How to Calculate Paint for a Room in 5 Simple Steps

Paint for a room in one line

Paint for a room is wall area times coats, divided by coverage per gallon, rounded up to whole gallons. A 12 ft by 10 ft room with 8 ft ceilings has 352 sq ft of wall; two coats make 704 sq ft of painted surface; at 350 sq ft a gallon that is 2.01 gallons, so you buy 3. The rest of this guide is about getting those three inputs honest.

Step 1: Measure the room

Measure the length, the width and the ceiling height, all in feet, wall to wall and floor to ceiling. The worked example is 12 ft long, 10 ft wide and 8 ft high — a normal bedroom. Measure at floor level, not around the skirting, and measure the height to the ceiling itself, not to a picture rail. If the room is not a rectangle, split it into rectangles and measure each one; the area step works on each part separately.

Step 2: Work out the wall area

There are two long walls and two short walls. Each long wall is length times height — 12 × 8 = 96 sq ft — and each short wall is width times height — 10 × 8 = 80 sq ft. Add them: 96 + 96 + 80 + 80 = 352 sq ft. The shortcut is the same maths in one line: 2 × (length + width) × height, or 2 × (12 + 10) × 8 = 352.

Step 3: Divide by coverage

Coverage is how far one gallon goes, and it is printed on the tin. A typical emulsion covers about 350 sq ft a gallon on smooth walls. One coat of this room is 352 ÷ 350 = 1.01 gallons — just over a single tin. If your tin states 300 sq ft a gallon, one coat is 352 ÷ 300 = 1.17 gallons instead; the area maths does not change, only the division, so always read the tin before trusting a round number.

Step 4: Multiply by coats

One coat rarely finishes a colour change evenly, so plan two coats as standard. Two coats double the painted surface: 352 × 2 = 704 sq ft, and 704 ÷ 350 = 2.01 gallons. A third coat, for a dark-to-light change, would make it 352 × 3 = 1,056 sq ft, or 3.02 gallons. Count coats honestly: a coat that is really a touch-up is not a coat, and a primer coat is a separate product, not a colour coat.

Step 5: Round up and buy

Paint is sold by the gallon, so round up: 2.01 gallons means buy 3 gallons. The spare gallon is not waste — it is the touch-up tin, and the first scuff will prove it. Buy all the tins in one batch if you can, since the same colour mixed twice can differ slightly. If the figure lands just over a whole number — 3.02 gallons, say — that is four tins, not three and a hope; a 0.02-gallon shortfall at the end of the last wall is a second trip.

Paint for a room in numbers

Suppose the room is 12 ft by 10 ft with 8 ft ceilings, the paint covers 350 sq ft a gallon, and the plan is two coats. Wall area: 2 × (12 + 10) × 8 = 352 sq ft. Painted surface: 352 × 2 = 704 sq ft. Paint needed: 704 ÷ 350 = 2.01 gallons. Buy: 3 gallons.

Common mistakes

The most common mistake is measuring the floor area instead of the wall area: 120 sq ft of floor is not 120 sq ft of wall, and this room has nearly three times as much wall as floor. The second is forgetting the second coat, which halves the estimate. The third is rounding down instead of up — 2.01 gallons is three tins, not two. The fourth is painting the ceiling without counting it: 120 sq ft of ceiling at two coats adds 0.69 gallons and can push the buy to four tins.

Worked example, start to finish

A bedroom, 12 ft by 10 ft, 8 ft ceilings, colour change from cream to sage, two coats, emulsion at 350 sq ft a gallon. Walls: 2 × (12 + 10) × 8 = 352 sq ft. Surface: 352 × 2 = 704 sq ft. Paint: 704 ÷ 350 = 2.01 gallons. Buy 3 gallons, one batch. Result: two coats cover evenly, the spare gallon stays sealed for touch-ups, and the job needs no second shop trip. If the ceiling joins the job, add 120 sq ft of ceiling first: 472 sq ft of surface per coat, 2.70 gallons for two coats, still 3 tins but with little spare.

What changes the answer most

The ceiling height changes the answer most: every extra foot adds the whole perimeter — 44 sq ft here — to the wall area, and that area is then multiplied by the coats. The coat count is second: each added coat adds the full wall area again, 352 sq ft in the example. The coverage rate is third: it changes the division, not the area, so a poorer-covering paint raises the gallons without changing the room. If the budget is tight, cut a coat only if the colour allows it — never cut the rounding-up.

Frequently asked questions