The units of dynamic viscosity
Dynamic viscosity measures a fluid's resistance to deformation: it is what makes honey fall slowly and water quickly. Its SI unit is the pascal second, but the one used day to day is the centipoise, and not by accident: water at twenty degrees is exactly one centipoise, so the figure tells you at a glance how many times thicker than water a fluid is.
| Unit | Symbol | Equals |
|---|---|---|
| Pascal seconds | Pa·s | 1 Pa·s |
| Millipascal seconds | mPa·s | 0.001 Pa·s |
| Poise | P | 0.1 Pa·s |
| Centipoise | cP | 0.001 Pa·s |
| Pound-force seconds per square foot | lbf·s/ft² | 47.88025898 Pa·s |
How to convert by hand
Every dynamic viscosity conversion follows the same procedure: take the amount to the base unit, then from there to the target unit. In practice it is a single multiplication, because the two steps combine into one factor.
Everything passes through pascal seconds. A poise is 0.1 and a centipoise 0.001, so 1 Pa·s is 1,000 cP. The Anglo-American equivalence, 1 lbf·s/ft² = 47.88 Pa·s, comes from dividing pound-force by the square foot.
The equivalents people look up most
| Conversion | Result |
|---|---|
| Water at 20 °C | 1 cP · 0.001 Pa·s |
| Air at 20 °C | 0.018 cP |
| Olive oil | 84 cP |
| Honey | 2,000 to 10,000 cP |
| Glycerine at 20 °C | 1,410 cP |
| Bitumen | 230 billion cP |
Common mistakes
- Confusing dynamic viscosity with kinematic. The kinematic one is this divided by density and is measured in stokes or mm²/s: converting between them means knowing which fluid you are talking about.
- Comparing viscosities measured at different temperatures. Engine oil drops tenfold between twenty and a hundred degrees, so a figure without its temperature says nothing.
- Taking the 10W-40 on an engine oil for a viscosity. Those are two grades on an SAE scale, not measurements: they describe behaviour cold and hot.