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
VBI ≈ 33.196
Oil 2
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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