Base-Oil Blend Viscosity Calculator

Estimate the kinematic viscosity of a two-base-oil blend from component viscosities at a common temperature and mass proportion.

Engineering screening tool—verify inputs and results independently. Use at your own risk.

Blend estimate

Empirical transformed-log blend result at the selected temperature.

Blend kinematic viscosity, ν_mix—mm²/s (cSt)

Enter valid inputs to calculate.

Oil 2 by weight—

Empirical constant K—

Equations used

Let ν₁ and ν₂ be component kinematic viscosities in mm²/s at the same selected temperature. p₁ is the entered oil-1 mass percentage, and the equation uses the dimensionless oil-1 mass fraction w₁ = p₁ / 100; oil 2 has mass fraction 1 − w₁.

A₁ = ln(ν₁ + K)

A₂ = ln(ν₂ + K)

Amix = A₂(A₁/A₂)^w₁

νmix = exp(Amix) − K

K = 1.9 mm²/s at −5 °C, K = 4.1 mm²/s at 40 °C, and K = 1.8 mm²/s at 100 °C. K uses the same numerical mm²/s convention as ν₁ and ν₂ inside the empirical transform. Natural logarithm and exponential functions are used. The result is rounded to two decimal places for display.

Symbols and sign conventions

ν₁, ν₂, and ν_mix are positive kinematic viscosities in mm²/s. p₁ is a percentage from 0% to 100%; w₁ is dimensionless and ranges from 0 to 1. T selects one discrete common temperature. K is the temperature-specific viscosity shift in mm²/s. A₁, A₂, and Amix are dimensionless empirical transformed quantities only under this fixed unit convention; do not substitute viscosity values expressed in another numerical unit. All viscosities must be strictly greater than zero; negative viscosities and mass fractions are not physical.

How to use

Select the common temperature: −5 °C, 40 °C, or 100 °C.
Enter oil 1 kinematic viscosity ν₁ in mm²/s (cSt) at that temperature.
Enter oil 2 kinematic viscosity ν₂ in the same units and at the same temperature.
Enter oil-1 mass percentage p₁ from 0% to 100%; the equation converts it to w₁ = p₁ / 100 and the displayed oil-2 percentage is the balance.
Select Calculate to update the blend estimate. Correct any red warning before using the result.
Read ν_mix as the estimated blend kinematic viscosity at the selected temperature. The shown value is rounded to two decimal places.
Select Reset example to restore 40 cSt, 20 cSt, 50 wt%, and 40 °C.
Do not use the estimate for incompatible oils, additive-reactive blends, non-Newtonian fluids, temperatures between the listed choices, or safety-critical specifications without measured blend data.

Worked example

1. At 40 °C, enter ν₁ = 40 mm²/s, ν₂ = 20 mm²/s, and p₁ = 50%, so w₁ = 0.5. 2. Calculate. 3. The unrounded estimate is 28.0757362 mm²/s and the displayed result is 28.08 mm²/s. This screens an equal-mass blend whose viscosity lies between the two component values.

Theory and method

Viscosity does not generally blend linearly with mass fraction. This calculator transforms each component viscosity with a temperature-specific shift and natural logarithm, combines the transformed values by the oil-1 weight fraction, and applies the inverse exponential transform. The endpoint behavior is exact within floating-point precision: 0% oil 1 returns ν₂ and 100% returns ν₁.

Assumptions and limits

The estimate assumes two fully miscible base oils, a homogeneous blend, Newtonian behavior, and component viscosities valid at one identical temperature. It does not convert mass fraction to volume fraction, account for density, additives, chemistry, shear thinning, pressure, uncertainty, or experimental scatter. The three K values are discrete; selecting a temperature does not calculate component viscosity at that temperature. Confirm critical formulations with laboratory kinematic-viscosity measurement.

Reference

For broader standard practice and applicability considerations, see ASTM D7152, Practice for Calculating Viscosity of a Blend of Petroleum Products, doi:10.1520/D7152-11R16. The implemented empirical equations and constants are stated explicitly above and should not be assumed equivalent to every standardized blending method.