How the paint calculator works
Enter the size of your room or wall, the number of coats and the type of wall, and the calculator tells you how much paint to buy and the best mix of cans, so you do not end up with too little or a large amount left over. It can also add primer and ceiling paint.
Choose Whole room to enter the room length, width and height and the number of doors and windows, or One wall for an accent wall.
How the paint amount is worked out
- Wall area: the distance around the room, times the wall height. For a single wall, the width times the height.
- Take off openings: the calculator subtracts about 21 sq ft for each door and 15 sq ft for each window.
- Multiply by the number of coats.
- Divide by the coverage of the paint. A gallon covers about 350 sq ft on average.
The coverage is printed on the paint can, so use that number if you have already chosen the paint.
Paint coverage by wall type
| Wall | Coverage per gallon | Per litre |
|---|---|---|
| Smooth, primed walls | about 400 sq ft | about 10 m² |
| Standard walls | about 350 sq ft | about 8.5 m² |
| Textured or porous walls | about 275 sq ft | about 6.5 m² |
How many coats do you need?
- Two coats is the usual answer, and gives an even colour that lasts.
- One coat works only if you are painting a similar colour over a clean, sound wall, or using a paint and primer in one on a very light shade.
- Three coats may be needed for a dark or bold colour, such as navy or deep red, over a light wall. A tinted primer usually helps.
Use primer on new drywall, on stains and patches, when you go from a dark wall to a light one, and on glossy surfaces.
Which cans should you buy?
Paint is sold in quarts (1/4 gallon), gallons and 5-gallon pails, or in 1, 2.5, 5 and 10 litre cans. A gallon costs less per quart than a quart, but a left-over gallon is a waste. The calculator tests the combinations of cans and picks the one with the least waste and the fewest cans.
Worked examples
A 12 ft × 10 ft room, 8 ft ceiling, one door and two windows
The walls are 352 sq ft, less 21 for the door and 30 for the windows, which leaves about 301 sq ft. Two coats at 350 sq ft per gallon need about 1.7 gal, so buy 2 gallons. A coat of primer needs another 0.86 gal, so buy 1 gallon. The 120 sq ft ceiling needs 0.69 gal for two coats, so buy 1 gallon of ceiling paint.
A 10 ft × 8 ft accent wall
The wall is 80 sq ft. Two coats need about half a gallon, so 2 quarts are enough.
Common mistakes
- Not allowing for two coats. One coat of paint rarely hides the old colour.
- Buying paint for the whole wall when a window takes up half of it. Take off the openings.
- Mixing cans from different batches. Colour can vary a little, so mix the cans together in one big bucket before you begin (“boxing”).
- Skipping the primer. It saves paint and time on bare, stained or dark walls.
- Forgetting trim and doors. They need their own paint, and often a different sheen.
Tips
- Buy a sample pot and paint a large piece of board first. Colours look different at different times of the day.
- Keep a little paint for touch-ups, in a small, airtight jar.
- Use a flat finish for ceilings, and eggshell or satin for walls that you need to wipe clean.
- Painting a textured wall? Use a roller with a thicker nap and allow more paint.
