People type this into search every day: how much paint do I need.
The usual answer is “a gallon covers 350 to 400 square feet.” Sherwin-Williams prints that range on its interior data sheets. That number is the coverage rate you divide by, once you know the area you will coat and how many coats you will roll.
Floor area is the wrong wall number
A 5 m by 3 m room has 15 m² of floor. The ceiling is about the same 15 m².
The walls add height. Walk the perimeter and multiply by the wall height.
Perimeter: 2 × (5 + 3) = 16 m
Wall rectangle: 16 × 2.5 = 40 m²
That 40 m² is more than twice the floor. Buy from the floor figure and you stop before the second coat is done.
Sherwin-Williams’ paint selection FAQ gives the method. Calculate the square feet of the surface you will coat. Divide by the square feet one gallon of that product covers.
Their example: 1,000 square feet at 400 square feet per gallon is 2.5 gallons.
Coverage comes from the can, at a film thickness
The SuperPaint Interior Latex product data sheet lists coverage as 350-400 sq. ft. per gallon at 4 mils wet and 1.5 mils dry.
Sherwin-Williams’ paint calculator FAQ repeats the 350-400 range. The same page says that range is enough to cover a 10-foot-by-10-foot room. That sentence uses the floor size as a room nickname. The 350-400 figure is still per gallon of coated area, at the film thickness on the data sheet.
The FAQ also says a gallon of primer covers about 200-300 square feet. Primer is a different product, so it gets its own coverage number.
The paint quantity calculator stores coverage as square metres per litre. The default is 10 m²/L. In US units the site shows that as about 407 ft²/gal, next to the high end of the SuperPaint range. Replace it with the number printed on your can.
The formula
The calculator uses:
((perimeter × height) − openings) × coats ÷ coverage
Openings are the doors and windows you will skip. The tool’s assumptions keep ceilings, trim, and primer out of this estimate. Calculate those with their own product and coverage.
The published result rounds up to the next 0.5 litre before the site converts units. Leftover paint is useful for later chips.
What the defaults look like
Here are the calculator defaults:
- room perimeter: 16 m
- wall height: 2.5 m
- doors and windows: 5 m²
- coats: 2
- coverage: 10 m²/L
Painted wall area: 16 × 2.5 − 5 = 35 m²
Coverage across both coats: 35 × 2 = 70 m²
Calculated amount: 70 ÷ 10 = 7 L
Rounded paint to buy: 7 L
That 16 m perimeter is a 5 m by 3 m rectangle, so the floor is 15 m². Using the floor in place of the walls would give 15 × 2 ÷ 10 = 3 L. You would be short 4 L.
On a US display those 7 L show as 1.8 gal. The 35 m² of wall shows as 376.7 ft². Two coats at Sherwin-Williams’ 400 sq. ft. per gallon: 376.7 × 2 = 753.4, then 753.4 ÷ 400 = 1.88 gal, next to the calculator’s 1.8 gal.
Coverage on the can is theoretical
Sherwin-Williams calls 400 square feet theoretical. Texture, and paint left on the roller, sit outside that division.
Rough plaster, new drywall, and a dark-to-light color change use more product. The SuperPaint sheet says two coats at the recommended film thickness help hide a previous coating or new drywall. A drastic color change gets a primer coat first, then the topcoat.
If you add a third coat on the same 35 m² at 10 m²/L:
35 × 3 ÷ 10 = 10.5 L
Put 3 in the coats field so the formula multiplies 35 × 3.
Ceilings and extra rooms use floor area
The square footage calculator adds up to three rectangular floors. Use it when the surface is a ceiling, or when you are adding rooms before you walk each perimeter.
A ceiling is close to the floor rectangle. Walls still need height.
If the new color is much darker, the same lamps can look weaker. The room lighting calculator turns floor area and a target lux into a fixture count. Run it after you pick the color, once the walls are measured.
If you scrape or sand old paint, the SuperPaint sheet points at the EPA lead page. That page is about health during prep, which this quantity estimate does not model.
Put the can coverage in, then round up
Open the paint quantity calculator. Walk the room once for perimeter, measure height, and subtract the door and window area you will skip. Set coats to the number you will roll. Set coverage to the figure on the can.
Buy the rounded result. If the store only sells 2.5 L or 1 gal cans, go up to the next can it stocks.