Paint Calculator

About the Paint Calculator

Buying paint is one of the most common home jobs there is, and one of the most commonly misjudged. Buy too little and the second trip may bring back a tin mixed on a different day, a shade off from the first. Buy too much and you are left with half-full tins taking up space for years.

The calculation itself is short. Work out the area to paint, divide by how far a litre (or gallon) of paint goes, and multiply by the number of coats. The two things that make it go wrong are coverage — which varies far more with the surface than most people expect — and rounding to the can sizes that are actually on the shelf.

This paint calculator works out the walls from the room's size, subtracts doors and windows, adds the ceiling if you want it, applies the coats and coverage, and tells you which combination of cans to buy.

How to Use the Paint Calculator

Choose metres or feet.

Enter the room's length, width and wall height, and the total area of doors and windows. A standard door is about 1.9 m² (21 ft²).

Tick include the ceiling if you are painting it.

Set the number of coats — two is normal.

Leave coverage blank to use a typical figure (12 m² per litre, or 350 ft² per US gallon), or enter the figure from your tin.

How the Paint Is Worked Out

  walls   =  2 × (length + width) × height − openings
  paint   =  (walls + ceiling) ÷ coverage × coats
  cans    =  the fewest cans, largest first, that hold at least that much

Step-by-Step Example

A 4 × 3 m room, 2.4 m walls, 3.5 m² of door and window, two coats at 12 m²/L.

  Walls:    2 × (4 + 3) × 2.4 − 3.5   =  30.1 m²
  Paint:    30.1 ÷ 12 × 2             =  5.0 L
  Buy:      1 × 5 L + 1 × 1 L

In feet: a 12 × 10 ft room with 8 ft walls, 40 ft² of openings and the ceiling included is 432 ft², which at 350 ft² per gallon for two coats is 2.47 gallons — two gallons and two quarts.

Coverage Is the Number That Varies

The coverage on a tin is measured on a smooth, sealed surface under good conditions. Real walls often take more paint.

New plaster is very absorbent and can take half as much area per litre on the first coat, which is why it is sealed first with a mist coat or primer.

Textured surfaces — woodchip, artex, rough render — have more surface area than their flat size suggests and can use a third more.

Dark colours over light, or light over dark, may need an extra coat to cover evenly.

Rollers and brushes differ too; a thick-nap roller holds and lays down more paint than a short one.

If you are unsure, lower the coverage figure. Leftover paint is useful for touch-ups; running out mid-wall is not.

Why Two Coats

A single coat of most emulsions leaves a slightly uneven finish, with the texture of the roller and faint differences in colour where it was thicker or thinner. The second coat evens this out and gives the colour its full depth. Repainting the same colour over sound paint is the one case where one coat can be enough.

Ceilings and Trim

Ceilings are usually painted in a flat white ceiling paint, not the wall colour, and trim — doors, frames, skirting — in a satin or gloss trim paint. The calculator splits walls and ceiling so each can be bought separately. Trim needs relatively little paint: a litre usually covers the skirting, door and frames of an average room with two coats.

Buying and Mixing

Larger cans are cheaper per litre, so the calculator uses the fewest cans, largest first. When several cans of a mixed colour are needed, pour them all into a larger container and stir before starting — a technique called boxing — so the colour is identical across every wall. Keep a little leftover, labelled with the room, for touch-ups later.

Preparing to Paint

Paint only looks as good as the surface under it, and preparation usually takes longer than the painting itself.

Clear and protect. Move furniture to the middle of the room, cover it and the floor with dust sheets, and take down fittings where you can.

Fill and sand. Fill cracks and holes, let the filler dry, sand it flush and wipe off the dust.

Clean the walls. Grease and dirt, especially in kitchens, stop paint gripping. Warm water with a little sugar soap works well.

Cut in first. Paint the edges and corners with a brush before rolling the main areas, so the roller can blend into wet edges.

Understanding Your Result

Paint needed is the total in litres or gallons.

Cans to buy gives the combination of can sizes.

Walls and ceiling splits the total when the ceiling is included.

Area shows the area and coats used.

Worth knowing covers coverage on new or porous surfaces.

When Should You Use This Calculator?

Before redecorating a room, to buy the right amount in one trip.

When comparing paints with different coverage figures.

For a whole house, room by room.

When pricing a job, alongside primer and trim paint.

Common Mistakes

Trusting the tin's coverage on new plaster. It needs sealing first.

Forgetting the second coat. Most walls need two.

Painting the ceiling with wall paint. Ceiling paint is flatter and hides flaws.

Buying mixed colours in several trips. Batches can differ.

Not boxing multiple cans. Mix them together for an even colour.