Paint Calculator: Gallons, Cans and Cost for a Room
Work out how much paint a room needs. Add each wall or the perimeter, subtract doors and windows, set the coats, and get gallons or litres, suggested cans and cost.
Enter the wall sizes, or the room perimeter and wall height.
About the paint calculator
The Paint Calculator works out how much paint you need for walls and, if you want, a ceiling. Choose US units (gallons and square feet) or metric (litres and square metres), enter the surfaces, subtract doors and windows, set the number of coats and the coverage of your paint, and the result appears in both systems at once. It also suggests which cans to buy, and the cost if you enter a price.
You can enter walls one by one, with a width and height for each and rows you can add or remove, or use the perimeter shortcut: the distance around the room multiplied by the wall height. Tick the ceiling option and give its area to include it in the total.
How to calculate how much paint you need
Steps
- Choose US (gallons, ft²) or metric (litres, m²).
- Enter the walls wall by wall, or enter the room perimeter and wall height.
- Optionally tick ceiling and enter its area.
- Enter the number of doors and windows and their areas. The defaults are 21 ft² per door and 15 ft² per window.
- Set the number of coats. The default is 2.
- Set the coverage from the paint label. The default is 350 ft² per gallon, or 10 m² per litre.
- Optionally enter a price per gallon or litre.
Formulas
Area and volume
is the room perimeter, the wall height, and the numbers of doors and windows, and and the area of each. The paintable area must stay above zero. One US gallon is 3.785411784 litres.
Suggested cans
The calculator picks the combination of standard can sizes with the smallest total volume that still covers the need. Among combinations with the same volume, it picks the fewest cans. US sizes are 1 quart, 1 gallon and 5 gallons. Metric sizes are 1, 2.5, 5 and 10 litres. For example, 3.2 gallons needed gives 3 one-gallon cans and 1 quart, or 3.25 gallons. If you enter a price, the cost is the volume of the suggested cans multiplied by that price.
Worked examples
Room with a door and windows. A room 14 ft by 12 ft has a perimeter of 52 ft. With 8 ft walls that is 416 ft². Subtract 1 door (21 ft²) and 2 windows (30 ft²) to get 365 ft², or 33.91 m². Two coats at 350 ft² per gallon need 365 x 2 / 350 = 2.086 gallons, or 7.90 litres. The suggestion is 2 one-gallon cans and 1 quart, 2.25 gallons in total (8.52 litres). At 85.50.
Adding a ceiling. The same room has a ceiling of 14 x 12 = 168 ft². The paintable area becomes 365 + 168 = 533 ft². Two coats need 3.046 gallons. The suggestion is 3 one-gallon cans and 1 quart, 3.25 gallons.
Metric. A room with a 20 m perimeter, a 2.4 m wall height and one 1.9 m² door gives 48 m² of wall. Two windows of 1.44 m² each leave 43.22 m². Two coats at 10 m² per litre need 8.64 litres. The suggestion is four 1 litre cans and one 5 litre can, 9 litres in total.
Practical tips
- Check the coverage on the can. Smooth, previously painted walls reach the label figure, while bare plaster, drywall and textured surfaces absorb more, so coverage can be much lower.
- Primer is needed on bare surfaces, stains and big colour changes. Its coverage is often different from topcoat, so run a separate calculation, usually with 1 coat. The room above needs 1.043 gallons of primer, so 1 gallon and 1 quart.
- Covering a dark colour with a light one often takes an extra coat.
- Cutting in around edges and trim uses a little more paint than rolling alone, so a small margin is sensible.
- Buy all the paint for a room together so the colour comes from the same batch.
Need to work out room sizes first? The Area Calculator handles unusual shapes. For the wall-focused version of this tool, see the Wall Paint Calculator. The Volume Converter converts litres, gallons and quarts if you need other units.
Accuracy and limits
The result is an estimate. Real coverage varies with the paint, the surface, the applicator and the person using it. Moulding, trim, closets, radiators and the second face of doors are not included unless you add them as extra wall area. Always compare your result with the manufacturer's stated coverage.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Frequently asked questions
How much paint do I need for a room?
Find the paintable area, multiply by the number of coats and divide by the coverage per gallon or litre. A 14 x 12 ft room with 8 ft walls, one door and two windows has 365 ft² to paint, so two coats at 350 ft² per gallon need about 2.09 gallons.
How do I include the ceiling?
Tick the ceiling option and enter its area. For a rectangular room that is length times width. The ceiling area is added to the wall area before doors and windows are subtracted.
What default areas are used for doors and windows?
A door is 21 ft² and a window is 15 ft² by default. Both can be edited. Use real measurements if you have large patio doors or small windows, since the deduction directly reduces the paint estimate.
How are the suggested cans chosen?
The calculator picks the standard can sizes that give the smallest total volume covering your need, and among those the fewest cans. US sizes are 1 quart, 1 gallon and 5 gallon. Metric sizes are 1, 2.5, 5 and 10 litres. A need of 3.2 gallons gives 3 gallon cans and 1 quart.
What coverage should I enter?
Use the figure on the paint can. The defaults are 350 ft² per gallon or 10 m² per litre, which is typical for many interior paints on smooth surfaces. Rough or porous surfaces take more paint, so use a lower coverage for them.
Do I need primer?
Primer helps on bare drywall, plaster, stains and large colour changes. Run a second calculation with 1 coat and the primer's coverage. A 365 ft² paintable area needs about 1.04 gallons of primer in one coat.
Is my data sent anywhere?
No. The paint sums run in your browser, so your room measurements and prices are never uploaded or stored on a server. They stay on this device and are not shared with anyone.