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).
Pick the reference temperature & enter viscosities
Oil 1
Oil 2
Results
Viscosity comparison
Bar lengths are normalised to log₁₀(ν) so oils with similar viscosity remain visually distinguishable.
One page with the inputs, results, equations, standards, and source URL.
Equations and standards
Converts each oil's kinematic viscosity into a viscosity blending index that adds linearly.
Weighted by mass fraction. For a two-oil blend, x2 = 1 - x1.
Inverts the index back to kinematic viscosity, at the same reference temperature as the inputs.
Symbols and units
| Kinematic viscosity of each oil, at the same reference temperature | mm²/s (cSt) | |
| Mass fraction of oil 1 | — | |
| Kinematic viscosity of the blend | mm²/s (cSt) | |
| Viscosity blending index | — |
Valid range and limitations
- ·Both viscosities must be at the same reference temperature — two values at 40 °C, or two at 100 °C. Mixing them makes the result meaningless.
- ·Defined only for nu > 0.2 mm²/s, since ln(nu + 0.8) must be positive.
- ·The Refutas equation is empirical. It works well for base-oil blends but deviates for multigrade oils containing polymeric VI improvers.
- ·Assumes mass fractions. Convert with density if you are blending by volume.
Reproduce in Excel
Blend viscosity, in a single cell
=EXP(EXP((($C$1*(14.534*LN(LN($A$1+0.8))+10.975)+(1-$C$1)*(14.534*LN(LN($B$1+0.8))+10.975))-10.975)/14.534))-0.8A1 = viscosity of oil 1, B1 = viscosity of oil 2, C1 = mass fraction of oil 1 (0–1). Both viscosities at the same reference temperature.
Viscosity blending index, on its own
=14.534*LN(LN(A1+0.8))+10.975A1 = kinematic viscosity nu in mm²/s.
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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