Viscosity Converter — Poise, Stokes, Centipoise, Centistokes & More

Dynamic viscosity (μ) measures a fluid's internal resistance to shear flow — how much force is needed to make it flow. Kinematic viscosity (ν) is dynamic viscosity divided by the fluid's density (ν = μ/ρ). Kinematic viscosity accounts for both the fluid's internal resistance AND its density, and represents how it flows under gravity alone. The practical difference: a thick, heavy oil might have high dynamic viscosity but moderate kinematic viscosity because its density is also high. Converting between the two requires knowing the fluid density at the measurement temperature.

You need the fluid density: cSt = cP ÷ density (in g/cm³). For example, SAE 30 oil at 100°C has a density of about 0.87 g/cm³ and dynamic viscosity of about 10 cP: kinematic viscosity = 10 ÷ 0.87 ≈ 11.5 cSt. For water at 20°C, density = 1.0 g/cm³, so 1 cP = 1 cSt — a coincidence that causes confusion. Always use the density at the same temperature as the viscosity measurement, because both density and viscosity change with temperature.

SAE (Society of Automotive Engineers) viscosity grades define engine oil flow characteristics at specific temperatures. Monograde oils (SAE 30, SAE 40): tested at 100°C only. Multigrade oils (SAE 5W-30, 10W-40): the "W" (Winter) number indicates low-temperature pumpability (tested at -30°C to -40°C), the second number indicates high-temperature viscosity at 100°C. For example, 5W-30 oil flows like SAE 5 when cold (good winter starting) but thickens to SAE 30 at operating temperature (good engine protection). Lower first numbers = better cold flow. Common recommendations: 0W-20 for modern fuel-efficient engines, 5W-30 for most gasoline engines, 10W-40 for older/high-mileage engines, 15W-40 for diesel engines.

Viscosity varies enormously: Water at 20°C: 1.0 cP. Olive oil at 20°C: 84 cP. SAE 30 motor oil at 20°C: 500-700 cP. SAE 30 motor oil at 100°C: 9-12 cP (viscosity drops dramatically with temperature). Honey at 20°C: 2,000-10,000 cP depending on type. Ketchup: 50,000-100,000 cP (shear-thinning — it flows more when squeezed). Peanut butter: 150,000-250,000 cP. Glass at room temperature: 10^20+ cP (it is technically an extremely slow-flowing liquid). Pitch (the famous "pitch drop experiment"): 2.3 × 10^11 cP — about 230 billion times more viscous than water.

Viscosity decreases as temperature increases for liquids (and increases for gases). For liquids, heat increases molecular motion, reducing intermolecular forces and allowing molecules to slide past each other more easily. Engine oil viscosity drops by roughly 90% from 20°C to 100°C. This dramatic change is why multigrade oils exist — they contain viscosity index improvers (long polymer chains) that expand at high temperature to partially compensate for the natural thinning. The relationship follows an approximate exponential curve: a 10°C rise typically cuts viscosity by 30-50% for most oils. This temperature sensitivity is a critical consideration in any engineering application involving fluid flow, from hydraulic systems to food processing to chemical reactors.