Paint Calculator

How the paint estimate is built

Every paint calculation is the same four steps, and doing them by hand is where people go wrong. First you find the perimeter of the room and multiply it by the wall height: 2 × (length + width) × height. That gives the gross wall area, including the parts you will never paint. Second you subtract the openings — this tool uses 21 square feet for a standard door (roughly 3 ft × 7 ft) and 15 square feet for an average window (about 3 ft × 5 ft). Third you add the ceiling if you are painting it, which is simply length × width. Fourth you multiply the whole thing by the number of coats and divide by the coverage rate of a gallon.

The coverage rate is where estimates diverge. Tins commonly claim 400 square feet per gallon. That figure comes from a laboratory draw-down on sealed, smooth stock — not from a real bedroom with corners to cut in, a tray you cannot scrape clean, and a roller sleeve holding a quarter pint hostage. This calculator uses 350 square feet per gallon per coat (32.5 m², or 8.6 m² per litre), which is what most professional painters plan around.

Worked example: a 12 × 10 ft bedroom

Take a typical US bedroom, 12 feet by 10 feet, with 8-foot ceilings, one door and two windows, painted in two coats with the ceiling left white.

The perimeter is 2 × (12 + 10) = 44 feet. Multiplied by the 8-foot wall height, that is 352 square feet of gross wall. Subtract one door at 21 and two windows at 15 each — 51 square feet of openings — and you are left with 301 square feet of paintable wall. Two coats means covering 602 square feet of surface, and at 350 square feet per gallon that is 1.7 gallons, or about 6.5 litres. Since paint is sold in gallons and quarts, the practical answer is one gallon plus three quarts. Most people in this situation buy two gallons and accept a little left over for touch-ups, which is a perfectly reasonable call.

Worked example: adding the ceiling

Same room, but now the ceiling gets two coats as well. The ceiling adds 12 × 10 = 120 square feet, bringing the paintable area to 421 square feet and the two-coat total to 842. Divided by 350 that is 2.4 gallons — nine litres. Note that this assumes the same paint everywhere. In practice most people use a dedicated flat ceiling paint, so run the tool twice: once with the ceiling box unticked for your wall colour, once with only the ceiling in mind. A 120-square-foot ceiling at two coats needs about 0.7 gallons, so a single gallon of ceiling paint does the job.

Why two coats is the default

One coat only works when you are repainting the same colour on a sound, previously painted wall, and even then the finish tends to look patchy in raking light. Two coats is the standard because the first coat is doing structural work — filling the surface, evening out absorbency — while the second delivers the colour and the uniform sheen. Choose three coats when you are going from a dark colour to a light one without primer, or when you are using a saturated red, orange or yellow. Those pigments are transparent by nature and notoriously need an extra pass.

Dark to light, and when primer earns its keep

Covering a deep charcoal wall with a pale grey is the classic case for primer rather than extra coats. A tinted grey primer costs less than finish paint, blocks the old colour in a single pass, and lets you get away with two topcoats instead of four. Primer is also non-negotiable on bare drywall, on patched or skim-coated repairs, over glossy or oil-based surfaces, and over stains such as water damage or smoke. Do not count primer as a coat in this calculator — estimate it separately using a one-coat run.

Texture, porosity and the fudge factor

Coverage rates assume a smooth, sealed surface. Anything rougher increases the actual surface area you are painting, sometimes dramatically. Add roughly 10% for lightly textured or orange-peel walls, 15 to 20% for knockdown texture, popcorn ceilings, or new drywall that has only been primed once, and up to 50% for masonry, stucco and rough brick where paint has to bridge every pit. Sprayers also use more material than rollers because of overspray — plan on 20 to 30% extra if you are spraying an interior.

What this calculator does not account for

It treats the room as a plain rectangular box. Bay windows, sloped or vaulted ceilings, stairwells, closets and alcoves all change the geometry, and the honest fix is to measure those surfaces separately and add them. Trim, skirting boards, doors and window frames are not included either — they take a different product entirely, and a gallon of trim enamel typically handles the woodwork of two or three average rooms. Finally, the door and window deductions are averages: if your room has a wall of glass or a pair of sliding doors, measure the openings yourself rather than counting them.

When you are on the fence between two quantities, buy the larger one. Paint is cheap relative to a second trip, and returning to the shop for a half-gallon mid-project risks a batch with a slightly different tint. Have the store record your colour formula on the receipt so a follow-up can be matched exactly.