Viscosity-Temperature Interpolation Calculator

How to use

1. Enter two measured kinematic viscosity and temperature pairs for the same lubricant, with an explicit unit for every value.
2. Enter the target temperature and select Calculate.
3. Read the result together with its interpolation, boundary, or extrapolation mode and any warning.
4. Edit a field and calculate again when you need a revised result.
5. Select Reset inputs when that control is shown; otherwise reload the calculator page to restore the published starting example. Calculate again before using the restored values.

Worked example: enter 40 degC / 68 cSt and 100 degC / 8.6 cSt, then calculate at 60 degC. The expected interpolation result is 28.49 cSt.

Equations used

T1 and T2 are distinct measured temperatures, T3 is the target temperature, nu1 and nu2 are measured kinematic viscosities for the same lubricant, and nu3 is the calculated kinematic viscosity. Temperatures are normalized to degC and viscosities to cSt (mm²/s).

The offsets 273 and 0.7 intentionally reproduce the validated TriboNet implementation.

Theory and method

The ASTM D341 / Ubbelohde-Walther relationship treats the same lubricant’s viscosity-temperature behavior as a straight line on transformed coordinates between two measured points.

• Supported temperature units: degC, degF, and K.
• Supported kinematic-viscosity units: cSt, mm²/s, St, and m²/s.
• Dynamic-viscosity units require density and must not be substituted.
• Extrapolation outside the measured range is a screening estimate; distant extrapolation needs a nearer measured point.
• The method is unsuitable for phase-changing or non-Newtonian fluids and for mixing measurements from different lubricants.

References: TriboNet Viscosity-Temperature Interpolation Calculator and ASTM D341-20(2025).