Viscosity Converter

Dynamic viscosity (μ, in centipoise cP = mPa·s) measures the fluid internal resistance to flow. Kinematic viscosity (ν, in centistokes cSt = mm²/s) is the dynamic one divided by density: ν = μ/ρ — it is the property used in pump and piping calculations. Saybolt Universal (SSU or SUS, in seconds) is the empirical reading of the Saybolt viscometer, widely used in oil catalogs. All three measure the same thing on different scales: 1 cP = 1 cSt × density; cSt and SSU relate through ASTM D2161.

The conversion follows the ASTM D2161 standard. For high viscosities (above ~70 cSt), the relationship is nearly linear: SSU ≈ cSt × 4.6324. For low viscosities there is an additional correction term. For example: 20 cSt ≈ 97.8 SSU; 50 cSt ≈ 232 SSU; 100 cSt ≈ 464 SSU. The calculator uses the full canonical ASTM D2161 form at 100 °F, so it is accurate across the whole range — and performs the inverse conversion (SSU → cSt) consistently. Note: the Saybolt conversion depends on the measurement temperature.

Dynamic viscosity (cP) is the kinematic one (cSt) multiplied by the fluid density in g/mL: cP = cSt × density. That is why the conversion requires the density — without it, cSt and cP are not interchangeable. For water at 20 °C (density ≈ 1.0 g/mL) the two values nearly coincide: 1 cSt ≈ 1 cP. For an oil with density 0.9, 100 cSt equals 90 cP. For glycerin (density 1.26), 100 cSt equals 126 cP.

Viscosity is the main criterion when choosing between a centrifugal pump and a positive-displacement pump. Centrifugal pumps (like the FBCN) lose efficiency quickly with viscous fluids, because viscosity dissipates the impeller kinetic energy. As a rule of thumb, above ~100 cP the positive-displacement pump — gear type (FBE/FBEI) — becomes the correct choice: it moves a fixed volume per revolution, regardless of viscosity, with stable efficiency. Oils, asphalt, resins, glycerin and polymers call for a gear pump.

Yes, and strongly — in liquids, viscosity drops as temperature rises. That is why asphalt and heavy oils are heated before pumping: heating lowers the viscosity to a pumpable value. A practical consequence: any viscosity value only makes sense together with the temperature at which it was measured, and the Saybolt (SSU) conversion is defined at a reference temperature (100 °F or 210 °F). When specifying a pump, always give the viscosity at the actual operating temperature.

FB Bombas’ FBE series (external gear, API 676) pumps viscous fluids up to 100,000 SSU — about 21,600 cSt. This range covers virtually all viscous industrial fluids: lubricating and fuel oils, heated asphalt and bitumen, epoxy and polyester resins, glycerin, molasses, chocolate and polymers. For each application, the viscosity at pumping temperature sets the pump speed and size — FB engineering sizes it from the real fluid data.