Buy the paint once
Running short of paint halfway through a wall is annoying, and a second batch can differ slightly in colour. Buying far too much is a waste. The amount comes down to four things: the area of the walls, the doors and windows you skip, the number of coats, and how far each gallon spreads.
This paint calculator works out the wall area of a room from its size, takes off standard doors and windows, multiplies by the coats, and divides by the coverage printed on your can. It then tells you what to buy in gallons and quarts, or in litre tins.
How to use it
- Measure the room length, width and wall height. For a single wall or an odd space, choose Known area.
- Count the doors and windows you will not paint.
- Choose the number of coats. New colours, dark colours and bare plaster often need two or more.
- Enter the coverage from the can, and read the amount and what to buy.
The paint formula
wall area = 2 × (length + width) × height − doors − windows
paint = wall area × coats ÷ coverage
add the ceiling (length × width) if you paint it tooDoors are counted as 21 sq ft each (3 by 7 ft) and windows as 15 sq ft (3 by 5 ft). If yours are much bigger or smaller, work out the area yourself and use Known area. Rough, textured or porous surfaces soak up more paint, so their real coverage is lower than the can says.
Worked example: a 12 by 12 foot room
- Walls: 2 × (12 + 12) × 8 = 384 sq ft
- Less 1 door and 2 windows: 21 + 30 = 51 sq ft, leaving 333 sq ft
- Two coats: 666 sq ft
- At 350 sq ft per gallon: 666 ÷ 350 = 1.9 gallons
The room needs 1.9 gallons, so buy 2 gallons. A little left over is handy for touch ups later.
Paint for some room sizes
| Room | Area to paint | Paint | To buy |
|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.54 gallons | 1 gallon and 3 quarts |
| 12 × 12 ft | 333 sq ft | 1.9 gallons | 2 gallons |
| 12 × 15 ft | 381 sq ft | 2.18 gallons | 2 gallons and 1 quart |
| 15 × 20 ft | 509 sq ft | 2.91 gallons | 3 gallons |
Four quarts make a gallon, so a gallon and three quarts is the same amount as 1.75 gallons. Gallons usually cost less per quart, so it can be worth rounding up to whole gallons.
Before you start
- Primer goes on first over bare plaster, new drywall or a big colour change, and has its own coverage.
- Trim, doors and ceilings are often painted in a different finish, so measure them separately.
- For floor area, see the square footage calculator.
Frequently asked questions
How much paint do I need for a room?
Work out the wall area: twice the length plus the width, times the height. Take off doors and windows, multiply by the number of coats, and divide by the coverage on the can. A 12 by 12 foot room with 8 foot walls needs about 1.9 gallons for two coats.
How many square feet does a gallon of paint cover?
Many wall paints say about 350 to 400 square feet per gallon on smooth, primed walls, but check the can, since products differ. Rough, porous or textured surfaces soak up more, so a gallon covers less. Use the figure printed on your paint for the most accurate answer.
Do I need two coats of paint?
Often, yes. Two coats give an even colour and finish, especially with a new colour, a dark shade or a big change from light to dark. A fresh coat of the same colour over sound paint may need only one. The calculator lets you choose any number from one to five.
Should I subtract doors and windows?
Yes, for rooms with several doors or large windows, since they can add up to a sizeable area. The calculator takes off 21 square feet for each door and 15 for each window. For a single small window it makes little difference, and a small margin is useful anyway.
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