Hong Life CalculatorsUnderstanding the water viscosity calculator — concept and principle
Viscosity describes how strongly a fluid resists flowing. Water becomes much thinner as it warms, which affects pumping, pipe pressure losses, filtration and laboratory measurements. This calculator gives the dynamic and kinematic viscosity of liquid water from −8 to 150 °C using the recommended correlation of Kestin, Sokolov and Wakeham.
How it is calculated
log₁₀(η ÷ η₂₀) = [(20 − t) ÷ (t + 96)] × [1.2378 − 1.303×10⁻³(20 − t) + 3.06×10⁻⁶(20 − t)² + 2.55×10⁻⁸(20 − t)³]
η₂₀ = 1002.0 µPa·s
ν = η ÷ ρ
- t is the temperature in °C. η is shown in mPa·s (= cP), Pa·s and µPa·s.
- For 0–40 °C the density comes from the Tanaka (2001) formula; outside that range, enter it yourself. ν is shown in mm²/s (= cSt) and m²/s.
The principle
In a liquid, molecules are held together by attractions, and flow requires them to slide past each other. Warmer molecules move faster and break these temporary bonds more easily, so viscosity drops quickly with temperature. In water the effect is strong because of hydrogen bonding: viscosity roughly halves between 20 °C and 50 °C. The correlation is an empirical fit to careful measurements, anchored at the 20 °C value. Dynamic viscosity measures internal friction; kinematic viscosity divides it by density and is the form used in the Reynolds number.
Worked example
Default inputs: 20 °C, density automatic.
- At 20 °C, 20 − t = 0, so log₁₀(η ÷ η₂₀) = 0 and η = η₂₀ = 1,002 µPa·s.
- That is 1.002 mPa·s (cP) = 0.001002 Pa·s.
- Density from Tanaka (2001) at 20 °C = 998.207 kg/m³.
- ν = 1,002 ÷ 998.207 = 1.0038 mm²/s = 0.000001004 m²/s.
- State: Liquid.
| Water temperature (°C) | Dynamic viscosity η (mPa·s = cP) |
|---|---|
| 5 | 1.5202 |
| 10 | 1.3075 |
| 15 | 1.1385 |
| 20 | 1.002 |
| 25 | 0.8901 |
| 30 | 0.7972 |
| 40 | 0.6529 |
How to read the result
Near room temperature, water's viscosity is close to 1 mPa·s and its kinematic viscosity close to 1 mm²/s, which makes it a common reference fluid. At 0 °C viscosity is about 1.8 mPa·s, and at 100 °C about 0.28 mPa·s. Lower viscosity means less pressure loss in pipes and faster settling of particles; higher viscosity at low temperature can noticeably slow filters and increase pumping energy.
Common mistakes
- Confusing dynamic viscosity (Pa·s) with kinematic viscosity (m²/s).
- Using the 20 °C value for hot or cold water.
- Forgetting that the result above 100 °C assumes pressurised liquid, not steam.
- Applying pure-water values to salty or sugary solutions.
Frequently asked questions
What is a centipoise?
A unit of dynamic viscosity equal to 1 mPa·s; water at 20 °C is about 1 cP.
Why does warm water flow more easily?
Faster molecular motion breaks hydrogen bonds more readily, so layers slide past each other with less resistance.
What is kinematic viscosity used for?
It appears in the Reynolds number and in many pipe-flow and settling calculations.
Does pressure change water's viscosity?
Only slightly in this temperature range.
Sources: Kestin J., Sokolov M., Wakeham W. A., "Viscosity of liquid water in the range −8 °C to 150 °C", J. Phys. Chem. Ref. Data 7, 941 (1978) · Tanaka M. et al., Metrologia 38 (2001) 301–309