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
Oil 1
Oil 2
2Results
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.
Related Calculators
Viscosity Index & Temperature Calculator
ASTM D2270 VI and the ASTM D341 (Walther) viscosity–temperature curve
Lambda Ratio Calculator
EHL film thickness and lubrication regime classification
Specific Wear Rate Calculator
Compute wear volume and specific wear rate k from surface data, load, and sliding distance