How much paint do I need? A room-by-room guide

Buying paint is the one home-improvement errand where both mistakes are equally common: you buy two gallons too little and find yourself in the paint aisle mid-roll, or you buy three gallons too many and store a can that settles into a solid block by the next room. The fix is a formula you can run in a minute, and it only needs three measurements.

The formula

1. Measure the perimeter of the room — the total length of all walls — and the wall height, then multiply them for the gross wall area.
2. Subtract the area of windows and doors you will not paint.
3. Multiply the net wall area by the number of coats you will apply.
4. Divide by the coverage rate — 350 square feet per US gallon per coat — and round up to a whole gallon.

In one line: cans = (perimeter × wall height − openings) × coats ÷ 350, rounded up.

Worked example: a 12 × 10 ft bedroom

Take a bedroom that is 12 ft by 10 ft with 8 ft walls:

• Perimeter: 2 × (12 + 10) = 44 ft. Gross wall area: 44 × 8 = 352 sq ft.
• One window (say 3 × 5 ft = 15 sq ft) and one interior door (about 20 sq ft): 35 sq ft of openings.
• Net wall area: 352 − 35 = 317 sq ft.
• Two coats: 317 × 2 = 634 sq ft.
• Gallons: 634 ÷ 350 = 1.8 → buy 2 gallons.

That is the whole trick. Most rooms land between 1.5 and 3 gallons for two coats, and rounding up to the next gallon absorbs most real-world variation.

Room-by-room quick table

These are typical room sizes with 8 ft walls, two coats, and about 35 sq ft of openings. Your numbers will differ, but the shape holds:

Room	Typical size	Wall area (2 coats)	Gallons to buy
Bedroom	10 × 10 ft	570 sq ft	2
Bedroom	12 × 10 ft	634 sq ft	2
Living room	14 × 12 ft	762 sq ft	3
Living room	16 × 14 ft	890 sq ft	3
Open-plan	20 × 16 ft	1,082 sq ft	4

For one coat, halve the painted area: the 10 × 10 bedroom at one coat needs 285 sq ft — comfortably under one gallon’s 350 sq ft — so a single gallon covers it.

Metric: liters instead of gallons

The same rate in metric is about 8.6 m² per liter per coat, derived from the same 350 sq ft per gallon. A 4 × 5 m bedroom with 2.5 m walls: perimeter 18 m × 2.5 m = 45 m²; minus about 4 m² of window and door leaves 41 m²; two coats make 82 m²; 82 ÷ 8.6 ≈ 9.6 liters. Paint in most countries sells in 2.5, 5, and 10 L cans, so the natural purchase is a 10 L can.

When to add waste

The formula assumes clean, primed walls. Add about 10% when the walls are textured or rough (knockdown, orange peel, brick), when you are painting a saturated color over white, when the surface has never been painted, or when you want leftover paint for touch-ups. On the bedroom above, 10% is 63 sq ft — about 0.18 gallons — which keeps you at 2 gallons; on a large open-plan room it can be the difference between buying 3 or 4.

Let the calculator do it

Our paint calculator runs this exact formula: it takes your room perimeter, wall height, openings, coats, and waste percentage, then multiplies by the price per can for a total cost. It works in feet or meters, applies the 350 sq ft per gallon rate automatically, and rounds up to whole cans the way a store would. Related estimators for the same job: flooring calculator and tile calculator.

The one rule

Measure twice, subtract openings, multiply by coats, divide by 350, round up. Five minutes with a tape measure beats a second trip to the paint aisle — and at $45 a gallon it is the difference between a $90 job and a $135 one.