Paint calculator: how much paint for a room?

Doors and windows are taken off at typical sizes (20 sq ft and 15 sq ft); change them under More options if yours are bigger. The coverage field starts at 350 sq ft per gallon, the low end of what most wall-paint data sheets list. Type the figure from your can for the best answer. New to this? Paint words explained.

How much paint for a room: the formula

Paint is bought by volume but used by area. Work out the wall area, multiply by the coats, and divide by how many square feet a gallon covers.

wall area = 2 × (length + width) × height − doors − windows
paint (gal) = wall area × coats ÷ coverage (sq ft per gal)
1 US gallon = 4 quarts ≈ 3.79 L  ·  350 sq ft/gal ≈ 8.6 m²/L

Example: a 12 × 12 ft bedroom with 8 ft walls has 2 × 24 × 8 = 384 sq ft of wall. Take off one door (20 sq ft) and two windows (30 sq ft) and 334 sq ft is left. Two coats at 350 sq ft per gallon is 668 ÷ 350 = 1.91 gallons, so you buy 2 gallons. That one is worked through in full on how much paint for a 12x12 room.

Worked examples

• How much paint for a 10x10 room: the small bedroom or office where a gallon is not quite enough.
• How much paint for a 10x12 room: the size most new-build second bedrooms come in.
• How much paint for a 12x14 room: where the order tips past 2 gallons.
• How much paint for a small bathroom: tile and the tub surround take a big bite out of the wall.
• How much paint for a 12x12 room and ceiling: walls and ceiling in one order.
• How many litres of paint for a 10x12 room: the same job in metric tins.

What the coverage on the can means

Every manufacturer prints coverage as a range per coat, measured at a set wet film thickness on a smooth, sealed surface. Rough, porous or unprimed surfaces drink more paint, and deep colors often need an extra coat to hide. These are the figures from the current data sheets of three common lines:

That spread is why the calculator asks for the number on your can. At 400 sq ft per gallon the 12 × 12 room above needs 1.67 gallons; at 250 it needs 2.67. When in doubt, plan on the low end of the range and keep the leftover for touch-ups.

How many coats?

• New drywall: one coat of primer, then two finish coats. That is the system on the ProMar 200 data sheet; the primer calculator does the first step.
• Same or similar color over sound paint: often one coat will do. Two give a more even sheen.
• Big color change, especially light over dark or a deep red or yellow: two coats, and the data sheets suggest a tinted primer first.
• Patched walls: spot-prime the patches, or they can show through as dull “flashing” in an eggshell or satin finish.

Tips and common mistakes

• Measure the room, not the floor plan. Bay windows, closets and soffits all add wall; a walk-in closet painted the same color is often another 100–150 sq ft.
• Don’t subtract small things like outlets and switch plates. Do subtract big openings, fitted wardrobes and tiled areas (use Other wall area under More options).
• Buy all the paint from the same batch, or mix the cans together before you start. Color can shift slightly between batches.
• Ceilings and trim are usually different products. Price them with the ceiling paint calculator and the trim paint calculator.
• Ventilate while you paint and for a day or two after: open windows and run a fan blowing out. Behr’s data sheets note that most emissions happen during painting and in the 48 hours after.

For whole-job numbers, the paint coverage chart lists gallons per room size, and how to paint a room walks through the order of work.