⁵·t³ + 1.0556302×10⁻⁷·t⁴ − 2.8054253×10⁻¹⁰·t⁵] / [1 + 16.879850×10⁻³·t], with t in °C (IPTS-68 scale), 1 atm pressure. This page’s implementation reproduces Table I of that paper to ~1×10⁻⁷ relative on 9 temperature points.

This is water’s anomaly: below ~4 °C, water molecules begin forming more open hydrogen-bonded structures (ice-like), so volume grows and density falls even as temperature drops. Above ~4 °C, ordinary thermal expansion dominates. The two together produce the density peak of 999.9720 kg/m³ at 3.9832 °C. That is why water is densest at 4 degrees. In short, the water density-maximum anomaly explained: right at 3.9832 °C, thermal expansion and hydrogen-bonded structure formation exactly cancel out, so density reaches its extremum.

To compute a fluid’s specific weight: multiply density by gravity (γ = ρ × g). For the specific weight of water in n/m³ at 20 degrees (= water’s specific weight at 20 degrees): 998.2041 × 9.80665 = 9,789.04 N/m³, equivalent to 9.78904 kN/m³ or 62.3158 lbf/ft³. Numerically, the value in lbf/ft³ always equals the density in lb/ft³. “Specific weight” mode computes this from temperature (Kell) or from a manual density, with adjustable gravity.

To convert kg/m³ density to lb/ft³: multiply by 0.062428 (exact value: 0.028316846592 / 0.45359237, from the definitions of foot and pound). Example: 998.2041 kg/m³ × 0.062428 = 62.3158 lb/ft³. Other exact conversions: 1 g/cm³ = 1000 kg/m³, and 1 g/cm³ = 1 g/L. “Unit conversion” mode supports all four units both ways plus the “to all units” table.

m = ρ × V = 998.2041 kg/m³ × 0.001 m³ = 0.99820413 kg (about 0.9982 kg — slightly under 1 kg because 20 °C water is slightly lighter than 1000 kg/m³). So how many kg is 1 liter of water at 20 degrees? The answer: 0.9982 kg. How many kg is one gallon of water? One US gallon (3.78541 L) of 20 °C water has a mass of 3.7786 kg. For comparison, what does 1 m³ of water weigh at 20 °C? Exactly 998.2041 kg. “Mass from volume” mode (water mass-from-volume calculator) is a 3-way solver: it can find mass, volume, or density.

No — the Kell correlation is for pure water only. Pure-water vs seawater density comparison: pure water at 20 °C = 998.2041 kg/m³, while seawater at 35‰ salinity is about 1025 kg/m³ (depending on temperature and salinity). For other fluids, use “Specific weight” or “Mass from volume” mode with a manual density.

0–40 °C. Inputs outside this range are rejected with an error message — never silently extrapolated. The original Kell (1975) correlation is actually valid 0–150 °C at 1 atm; the 0–40 °C limit is chosen for maximum accuracy in common use (drinking water, aquariums, HVAC, labs).

This is the only calculator using the Kell (1975) temperature correlation for pure water. Density-conv only converts density units without temperature; volume-to-density computes ρ = m/V for general materials; buoyant-force computes Archimedean buoyant force; specific-gravity computes specific gravity relative to water. Choose what fits your need.