Oil viscosity blending calculator
Refutas / ASTM D341 blend-ratio solver. In Blend mode it finds the volume ratio of two ISO VG oils to hit a target viscosity; in Reservoir Correction mode it back-solves a drain-and-refill after the wrong oil was added.
How this calculator works
Each oil's viscosity at 40 °C is taken as the nominal ISO VG of its grade (ISO 46 = 46 cSt, and so on). Viscosities are converted to a blending number with the Refutas equation VBN = 14.534 × ln(ln(ν + 0.8)) + 10.975. The blend's VBN is the volume-fraction-weighted average, which is converted back to a viscosity.
Blend Two Oils searches for the fraction of Oil A that hits your target and splits the total volume. The target must lie between 90% of the lighter grade and 110% of the heavier one. A warning shows if the closest blend is more than 2 cSt from the target.
Reservoir Correction works out the current mix of the correct oil and the wrong oil (the added volume is converted to the reservoir's units). It then tests draining 0–90% of the reservoir in 0.5% steps and refilling the same volume with the correction oil. It picks the drain that lands closest to the target grade's midpoint. A mix is in spec within ±10% of the nominal grade. The calculator says when the current mix is already in spec, when the correction oil can't bring the mix into spec, and when more than half the reservoir would have to be drained.
Inputs explained
- Oil A and Oil B (Blend): ISO VG 22 to 1000. Defaults are 46 and 220.
- Target viscosity (Blend): cSt at 40 °C. Defaults to 100.
- Total volume needed (Blend): gallons or litres. Defaults to 100.
- Reservoir volume, target ISO VG, wrong oil added and its volume, correction oil (Correction): defaults are 80 gal of ISO 220 with 10 gal of ISO 46 added, corrected with ISO 320.
Worked example
Blend: ISO 46 and ISO 220 mixed to 100 cSt, 100 gal total. Calculator output: “46.3 gal of ISO 46 + 53.7 gal of ISO 220”, resulting viscosity 100.0 cSt at 40 °C (46.3% / 53.7% by volume).
Correction: an 80 gal ISO 220 reservoir got 10 gal of ISO 46 and will be corrected with ISO 320. Calculator output: current mix 175.7 cSt (out of spec) · “Drain 31.2 gal, refill with ISO 320” · corrected viscosity 220.3 cSt (+0.1% from midpoint), spec range 198.0–242.0 cSt.
Frequently asked questions
How do you calculate the viscosity of an oil blend?
The calculator uses the Refutas method. Each oil's 40 °C viscosity ν is converted to a blending number, VBN = 14.534 × ln(ln(ν + 0.8)) + 10.975. The numbers are averaged by volume fraction, and the average is converted back to a viscosity. For example, 46.3% ISO 46 and 53.7% ISO 220 gives 100.0 cSt at 40 °C.
What viscosity does the calculator assume for each ISO grade?
It uses the nominal ISO VG midpoint at 40 °C, so ISO 46 counts as 46 cSt, and so on. Real oils can sit anywhere within ±10% of the midpoint, so check the product data sheets if you need a closer answer.
How do I fix a reservoir topped up with the wrong oil grade?
Use Reservoir Correction. Enter the reservoir volume, the grade that should be in it, the wrong grade and how much was added, and the grade you will refill with. The calculator finds the drain-and-refill volume that brings the mix closest to the target grade's midpoint. It suggests a full drain, or a different correction grade, when that can't reach ±10% or when more than half the reservoir would need draining.