Understanding your result
The headline is the thinner to add, in fluid ounces or millilitres, and the sub-line gives the paint, the total mixed and whether you are over the limit. The three figures show the paint-to-thinner ratio, the total mixed and whether the mix is within the label limit.
The table lists the paint, the thinner and its percentage, the total, the ratio, the label limit in the same units, the yes or no, and the film per coat compared with unthinned paint. That last line is the honest cost of thinning: at 10% it is 91%, so ten coats of thinned paint lay down about what nine coats of unthinned paint would.
Thinning is usually a sprayer question. The spray tip calculator matches a tip to unthinned paint, which is often the better answer than thinning; the sprayer usage calculator includes a thinning percentage in its gallons; and the quantity converter turns fluid ounces into cups or millilitres if your measuring jug is marked differently.
How we calculate this
The percentage is of the paint volume, as labels state it, and the arithmetic is exact: a US gallon is 128 fl oz, so 10% is 12.8 fl oz; a litre is 1,000 mL, so 5% of 4 L is 200 mL. The film-per-coat line follows from the dilution: the solids in a gallon of paint are spread through 1.1 gallons of mix, so each unit volume carries 1 ÷ 1.1 of the solids, and the same wet film dries to 91% of the thickness.
The film thickness guide explains why the dry film is what decides hiding and durability, and the spray tips guide covers the alternative of a larger orifice. Label limits come from the products themselves; the default of 10% is typical of latex wall paints.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Thinner percentage | 10% of the paint volume | A typical maximum on latex wall paint labels |
| Label limit | 10% | Typical latex limit; oil-based labels often allow 5–15% |
| US gallon | 128 fl oz | US customary definition |
| Litre | 1,000 mL | SI |
| Film per coat | paint ÷ (paint + thinner) | Dilution arithmetic |
Assumptions last reviewed October 8, 2026.
The calculator does not know your product’s label limit, which can be zero for some coatings and for most two-part products; enter it from the can. It does not model temperature, which changes how much a paint needs, or the VOC content of the thinner, which matters where VOC limits apply to the applied mix.
Two worked examples
A gallon of latex for a sprayer
1 gallon, 10% water, label limit 10%.
- Water: 1 gallon × 10% = 12.8 fl oz
- Total mixed: 1.1 gallons; ratio 10 : 1
- Film per coat: 1 ÷ 1.1 = 91% of unthinned
Within the limit. At 15% the same gallon would take 19.2 fl oz, a 6.7 : 1 ratio, over the limit: the result says so and suggests a larger tip instead.
Oil enamel in metric
4 L of oil-based enamel, 5% mineral spirits, label limit 10%.
- Mineral spirits: 4 L × 5% = 200 mL
- Total mixed: 4.2 L; ratio 20 : 1
- Film per coat: 95% of unthinned; the label would allow up to 400 mL
A light thinning like this helps a brushed enamel level out its brush marks without costing much hide.
Where to find your inputs
Paint to thin. Thin only what you will use today, in a separate bucket, not the whole can.
Thinner percentage. From the sprayer manual, the label’s application section, or a few percent at a time until the paint flows.
Label limit. On the can or the technical data sheet, under “Thinning” or “Reduction”. “Do not thin” means zero.
Thinner. Water for latex and acrylic; mineral spirits (or the named solvent) for oil and alkyd.
Common mistakes
- Thinning to stretch the paint. Each coat hides less; you pay it back in coats.
- Thinning the whole can. Thin a working bucket; the rest stays as made.
- Water in oil paint, or spirits in latex. They do not mix; the paint is ruined.
- Going past the label limit. Runs, poor hide, lost warranty.
- Not stirring enough. Thinner sits on top; stir until uniform.
- Thinning instead of changing the tip. A larger orifice sprays full-bodied paint properly.
Questions people ask
- How much water do I add to latex paint?
- Most latex paint labels allow up to about 10%, which is 12.8 fl oz per gallon (roughly a pint and a half cup short) or 100 mL per litre. Many do not need any for brushing or rolling; thinning is mainly for sprayers that cannot handle full-bodied paint. Check the can, because some products say do not thin.
- How much mineral spirits for oil-based paint?
- Labels typically allow 5 to 15%, often expressed as about a pint per gallon (12.5%). Add it a little at a time, stir and test. A 5% addition to 4 L is 200 mL.
- Is the percentage of the paint or of the mix?
- Of the paint, which is how labels state it. Ten percent means 10 parts thinner to 100 parts paint, a 10 to 1 ratio, and the mix is 110 parts. The calculator shows the ratio and the total.
- Does thinning reduce coverage?
- It increases the area a gallon of mixed paint covers but reduces the film each coat leaves, and so the hiding. At 10% thinner each coat lays down about 91% of the film it would unthinned. That is why over-thinned paint needs an extra coat.
- When should I thin paint?
- When the sprayer manufacturer's tip and pump call for a thinner material, when a brushed enamel will not level, on a hot dry day when paint drags, or for a first coat into bare porous wood if the label recommends it. Not to stretch a gallon.
- What happens if I thin past the label limit?
- The paint runs and sags, hides poorly, can lose adhesion, and may no longer meet the VOC limits it was sold under. Manufacturers' warranties usually require the label limits. If the paint will not spray, use a larger tip or a pump rated for it.