Paint Thinning Calculator

How the paint thinning calculator works

Tell it how much paint you have and what percentage of thinner you want, and it gives you the amount to measure out and the volume of the finished mix. Thinner = paint × percentage ÷ 100. One US gallon thinned at 5% takes 6.4 fl oz of thinner and makes 1.05 gal; a litre at 5% takes 50 ml. Small amounts come out in fluid ounces or millilitres because that is what a measuring jug shows.

The sum is the easy part. The percentage is where jobs go wrong, and it should come from the paint, not from a rule of thumb.

Start from the data sheet, not a typical range

Many modern paints don't need thinning at all, and the ones that do set a limit. Benjamin Moore's data sheet for Regal Select flat, a water-based wall paint, says thinning is unnecessary and caps any water or extender at 8 fl oz per gallon, which is 6.25%. Rust-Oleum's sheet for its oil-based Stops Rust brush enamel says thinning isn't normally needed for brush, roller or airless spray, but allows up to 15% for conventional air spray, and specifies acetone as the thinner while warning against gasoline, lacquer thinner and turpentine.

Those two products alone show why a single "safe" percentage doesn't exist. The right figure depends on the paint, the spray equipment, and the thinner the manufacturer names. So the result card now simply prints your ratio in the units data sheets use, fluid ounces per gallon or millilitres per litre, for a direct comparison with the maximum on the sheet.

Converting the ways thinning is written

Data sheets and sprayer manuals express the same idea several ways. Fluid ounces per gallon divide by 128: 8 fl oz per gallon is 6.25%, and a pint per gallon (16 fl oz) is 12.5%. Millilitres per litre divide by 10, so 100 ml per litre is 10%. A pint per gallon is also 12.5% in old British imperial measure, because an imperial pint is an eighth of an imperial gallon just as a US pint is an eighth of a US gallon; the ratio survives even though the gallons differ. A paint-to-thinner ratio of 4:1 means one part thinner to four of paint, or 25%.

Mixed units trip people up too. If the sheet is American and your jug is metric, one US fluid ounce is 29.6 ml, so the 8 fl oz Benjamin Moore limit is about 237 ml per US gallon, and the calculator's default 6.4 fl oz is about 189 ml.

Percent of the paint or of the mix?

This calculator takes the percentage as a share of the paint, which is how the two data sheets above write it. Some instructions mean a share of the finished mix instead. If a guide says thinner should be 10% of the final mix, the thinner is 10 ÷ 90 = 11.1% of the paint: 14.2 fl oz for a gallon rather than 12.8. The difference is small at low ratios and grows as they rise, so check which one your source means.

Worked examples

A 5 gallon pail of oil-based enamel for air spray at the full 15% needs 96 fl oz of thinner, three quarters of a gallon, and makes 5.75 gal of mix. The metric version, 5 litres at 15%, is 750 ml of thinner. At the other end, a 2.5 litre tin of water-based paint conditioned at the 6.25% Benjamin Moore maximum takes 156 ml of water.

Some sprayer manufacturers skip percentages and give a viscosity test instead: a small cup with a hole in the base, and a target time for it to drain. Where your manual does that, thin in small measured steps and retest, and note the percentage that hits the target so the next batch can go straight to it with this calculator.

Thin only what you will spray in the session. Pour off the paint you need, work out the thinner for that amount, and keep the rest of the tin as supplied. A half-used can of thinned paint is hard to judge later, and more thinner on top of it is how people drift past the limit.

What thinning changes besides flow

Adding thinner spreads the same solids over more liquid, so each coat dries thinner. Past the manufacturer's limit that can mean poorer hiding, runs, an extra coat, and a film that doesn't perform as the data sheet describes. With solvent-borne paints it also adds solvent to the air: the US EPA lists paints, paint strippers and other solvents among indoor sources of volatile organic compounds and advises increasing ventilation and following label precautions when using them. Spray in a ventilated space, keep solvent thinners away from ignition sources, and wear the protection the safety data sheet specifies.

Temperature changes things too. Cold paint is thicker and warm paint thinner, and both data sheets cited here set application temperatures of 50 to 90°F (10 to 32°C). A tin that has sat in a cold van will spray badly, and bringing it up to room temperature first can remove the urge to thin it at all.