Calculators
Paint quantity calculator
Paint is the one material where buying 10% too much costs a few pounds and buying 5% too little costs you a batch mismatch on the longest wall in the room.
On this page (3 sections)
Short answer
Wall area = perimeter x wall height, minus about 21 sq ft (2.0 m2) per door and 15 sq ft (1.4 m2) per window. Divide by 350 sq ft (33 m2) per US gallon and multiply by the number of coats.
The numbers, at a glance
- Coverage per US gallon
- 350 sq ft (33 m2)
- Coverage per litre
- About 8.6 m2 (93 sq ft)
- Deduction per door
- 21 sq ft (2.0 m2)
- Deduction per window
- 15 sq ft (1.4 m2)
Paint quantity calculator#
Counts coats, primer and the openings you do not have to paint, and gives the answer in both US gallons and litres.
Nothing you type leaves your browser: the maths runs on this page, and no values are stored or sent anywhere.
The method#
Wall area is the room's perimeter multiplied by the wall height. The calculator derives the perimeter from the room's width and length, which is also what lets it work out ceiling area if you ask for it.
Openings come off next. A standard door is about 21 sq ft (2.0 m2) of opening and a typical domestic window about 15 sq ft (1.4 m2), so a room with one door and two windows loses roughly 51 sq ft before you buy anything. In a small room that is 15% of the job.
Coverage is set at 350 sq ft (33 m2) per US gallon, which is a realistic figure for a sound, previously painted wall. Manufacturers quote higher, and on a good day they are right; on new plaster or a deep colour you will not see it.
Buying advice that survives contact with the job#
- Round up to whole tins and buy them together. Two 1 gallon tins from the same batch beat one gallon plus a quart bought a week later.
- Box the paint if the job needs more than one tin of the same colour. Five minutes of mixing removes any risk of a visible seam.
- Order 10% over for a room with a lot of cutting in. Brushwork uses more per square foot than rolling.
- Keep the last half litre labelled with the room, the finish and the date. Touch-ups a year later are the reason.
- Metric conversion: 1 US gallon is 3.785 litres, and a coverage of 350 sq ft per gallon is about 8.6 m2 per litre.
Sheen changes the calculation too#
Matt and flat finishes are the most absorbent and the least forgiving of a thin second coat. Eggshell and satin cover slightly further and hide roller marks better. Gloss and semi-gloss cover the furthest per litre but show every imperfection in the substrate, so the preparation time goes up as the paint quantity goes down.
Frequently asked questions#
How much paint for a 12 x 12 ft room?
A 12 x 12 ft room with 8 ft walls has 384 sq ft of gross wall. Take off one door and two windows, about 51 sq ft, and you have 333 sq ft. At two coats and 350 sq ft per gallon that is 1.9 gallons, so buy two. Add about 1.2 gallons if you are painting the ceiling as well.
Does one gallon really cover 350 square feet?
On a smooth, previously painted, mid-tone wall, yes. On new drywall, a chalky matt finish, a deep colour or a textured surface, expect 250 to 300 sq ft. Textured or heavily absorbent surfaces can drop it to 200. The tin's stated figure is a laboratory number on an ideal substrate.
Do I need primer?
Prime bare drywall, bare wood, patched areas, glossy surfaces you are painting over, and any dramatic colour change, particularly light over dark or over a strong red or yellow. A tinted primer under a deep colour usually saves a full topcoat, which is both cheaper and flatter.
Why does my second tin look slightly different?
Batch variation. Tinted paint is mixed to a tolerance, and two tins from different batches can differ enough to show in raking light along a long wall. Buy the whole job at once and box it: pour all the tins into one larger container, mix, and pour back.
Sources and standards referenced#
- Manufacturer spread rates for interior emulsion and latex paints Trade practiceTypically 350 to 400 sq ft per US gallon, 12 to 14 m2 per litre claimed, 8 to 10 m2 achieved
- Standard door and window opening sizes Industry standardBasis for the 21 sq ft and 15 sq ft deductions
We cite published codes, industry planning guidelines and manufacturer specifications. Where a figure is our own recommendation rather than a published standard, the text says so. See how we measure.