Show the formula and working
This mist coat calculator works out how much matt emulsion and how much water you need to seal freshly plastered walls and ceilings, for homeowners and decorators in the UK. Enter the room size, the water to paint ratio from your tin and the coverage, and it gives the litres of mixed mist coat, the split between paint and water, and the number of tins to buy.
Quick answer: Divide the wall and ceiling area by the coverage and add about 10% for thirsty plaster. A 4 m by 3 m room with 2.4 m walls and a ceiling is 45.6 m², which needs 2.79 litres of mix at 18 m² per litre: the mist coat calculator splits this into 2.09 litres of paint and 0.70 litres of water.
What Is a Mist Coat Calculator?
A mist coat calculator is a tool that works out the paint and water needed for the first, watered down coat of emulsion that seals new plaster. Mixed mist coat in litres = area in m² x coats / coverage in m² per litre x (1 + waste percentage). The paint share is the parts of paint divided by the total parts, so a 1 to 3 water to paint mix is three quarters paint and one quarter water.
The thinned paint soaks into the surface of the gypsum plaster and bonds with it, giving the following coats a sealed, even surface. Without it, dry plaster pulls the water out of normal emulsion too quickly, which can lead to patchy, flaking paint.
How Do You Use the Mist Coat Calculator?
- Enter Room length, Room width and Wall height in metres, or switch to feet.
- Tick Mist coat the walls, Mist coat the ceiling or both, and enter Doors and windows to leave out.
- Enter Parts water and Parts paint from your tin’s thinning instructions, for example 1 and 3.
- Check Coverage of the mixed mist coat. The 18 m² per litre default is the paint’s best case figure, so lower it for very thirsty plaster.
- Set Mist coats, Extra for thirsty plaster and waste and the Paint tin size.
- Read the litres of mix, paint and water, then open “Show the formula and working” to see each step.
What Is the Ratio for a Mist Coat?
A common mist coat ratio is up to 1 part water to 3 parts matt emulsion, which is the figure Dulux Trade gives for the first coat of Dulux Trade Supermatt on new or bare surfaces. That is about 333 ml of water for every litre of paint. Other paints give different ratios, so always use the thinning instructions on your own tin.
| Water to paint ratio | Water per litre of paint | Paint in 1 litre of mix | Typical use |
|---|---|---|---|
| 1 to 4 | 250 ml | 800 ml | Upper thinning limit Dulux Trade gives for Supermatt finishing coats |
| 1 to 3 | 333 ml | 750 ml | Upper thinning limit Dulux Trade gives for Supermatt on new plaster |
| 1 to 2 | 500 ml | 667 ml | Thinner than the Dulux Trade guidance; check your tin |
How Much Paint Do I Need for a Mist Coat?
A typical 4 m by 3 m room with 2.4 m walls and a ceiling needs about 2.1 litres of emulsion and 0.7 litres of water for one mist coat, assuming 18 m² per litre and 10% extra. One 2.5 litre tin covers it, but remember you will need more paint for the finishing coats, which our paint calculator and ceiling paint calculator work out.
When Can You Mist Coat New Plaster?
Apply the mist coat only when the plaster is completely dry, which you can see when it has turned an even pale colour with no darker damp patches. Drying time depends on the plaster thickness, the background, ventilation and the season, so follow the plasterer’s and the plaster maker’s guidance rather than a fixed number of days, and keep the room ventilated without using strong heat to speed it up.
How Many Mist Coats Do You Need?
One mist coat is usually enough to seal new plaster, followed by full strength finishing coats. Dulux Trade recommends two coats for Supermatt finishing coats, thinned with up to 1 part water to 4 parts paint. Add a second mist coat only if the first has soaked in very unevenly, and allow the recoat time on the tin, which is 2 to 4 hours for Supermatt.
Worked Example: New Plaster
Example: a 4 m by 3 m bedroom has been skimmed, with 2.4 m walls and the ceiling. Mix 1 part water to 3 parts matt emulsion, use 18 m² per litre, add 10% for thirsty plaster and buy 2.5 litre tins.
- Walls: 2 x (4 m + 3 m) x 2.4 m = 33.6 m². Ceiling: 4 m x 3 m = 12 m². Total 45.6 m².
- Mix: 45.6 m² x 1 coat / 18 m² per litre x 1.10 = 2.79 litres.
- Paint: 2.79 x 3 / 4 = 2.09 litres. Water: 2.79 minus 2.09 = 0.70 litres.
- Tins: 2.09 / 2.5 = 1 tin of 2.5 litres.
Mix the paint and water thoroughly in a scuttle before you start, so every wall gets the same mix. Other decorating tools are in our paint and decorating calculators section.
Mist Coat Calculator FAQs
How much water do I add to 5 litres of paint for a mist coat?
At 1 part water to 3 parts paint, add about 1.67 litres of water to 5 litres of paint, giving about 6.67 litres of mist coat. At 1 to 4, add 1.25 litres. Always check the maximum thinning on your tin first.
What paint should I use for a mist coat?
Use a matt emulsion whose tin says it is suitable for new or bare plaster and gives thinning instructions for a first coat. Dulux Trade Supermatt is one example, thinned with up to 1 part water to 3 parts paint on new surfaces.
Do I need to mist coat a ceiling?
Yes, a newly plastered ceiling needs sealing just like walls. A 4 m by 3 m ceiling is 12 m², which needs about 0.73 litres of mixed mist coat at 18 m² per litre with 10% extra.
Can I use primer instead of a mist coat?
Yes, a primer or sealer made for new plaster is an alternative to a mist coat. It costs more per litre but saves mixing. Check that the primer is compatible with the emulsion you plan to use for the finishing coats.
Does a mist coat cover more than normal paint?
Each litre of mix contains less paint, but bare plaster absorbs a lot, so the area covered per litre is often similar to or lower than the tin’s figure. That is why the calculator uses coverage per litre of mix and adds an allowance.
How long after a mist coat can I paint?
Wait for the recoat time on the tin before the first finishing coat. Dulux Trade gives 2 to 4 hours for Supermatt in normal conditions. Cold or damp rooms take longer, so make sure the mist coat is fully dry first.
Checked October 2026. Sources: Dulux Trade Supermatt (coverage, thinning and recoat time), Wikipedia: Plaster.