Dynamic Viscosity of Water at 20 °C
| Value | 0.001002 Pa·s |
| Status | Measured: ± 0.000005 Pa·s (0.005 relative) |
| Source | IAPWS / CRC Handbook of Chemistry and Physics, 104th ed. |
| Categories | Material PropertiesEngineering & Tradefluids |
| pascal second | 0.001002 Pa·s |
| millipascal second | 1.002 mPa·s |
| centipoise | 1.002 cP |
| poise | 0.01002 P |
Learning zone
Water at 20 °C is very nearly exactly 1 centipoise, which is not a coincidence: the poise scale was set up so that water would land there. The kinematic viscosity is 1.004 × 10⁻⁶ m²/s, or 1.004 centistokes. Both feed the Reynolds number, and therefore every friction factor, settling velocity and pump-suction calculation you will do with water.
Temperature dominates, and by more than most people expect. Water at 5 °C is 1.52 mPa·s and at 80 °C only 0.355 — a factor of four across ordinary building temperatures. Since Re scales as 1/µ, a chilled-water line and a heating line at the same velocity are at very different Reynolds numbers, and laminar-regime behaviour in cold, small-bore or glycol-laden lines is far more common than designers assume. Pressure barely matters for liquids. Add 50 % propylene glycol and viscosity at 0 °C rises to roughly 15 mPa·s, at which point coil film coefficients collapse and pump curves stop matching the catalogue.