In short: water's dynamic viscosity is about 1.00 cP (0.00100 Pa·s) at room temperature (20 °C), dropping to roughly 0.28 cP at boiling and rising to about 1.75 cP near freezing. It falls steadily and non-linearly as temperature rises — hot water flows noticeably more easily than cold.
These figures are planning aids based on a published reference equation for pure water. For process design, dosing or safety-critical work, confirm with a qualified engineer.
How to calculate the viscosity of water
Viscosity measures how strongly a fluid resists flowing — thick syrup has high viscosity, water has low viscosity. For water, viscosity depends almost entirely on one variable: temperature. This calculator uses the reference equation adopted by the IAPWS for the temperature dependence of water's viscosity, in the same family as the classic Vogel-Fulcher-Tammann form used for liquids generally:
To get kinematic viscosity — the form used in the Reynolds number and most pipe-flow equations — divide by density: v = µ / ρ. This calculator looks up ρ from the Kell equation at the same temperature, so the two figures always stay consistent with each other.
Worked example. Take water at 40 °C, a typical warm shower temperature. In kelvin, T = 313.15. The exponent is 247.8 / (313.15 − 140) = 1.4312, so 10^1.4312 ≈ 26.99. Multiplying: µ = 2.414×10⁻⁵ × 26.99 ≈ 6.51×10⁻⁴ Pa·s, or 0.651 cP. Water's density at 40 °C is about 992.2 kg/m³, so kinematic viscosity is 6.51×10⁻⁴ / 992.2 ≈ 6.57×10⁻⁷ m²/s, or 0.657 cSt — matching what the calculator above shows at that temperature.
Why centipoise? The centipoise (cP) is the most commonly quoted unit for everyday fluids because water at 20 °C works out so close to 1.0 cP — it makes water a natural mental reference point. Scientifically, the SI unit is the pascal-second (Pa·s); 1 cP = 1 mPa·s = 0.001 Pa·s exactly.
Why temperature changes water's viscosity
In a liquid, molecules constantly bump, drag and briefly bond with their neighbors, and viscosity is a measure of how much that jostling resists flow. Heat adds energy: molecules vibrate and move faster, weakening the brief hydrogen bonds between water molecules and letting layers of fluid slide past each other more freely. The result is a viscosity that falls sharply — but smoothly and predictably — as temperature climbs.
The drop is not linear. Between 0 °C and 20 °C, viscosity falls by about 43%; between 60 °C and 100 °C, a similar 20-degree jump only removes about 40% relatively, but from a much smaller starting number. Across the full liquid range, water at freezing is about 6 times as viscous as water at boiling. That is why the same pump or pipe pushes hot water with less friction loss than cold water — see our Reynolds number calculator to see how this plays into whether flow is laminar or turbulent.
Viscosity of water by temperature
The values this calculator uses, from freezing to boiling. Density comes from the Kell equation, so kinematic viscosity here matches the tool exactly.
| Temperature | Dynamic (cP) | Dynamic (Pa·s) | Density (kg/m³) | Kinematic (cSt) |
|---|---|---|---|---|
| 0 °C / 32 °F | 1.753 | 1.753×10⁻³ | 999.84 | 1.753 |
| 4 °C / 39 °F (densest) | 1.547 | 1.547×10⁻³ | 999.97 | 1.547 |
| 10 °C / 50 °F | 1.300 | 1.300×10⁻³ | 999.70 | 1.300 |
| 20 °C / 68 °F (room) | 1.002 | 1.002×10⁻³ | 998.20 | 1.004 |
| 25 °C / 77 °F | 0.890 | 8.904×10⁻⁴ | 997.05 | 0.893 |
| 37 °C / 99 °F (body) | 0.690 | 6.904×10⁻⁴ | 993.33 | 0.695 |
| 60 °C / 140 °F | 0.463 | 4.631×10⁻⁴ | 983.20 | 0.471 |
| 80 °C / 176 °F | 0.351 | 3.510×10⁻⁴ | 971.80 | 0.361 |
| 100 °C / 212 °F (boiling) | 0.279 | 2.790×10⁻⁴ | 958.36 | 0.291 |
Quick reference: how water compares to other fluids
For a sense of scale — commonly published approximate figures at or near room temperature. These other fluids are outside what this calculator computes; use it only for context.
| Fluid | Approx. viscosity (cP) |
|---|---|
| Water (20 °C) | 1.0 |
| Blood plasma (37 °C) | ~1.5 |
| Olive oil | ~80 |
| SAE 30 motor oil | ~200 |
| Corn syrup | ~2,000–3,000 |
| Honey | ~2,000–10,000 |
Worked examples
Cold tap water in winter
1.55 cP
Groundwater-fed tap water at 4 °C has a viscosity of about 1.55 cP — over 50% higher than at room temperature, which is one reason cold showers feel like they take longer to drain.
A warm shower
0.651 cP
At a comfortable 40 °C shower temperature, viscosity drops to about 0.651 cP — roughly a third lower than cold tap water, letting it move through pipes and showerheads with less resistance.
Water at a rolling boil
0.279 cP
At 100 °C, water's viscosity is only about 0.279 cP — around 6 times lower than at freezing — part of why boiling water pours and splashes so much more freely than ice-cold water.
What else affects the viscosity of water
Dissolved salts and minerals. Seawater at typical ocean salinity (~35 g/kg) is roughly 5–8% more viscous than fresh water at the same temperature. Everyday tap water, with far lower mineral content, is close enough to pure water that the difference rarely matters outside a laboratory. For density changes from salinity, see the water density calculator.
Pressure. Liquid water is nearly incompressible, and its viscosity changes very little with pressure until you reach hundreds of atmospheres — well beyond any household or typical industrial system. For practical plumbing and process work, pressure can be ignored.
Dissolved gas and purity. Small amounts of dissolved air have a negligible effect on viscosity, which is why this calculator — like the reference equation it is based on — models pure, air-free water.
Frequently asked questions
What is the viscosity of water at room temperature?
At 20 °C (68 °F), water has a dynamic viscosity of about 1.00 centipoise (cP), which equals 0.00100 pascal-seconds (Pa·s). This is the reference value viscosity is often measured against — 1 cP was historically defined as close to the viscosity of water at this temperature.
Does hot water have lower viscosity than cold water?
Yes. Water at 100 °C is about 0.28 cP, roughly 6 times less viscous than water at 0 °C, which is about 1.75 cP. Warmer molecules move faster and slide past each other more easily, so viscosity drops sharply as temperature rises.
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity (µ) measures a fluid's resistance to shear, in units like centipoise or Pa·s. Kinematic viscosity (v) divides that by density, giving units like centistokes or m²/s. Kinematic viscosity is what shows up in the Reynolds number and most pipe-flow equations.
How is the viscosity of water calculated?
This calculator uses the empirical Vogel-Fulcher-Tammann style equation for water, µ = 2.414×10⁻⁵ × 10^(247.8/(T−140)), with T in kelvin, consistent with the IAPWS reference formulation. Kinematic viscosity then divides the result by water's density at the same temperature, from the Kell equation.
Does salt or dissolved minerals change water's viscosity?
Slightly. Seawater at typical ocean salinity is roughly 5 to 8 percent more viscous than fresh water at the same temperature, and hard tap water is only marginally different from pure water. This calculator models pure, air-free water; treat the result as a close approximation for tap water and a modest underestimate for brine or seawater.
Why does viscosity matter for pipe flow and pumps?
Viscosity governs friction losses and the Reynolds number, which determines whether flow is laminar or turbulent. Because water is far less viscous when hot, hot-water lines can carry the same flow rate with somewhat lower friction loss than cold-water lines of the same size. Check this with the Reynolds number calculator.
Sources & references
- Vogel, H. (1921). Das Temperaturabhangigkeitsgesetz der Viskositat von Flussigkeiten. Physikalische Zeitschrift, 22, 645–646.
- International Association for the Properties of Water and Steam (IAPWS). R12-08: Release on the IAPWS Formulation for the Viscosity of Ordinary Water Substance.
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)