Paint Calculator

How to Use the Paint Calculator

Most people buy too much paint, and the reason is almost always the same: they multiply length by width by height and paint the volume of the room rather than its surface. Walls are a perimeter times a height, and the difference between those two mistakes is enormous.

1. Enter the room dimensions. The calculator works out the perimeter itself and multiplies by the ceiling height, which is what the wall area actually is.
2. Count the doors and windows. Doors are deducted at 21 square feet and windows at 15, which are standard sizes. Five openings in an ordinary room remove around 87 square feet — a meaningful fraction of the job.
3. Set the number of coats. Two is standard over an existing similar colour. Bare drywall, a strong colour change or a dark shade going light all need three, and primer besides.
4. Check the coverage on your tin. The 350 square feet per gallon default is typical for interior emulsion, but it varies from about 250 to 400 by product and finish.

All of this happens in your browser. Nothing about your house is transmitted or stored anywhere.

Paint Coverage Formula

Perimeter, not volume. That single distinction is what this calculation exists to get right.

Coverage is the input most worth checking rather than accepting. The figure on the tin is measured on a smooth, sealed, primed surface in laboratory conditions. Textured plaster, bare drywall, unfilled woodchip and anything porous all drink more paint — sometimes twenty-five per cent more on the first coat alone.

The opening deduction is worth taking seriously in small rooms. A bathroom with a door and a window loses 36 square feet from perhaps 250 — nearly fifteen per cent of the job, and easily the difference between needing one gallon and two.

Example

Worked example one: a 20 ft by 15 ft room with a 9 ft ceiling. The perimeter is 2 × (20 + 15) = 70 feet. Multiplied by the 9-foot ceiling that is 70 × 9 = 630 square feet of gross wall. Note what it is not: 20 × 15 × 9 = 2,700, the volume, which is more than four times too much.

Two doors at 21 square feet and three windows at 15 remove 42 + 45 = 87 square feet, leaving 543 square feet to paint. Two coats means covering 543 × 2 = 1,086 square feet. At 350 square feet a gallon that is 1,086 ÷ 350 = 3.10 gallons, so you buy 4 gallons — or three gallons and a quart, if the shop sells quarts.

At $38 a gallon, four gallons costs $152. In metric terms 3.10 gallons is 11.75 litres, and the area is 100.89 square metres.

Worked example two: three coats instead of two. Going from a dark colour to a light one usually needs three. That is 543 × 3 = 1,629 square feet, or 1,629 ÷ 350 = 4.65 gallons — five gallons to buy, and $190.

The extra coat costs $38, which is 25% more paint for a job whose labour has gone up by half. That is the arithmetic behind the standard advice to use a tinted primer under a big colour change: one coat of primer plus two of finish usually covers better than three of finish, and costs less because primer is cheaper.

Reading the Gap Between Needed and Bought

The gap between gallons needed and gallons to buy is where the waste lives. This job needs 3.10 gallons and forces you to buy four, so 0.90 of a gallon — nearly thirty per cent of a tin — is left over. Buying three gallons and one quart gets you to 3.25 and cuts that surplus to a sixth of what it was.

Some surplus is worth keeping. A pint of the exact colour, properly sealed and labelled with the room it belongs to, is the difference between touching up a scuff in five minutes and repainting a wall. Anything beyond that is storage.

If you are painting the ceiling as well, switch the segment rather than running the calculation twice. Ceilings usually take a different paint — flatter, often cheaper — so knowing the two areas separately is more useful than a combined total. This room's ceiling is 300 square feet, which is a single gallon for one coat with plenty spare.

Coverage claims deserve scepticism in one specific direction. The tin's figure is achievable on a smooth primed wall with a roller and a steady hand. Spraying uses more, cutting in by brush uses more, textured surfaces use more, and a first coat on bare plasterboard can use half as much again. When in doubt, treat the tin's number as the best case.

For the room measurement, the square footage calculator handles awkward outlines, and the flooring calculator covers the floor of the same room if the whole space is being redone.

Five Things That Raise the Real Number

Five things that push the real number above the calculated one.

Surface porosity. New plaster, bare drywall and old distemper all absorb the first coat rather than sitting on top of it. A mist coat of thinned paint or a proper sealer costs less than the extra finish paint it saves, and it produces a better result besides.

Texture. Anything other than a flat wall has more surface than its measured area. Woodchip, heavy artex, brick and rough render can add twenty to fifty per cent to the paint required, and no measurement of the wall's outline will reveal that.

Colour change. Covering a dark colour with a light one, or covering anything with a strong red or yellow, typically needs three coats rather than two. Pigment density varies by colour, and reds and yellows are the notorious cases — some need four.

Application method. A roller is the most efficient. Brushes use more, and airless spraying can use forty per cent more paint for the same surface because a proportion never reaches the wall. Spraying is faster and it is not cheaper.

Trim, doors and ceilings are separate jobs. This calculator covers walls, with the ceiling as an option. Skirting, architrave, doors and window frames use a different paint in different quantities and are usually estimated by length rather than by area — a rough figure is a quart per two doors including frames.