Preparation
Thinning and mixing ratio
- where
- thinner to add (L)
- paint volume (L)
- thinning as % of the paint
- total mix (L)
- thinner as a share of the mix (%)
- parts of paint per part of thinner
Explain this equation
Works out how much thinner to add to paint for a given thinning percentage, and shows the same thing as a ratio.
Thinning is usually quoted as a percentage of the paint, so the thinner is that percentage of the paint volume. The mix is paint plus thinner. As a share of the mix the percentage is slightly lower, because the mix is bigger than the paint. 100/p expresses it as one part thinner to n parts paint.
Percentages and ratios of a simple mixture.
Sense check: 5 litres thinned 10% needs 0.5 litres of thinner, 5.5 litres of mix; that is 1 part to 10, or about 9% of the mix.
Watch out: "10% thinned" on a data sheet almost always means 10% of the paint volume. Over-thinning reduces film build and hiding.
Works out how much water or thinner to add for a given thinning percentage, and the equivalent “1 part to n parts” ratio.
Enter your values to see the result.
Good to know
Data sheets give thinning either as a percentage of the paint or as parts: Johnstone’s Covaplus up to 10% water on absorbent surfaces (1:10); Dulux Trade Weathershield Textured normally no more than 1 part water to 10 parts paint, and 1:5 (20%) for sealing new or bare surfaces. Thin water-based paints with clean water and solvent-borne paints only with the thinner on the data sheet. Spray thinning follows the sprayer and paint maker’s guidance.
Common mistake
Confusing “10% of the paint” with “10% of the mix”: 1:10 is about 9.1% of the final mix.
Limits
Arithmetic only. Over-thinning reduces film build and opacity; never exceed the data sheet limit.
Sources
- Johnstone’s Trade Covaplus Vinyl Matt (up to 17 m²/L depending on porosity; seal absorbent surfaces with a coat thinned up to 10% with water; 1, 2.5, 5, 10 L)
- Dulux Trade Weathershield Textured Masonry Paint data sheet (6–8 m²/L on smooth render; 3–4 m²/L on pebbledash; normal thinning max. 1 part water to 10 parts paint)
- Dulux Trade Vinyl Matt product data sheet (up to 17 m²/L; 2 full coats; seal new or bare surfaces with up to 1 part water to 5 parts paint; packs 1, 2.5, 5 and 10 L)
- BS 6150:2019 Painting of buildings. Code of practice
Results are calculations from the published methods and figures shown, and depend on the data you enter. Coverage depends on the product, surface and application. Always follow the paint and wallcovering manufacturers’ data sheets and BS 6150 / BS 8000-12. Check prices and VAT with your own records and HMRC.