Standard Practice for Viscosity-Temperature Charts for Liquid Petroleum Products

If you know the viscosity of a particular type of oil at two temperatures (like 40 and 100°C) , it is possible to calculate it at a different temperature (ASTM D341). In addition to this, one can calculate the oil viscosity at the operating temperature of the equipment. See examples given at the end of this page.

Calculator for kinematic viscosity extrapolation

Viscosity 1 (mm2/s) cSt
At Temperature (°C)     
Viscosity 2 (mm2/s) cSt
At Temperature (°C)     
Calculate Viscousity at (°C)

Example 1: Calculating viscosity at the operating temperature

SAE-140 gear oil has a 28 cSt kinematic viscosity at 100°C and 404 cSt at 40°C. Calculate lubricant viscosity at 60°C operating temperature.

Viscosity at 60°C=132.126 (mm2/s)

Example 2: Calculate viscosity index

ASTM D2270 covers the procedures for calculating the viscosity index of oils from the kinematic viscosities at 40 and 100°C. For low flash point lubricants when conditions prevent testing at 100°C. An estimate of 100°C kinematic viscosity can be calculated by testing the lubricant at low temperatures.

Measured viscosity (ASTM D445) at 40°C= 68 cSt
Measured viscosity (ASTM D445) at 80°C= 14.85 cSt
Calculated viscosity at 100°C=8.8 cSt (use above calculator)
Use Viscosity index calculator to calculate viscosity index from kinematic viscosities at 40 (68 cSt) and 100°C (8.8 cSt).

Viscosity index=102


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