Oil Blend Viscosity Calculator

Compute blended kinematic viscosity for two oils using the Refutas equation

Enter the kinematic viscosity of two oils at the same reference temperature (40 °C or 100 °C) together with their mixing ratio. The tool computes the blended viscosity via the Refutas Viscosity Blending Index (VBI).

1Pick the reference temperature & enter viscosities

Reference temperatureBoth viscosities must be at the same temperature.

Oil 1

ν40 [mm²/s (cSt)]

VBI ≈ 33.196

Oil 2

ν40 [mm²/s (cSt)]

VBI ≈ 25.730

Oil 1 fraction

50%/50%(Oil 1 / Oil 2)

2Results

Blended viscosity at 40 °C34.65mm²/s

Blend50% · 100.0 mm²/s50% · 15.00 mm²/s

Viscosity comparison

Oil 1

100.0mm²/s

Blend

34.65mm²/s

Oil 2

15.00mm²/s

Bar lengths are normalised to log₁₀(ν) so oils with similar viscosity remain visually distinguishable.

Composition vs blended viscosity

Assumptions & Limitations

The Refutas equation is empirical and assumes the two oils are miscible. It is a reasonable approximation for base-oil blends but does not capture additive packages or viscosity-index improvers (VII).
Both viscosities must be entered at the same reference temperature (40 °C or 100 °C). Mixing values at different temperatures produces meaningless output.
For very low viscosities the inner ln(ν + 0.8) can drop below 1, pushing the Refutas formula out of its domain. The tool requires ν > 0.2 mm²/s; values above ~2 mm²/s are recommended in practice.
Real blends may deviate from the Refutas estimate due to additive interactions, water content, oxidation by-products, or shear history. Validate empirically before any safety-critical decision.

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Assumptions & Limitations

The Refutas equation is empirical and assumes the two oils are miscible. It is a reasonable approximation for base-oil blends but does not capture additive packages or viscosity-index improvers (VII).

Both viscosities must be entered at the same reference temperature (40 °C or 100 °C). Mixing values at different temperatures produces meaningless output.

For very low viscosities the inner ln(ν + 0.8) can drop below 1, pushing the Refutas formula out of its domain. The tool requires ν > 0.2 mm²/s; values above ~2 mm²/s are recommended in practice.

Real blends may deviate from the Refutas estimate due to additive interactions, water content, oxidation by-products, or shear history. Validate empirically before any safety-critical decision.