Paint Calculator

Buying paint by guesswork ends one of two ways: a second trip to the shop mid-job, or half-used tins that dry out in a cupboard. This calculator works out the paintable area of a room — walls, optionally the ceiling, minus doors and windows — multiplies by the number of coats, and divides by how far a litre actually goes. Coverage is an editable input rather than a hidden constant, because fresh plaster drinks nearly twice what a repainted wall does.

How to use it

1. Enter the room length, width and wall height in feet.
2. Set the number of coats and how many doors and windows to deduct.
3. Adjust the coverage if you are painting fresh plaster, then read the litres to buy.

Examples

• A 12 x 10 room with 10 ft walls and 3 openings: 377 sq ft to cover, about 7 litres for two coats.
• The same room on fresh plaster at 75 sq ft per litre needs about 10 litres.

Frequently asked questions

How much paint do I need for one room?

For a typical 12 by 10 room with 10 foot walls, roughly 7 litres covers two coats once doors and windows are deducted. The figure moves with wall height and the number of coats more than with floor area, which is why this calculator asks for height separately rather than assuming a standard room.

How many coats does a wall actually need?

Two is the normal answer for a repaint in a similar shade. Go to three when covering a dark colour with a light one, or when the new colour is strongly saturated — reds and deep yellows have poor hiding power and often need an extra coat regardless of quality.

Does fresh plaster need more paint?

Considerably. New plaster is porous and absorbs the first coat, so coverage can fall from around 110 sq ft per litre to 70 or 80. The usual practice is a primer or a thinned first coat to seal the surface, after which normal coverage applies. Set the coverage input to match what you are actually painting.

Should I include the ceiling?

Only if you are painting it, and it is usually worth doing at the same time — a fresh wall makes a yellowed ceiling obvious. Tick the ceiling option and the calculator adds the floor area to the paintable surface. Ceilings often need an extra coat because they are viewed in raking light, which shows every patch.