Paint Calculator

Paint coverage is quoted per litre on the tin, but the tin's figure is a laboratory number measured on a smooth, sealed, non-absorbent surface. Real walls are none of those things. The gap between the stated coverage and what actually happens is why so many decorating jobs finish one litre short.

This calculator measures the walls, deducts the openings, applies your number of coats, and converts the result into the cans paint is actually sold in — because a paint shop will not sell you 7.1 litres, and knowing you need two 4 litre cans rather than three changes the budget.

Why the tin's coverage figure is optimistic

A coverage rate of 10 square metres per litre assumes a smooth surface with the same porosity throughout, applied at exactly the specified film thickness. Bare plasterboard and fresh plaster absorb paint far more readily than the third coat on an old wall, so the first coat on new work can consume 20–30% more than the tin claims.

The colour matters too. Going from a dark colour to a light one, or painting red over anything, usually takes three coats rather than two — red and yellow pigments are naturally translucent and simply do not cover opaquely in two passes. If you are making a big jump in tone, plan for an extra coat or use a tinted primer.

This calculator lets you pick the coverage rate you expect rather than forcing the tin's figure on you. If you are painting new plaster, choose the exterior masonry rate of 8 m²/L as a conservative stand-in, or enter a lower number under the same options — the result will be closer to reality.

Coverage rates for common paints

The defaults in the calculator reflect typical manufacturer figures for a properly prepared surface:

• Interior emulsion — 10 m² per litre. The general-purpose choice for walls and ceilings. Vinyl and acrylic emulsions are scrub-resistant and are what most interior walls get.
• Gloss and enamel — 14 m² per litre. Higher coverage because it is applied in a thinner, harder film. Used on trim, doors, skirting and joinery, where durability matters more than a flat finish.
• Exterior masonry — 8 m² per litre. Lower coverage because masonry is porous and textured and drinks the first coat. Also the right figure to use for bare interior plaster.
• Primer and sealer — 12 m² per litre. Seals a new or patchy surface so the topcoats behave predictably and cover evenly.

Always check the tin. Coverage rates vary between manufacturers and between products within one manufacturer's range, and the difference between 8 and 12 square metres per litre on a large room is several litres.

Worked example

A room 12 feet by 12 feet with 9 foot walls. The wall area is 2 × (12 + 12) × 9 = 432 square feet. Deduct one door at 21 square feet and two windows at 15 square feet each, and the paintable area is 381 square feet — 35.4 square metres.

At 10 square metres per litre for interior emulsion that is 3.54 litres per coat; two coats need 7.08 litres. In cans, that is two 4 litre cans with a little to spare, or one 20 litre drum. If you buy the same volume as five 1 US gallon (3.785 litre) cans you will pay noticeably more per litre for the same paint — the smaller the container, the higher the price per litre, on essentially every product.

Now suppose the ceiling is included. Adding 12 × 12 = 144 square feet brings the total to 525 square feet, or 48.8 square metres — 9.75 litres for two coats, which is three 4 litre cans. Including the ceiling changes the order, which is why it is a checkbox rather than an assumption.

Doors, windows and the deduction

Deducting openings is worth doing on a room with several of them, but it is easy to over-deduct. You do not paint the door, but you usually do paint the frame and the architrave around it — and on a standard doorway those edges add back a good part of the area you just removed. Windows likewise come with reveals and sills that need cutting in.

The calculator uses roughly 2 square metres per door and 1.4 square metres per window, which are the conventional allowances. If you are deducting openings and also painting all the trim in a separate colour, price the trim separately — a trim paint in gloss or enamel over doors, skirting and frames can easily consume a couple of litres on its own.

Buying and storing

Buy all the paint for one room in a single purchase, and mix cans from different batches together in one large container before you start. Paint shade varies slightly between batches, and a wall that changes colour halfway across is a mistake you cannot undo without repainting the whole thing.

Estimate generously for the first coat on new or repaired surfaces and buy one can more than the calculation suggests. Unopened water-based paint keeps for years in a frost-free store, and leftover paint in the original tin, labelled with the room and the date, is what makes a later touch-up possible. Keep a note of the colour name, the product and the sheen.