How much paint do I need for a room or a job

Measure the walls and take out the openings

Paint is bought by the area it covers, so start with the area of the walls. A tape measure and a notebook are enough. Use feet or metres, but use one of them all the way through.

1. Measure the length of each wall and the height from floor to ceiling.
2. Multiply length by height for each wall.
3. Add the walls together.
4. Measure each door and window the same way, and add those up.
5. Subtract the openings from the wall total. That is the area to paint.

Do not paint the ceiling with the same figure. Measure the ceiling on its own, because it often needs a different paint.

Count the coats and read the can

Most walls need two coats. A big colour change, bare drywall or a dark wall may need more. Decide the number of coats before you do the maths.

Every can states how much area it covers. Rough walls soak up more paint than smooth walls, so the figure on the label is a guide, not a promise. Use the number printed on the can you plan to buy.

• Area to paint, times the number of coats, gives the total area to cover.
• Total area to cover, divided by the coverage on the label, gives the amount of paint.
• Round up to whole cans. Keep a little extra for touch-ups.

Worked example: a bedroom (made-up figures)

The four walls add up to 400 square feet. One door and one window take out 40 square feet. That leaves 360 square feet to paint.

You plan two coats, so 720 square feet to cover. The can label says 350 square feet per gallon (an example figure, not a rule). 720 divided by 350 is about 2.06, so you buy 3 gallons.

At $60 a gallon that is $180 before Tax A and Tax B. Add the tax lines you take from the official source on top.

Common mistakes

• Forgetting the second coat when buying.
• Measuring in feet for the walls and metres for the label.
• Skipping primer on bare or repaired spots.
• Mixing cans from different batches for one wall. Pour them together in a larger pail first.

Check your figures with the paint coverage calculator

Type the same figures into the paint coverage calculator if you want a second check. It is free to start, you type every figure yourself, and nothing is sent. Keep your page of measurements with the job, so you can quote the next room faster.

Built in Canada for Canadian rules.