Paint Calculator

Paint coverage is a range, not a fixed number. The gallon that stretches across a smooth, previously painted wall disappears far faster into fresh drywall, bare plaster or a textured surface. This calculator builds up your wall area one wall at a time, subtracts the doors and windows you are not painting, multiplies by the number of coats, and divides by whatever coverage figure your paint actually claims — so you can adjust the assumption instead of trusting a generic answer.

How the calculation works

The tool keeps a list of walls rather than one set of dimensions. Each time you press Add Wall, the current height and width are pushed onto that list and the totals are rebuilt:

• Total wall area is the sum of height × width for every wall listed. The page opens with four walls already added at the default 8 ft × 12 ft, giving 384 sq ft — a square room with 8 ft ceilings.
• Openings are deducted at a flat 21 sq ft per door and 15 sq ft per window. The default one door and two windows removes 51 sq ft, leaving 333 sq ft of paintable wall.
• Paintable area on the result card is that figure multiplied by the coats, so two coats over 333 sq ft reads 666. It is the surface the roller must cover, not the size of the room.
• Gallons needed is the coated area divided by your coverage: 666 ÷ 350 gives 1.90 gallons. Quarts is that figure × 4, since a US quart is a quarter gallon.

Watch one thing: typing new numbers into the height and width boxes does not change the total by itself. Those fields are a staging area — the wall counts only once you press Add Wall, and each listed wall has a red × to remove it.

Building an accurate wall list

• Measure each wall separately rather than averaging: floor to ceiling for height, corner to corner for width. Enter decimals, so a 9 ft 6 in wall is 9.5.
• Clear the list before switching units. The tabs reset the input boxes and the coverage default, but walls already added keep their original numbers — remove them first, or you will be adding metres to feet.
• The door and window deductions are fixed in square feet and are not converted in metres mode. Working in metric, set both counts to zero and account for openings yourself.
• Leave small openings in. Not deducting a window builds in a useful cushion, and paint cut in around a frame is rarely saved anyway.
• Ceilings are not included. To paint one, add it as an extra "wall" using the room's length and width, but buy ceiling paint separately.

Why coverage varies so much

Coverage defaults to 350 sq ft per gallon, or 10 m² per litre in metric mode — common rule-of-thumb figures for one coat on a smooth, sealed, previously painted wall, and the biggest assumption in the calculation. Replace the number with whatever the tin states, then lower it when any of these apply:

• Porous surfaces drink paint. New drywall, bare plaster, joint compound and untreated wood absorb the first coat instead of holding it on the surface. Priming first is usually cheaper than using finish paint as primer.
• Texture adds real area. Knock-down, orange peel, woodchip and rough render have far more surface than their flat dimensions suggest.
• Sheen changes behaviour. Flat and matt finishes forgive a patchy wall; higher-gloss finishes show every imperfection and demand better preparation.
• Big colour changes need more coats. Dark to pale, or covering stains, often means primer plus two topcoats — increase the coats field rather than adjusting coverage.
• Spraying wastes more than rolling, thanks to overspray and paint left in the equipment.

Reading the result and buying sensibly

The gallons figure is a decimal, so 1.90 gallons means two gallons in practice — paint is sold in whole containers. When the number lands just over a whole unit, compare prices: a gallon plus a quart is sometimes cheaper than two gallons, which is what the quarts card is for. Buy the whole job at once where you can, so every container comes from the same batch and dries to an identical shade. Refreshing the floor too? The carpet calculator covers room area and waste, and the volume converter moves a result between gallons, quarts and litres.

How to use the Paint Calculator

1. Pick Feet or Meters first, and delete existing walls if you switch later.
2. Remove the four sample walls with the red × unless the 8 ft × 12 ft default matches your room.
3. Type the height and width of a wall, press Add Wall, and repeat for each wall. Every one appears in the list with its own area.
4. Set the doors and windows you are not painting, remembering they deduct 21 and 15 square feet each.
5. Set the coats — two for a colour change, one for a refresh in the same shade.
6. Replace the coverage figure with the number on your tin, lowering it for porous or textured surfaces.
7. Read the gallons needed, round up to whole containers, and check the quarts figure for a cheaper mixed purchase.

Frequently Asked Questions

How much paint do I need for a 12 ft × 12 ft room?

That is the default setup: four 8 ft × 12 ft walls, one door, two windows and two coats at 350 sq ft per gallon comes to 1.90 gallons — so two gallons, with some left for touch-ups.

Why didn't my total change when I typed a new height?

The height and width boxes only feed the Add Wall button. Nothing counts until you press it and the wall appears in the list below.

Does it work properly in metric?

Mostly. Dimensions, wall area and coverage all switch to metres and litres correctly, but the door and window deductions stay fixed at 21 and 15 square feet. In metric mode, set both to zero and subtract openings yourself.

What coverage number should I enter?

Whatever your paint's label states, since it varies by product. The 350 sq ft per gallon default is a widely used rule of thumb for one coat on a smooth, already-painted wall; drop it substantially for bare or textured surfaces.

Should I count primer as a coat?

Only if you are estimating the primer itself. Primer and topcoat have different coverage, so it is cleaner to run the calculation twice — once for the primer, once for your topcoat and its coats.

Does it include ceilings, trim or the doors themselves?

No. It covers walls only, and it actively subtracts door and window area. Ceilings, skirting and door faces need their own estimate, usually in quarts rather than gallons.

Should I round up or down?

Up, always. Running out mid-wall risks a slightly different batch, while a spare quart is genuinely useful for later repairs.