Viscosity Units Explained: cP, mPa·s and cSt — with Conversion Tables

Dynamic vs kinematic viscosity — the one distinction to get right

Dynamic (absolute) viscosity is a fluid's resistance to flow when a force is applied — units mPa·s or cP (SI: Pa·s). This is what a tuning-fork or rotational viscometer measures, and the unit on most product and QC specifications. Kinematic viscosity is dynamic viscosity divided by density — how a fluid flows under its own weight — with units of cSt (= mm²/s). This is the petroleum world's unit: lubricants, fuels and oils are graded by gravity-driven flow.

The bridge between them: kinematic (cSt) = dynamic (mPa·s) ÷ density (g/cm³). A worked example: an oil of 90 mPa·s with density 0.9 g/cm³ has kinematic viscosity 90 ÷ 0.9 = 100 cSt. For water (density ≈ 1.0 g/cm³), cSt and mPa·s are numerically almost identical.

Dynamic viscosity — exact conversions

Petroleum labs may also meet Saybolt seconds (SUS/SSU) — an older flow-time unit whose conversion to cSt is non-linear and table-based, not a multiplication.

Kinematic viscosity — exact conversions

Typical viscosity values — approximate, at about 20 °C

Every one of these values moves strongly with temperature — which is the next point.

Why a viscosity number without a temperature is meaningless

Viscosity falls as temperature rises — for many liquids by several percent per degree Celsius, and honey or glycerine change far faster than that. A QC spec of 500 mPa·s is only testable as 500 mPa·s at a stated temperature. Practical rules: always record the sample temperature with the reading · compare readings only at the same temperature · in Gulf facilities, remember a sample carried from a hot warehouse into an air-conditioned lab is changing viscosity as it sits on the bench.

Newtonian vs non-Newtonian: when one number isn't enough

Water and simple oils are Newtonian — viscosity is constant regardless of how fast you stir. Many real products are non-Newtonian: ketchup, mayonnaise, paints, gels, creams and slurries thin or thicken as shear increases, so their viscosity depends on how vigorously they are being worked. For these, a single reading only makes sense at a stated shear condition — and comparing instruments that shear the sample differently will legitimately give different numbers. This is why formulation labs use a rheometer rather than a single-point viscometer: A&D's RV-10000A vibro rheometer measures 0.3 – 25,000 mPa·s continuously across shear rates with real-time graphing, for exactly these products.

How viscosity is measured — and what your instrument will read

A&D's SV series uses the sine-wave vibro tuning-fork method — two sensor plates vibrating at 30 Hz — and A&D is the only manufacturer of tuning-fork viscometers. It reads continuously in mPa·s (or cP; they are the same number). The family spans 0.3 to 100,000 mPa·s across stand-type SV-A models (the SV-1A measures samples as small as 2 mL) and the handheld SV-H line for on-site checks. How the method compares with rotational (Brookfield-type) instruments is covered in our A&D vs Brookfield comparison — the short version: continuous reading, wide range without changing spindles, and small samples.