Viscosity basics
Viscosity describes how “thick” or “resistant to flow” a fluid is. Two measures are commonly used in engineering:
- Dynamic (absolute) viscosity (μ): resistance under shear. SI: Pa·s. Lab/industry: cP, mPa·s, Poise.
- Kinematic viscosity (ν): dynamic viscosity divided by density. SI: m²/s. Lab/industry: St, cSt.
Core formulas
Dynamic viscosity: 1 Pa·s = 1000 mPa·s = 1000 cP.
Poise: 1 P = 0.1 Pa·s → 1 cP = 0.001 Pa·s.
Kinematic: ν = μ / ρ
If μ in Pa·s and ρ in kg/m³ → ν in m²/s.
Stokes: 1 St = 10⁻⁴ m²/s → 1 cSt = 10⁻⁶ m²/s.
Why converters differ
Some industrial converters assume ρ = 1000 kg/m³ (water) and treat cP and cSt as numerically equal. This is only valid near water density. For oils, fuels or polymer solutions, always enter the actual density.
FAQ
1. Can I convert cP to cSt without density?
Only approximately, assuming ρ ≈ 1 g/cm³. For engineering calculations, enter density.
2. What unit should I store in my database?
Store SI (Pa·s for dynamic, m²/s for kinematic) and convert to display units. This avoids rounding cascades.
3. Can I use this for high-temperature fluids?
Yes, but viscosity and density vary strongly with temperature. Use property tables from the fluid supplier and then convert.
Full original guide (expanded)
You can add below a table of typical viscosities (water 1 cP, motor oil 100–400 cP, glycerin 1000+ cP) and density ranges to help users pick a realistic ρ.