Viscosity is one of those properties everyone has an intuition for — honey is thicker than water — but it is also where the unit names get genuinely confusing. Poise, stokes, pascal-seconds, SAE grades and ISO VG numbers all describe the same thing from different angles. This guide untangles them.
Dynamic vs Kinematic Viscosity
| Unit | Type | SI Equivalent | Common Use |
|---|---|---|---|
| Pa·s | Dynamic | 1 Pa·s | SI standard |
| Poise (P) | Dynamic | 0.1 Pa·s | Legacy CGS |
| Centipoise (cP) | Dynamic | 0.001 Pa·s | Lab and industry |
| m²/s | Kinematic | 1 m²/s | SI standard |
| Stokes (St) | Kinematic | 10⁻⁴ m²/s | Legacy CGS |
| Centistokes (cSt) | Kinematic | 10⁻⁶ m²/s | Lubricants, fuels |
Reference Values at 20 °C
Some numbers that are worth committing to memory. Air sits at about 0.018 cP. Water is almost exactly 1 cP, which is why the centipoise feels so natural in everyday lab work. Light machine oil falls in the 30–50 cP range; SAE 30 motor oil is about 200 cP at room temperature but drops to roughly 10 cP when hot. Honey is around 10,000 cP, and pitch — famously slow — is around 2.3 × 10¹¹ cP at room temperature.
Converting Between Dynamic and Kinematic
The link between dynamic viscosity (μ) and kinematic viscosity (ν) is density: ν = μ / ρ. For water, μ ≈ 1 mPa·s and ρ = 1000 kg/m³, so ν = 10⁻⁶ m²/s = 1 cSt. For a fuel oil with μ = 50 cP and ρ = 0.9 g/cm³, ν ≈ 55.6 cSt. Always check whether a quoted value is dynamic or kinematic before plugging it into a flow calculation.
- 1 cP = 1 mPa·s exactly.
- 1 cSt = 1 mm²/s exactly.
- For any fluid with density ≈ 1 g/cm³, the cP and cSt values are numerically the same.
Common Pitfalls When Quoting Viscosity
Most viscosity errors come from missing context rather than arithmetic. Watch for these:
- No temperature stated. A viscosity number is meaningless without a temperature — oil can change tenfold between cold start and operating heat. Always pair the value with its reference temperature (commonly 40 °C or 100 °C for lubricants).
- Confusing dynamic and kinematic. A datasheet that lists “50” could be 50 cP or 50 cSt — very different once density is involved. Check the unit before using it in a Reynolds-number or flow calculation.
- Assuming Newtonian behaviour. Ketchup, paint, and polymer melts are non-Newtonian: their viscosity changes with shear rate, so a single number only applies at the measurement conditions.
- Mixing unit systems. Keep everything in SI (Pa·s, m²/s) or everything in cgs (P, St); converting halfway through is where factors of 100 or 1000 sneak in.
Convert Pressure Too
Flow problems usually need pressure conversions — UnitSnap handles Pa, bar, psi and more.
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