A 10x12 bedroom with 8 ft ceilings has about 352 sq ft (32.7 m²) of wall surface. According to two Canadian paint manufacturers’ estimating guides, one 3.70 L (one-gallon) can of wall paint covers roughly 375 to 400 sq ft. That means one coat should fit in a single can, with little room to spare, and two coats need two cans. Doors and windows are subtracted from the wall total, while the ceiling and trim are calculated separately. The steps below show the method so you can apply it to your own room.
How do you measure the wall area of your bedroom?
Paint covers the walls between the floor and the ceiling, so start by measuring the room at floor level and the height from the floor to the ceiling. The guides consulted use this formula: 2 × (width × height) + 2 × (length × height). In plain terms, multiply the width by the height to get the area of the two short walls, multiply the length by the height for the two long walls, double each result and add them together.
For a 10 ft by 12 ft room with 8 ft ceilings, the calculation is:
2 × (10 ft × 8 ft) = 160 sq ft for the two short walls 2 × (12 ft × 8 ft) = 192 sq ft for the two long walls Total wall area = 352 sq ft
The 8 ft height is a working assumption for this example. Measure your own ceiling, because each extra foot of height adds 44 sq ft to a room of this size. As a check on the method, one manufacturer’s guide works through a 12 ft by 14 ft room with 8 ft ceilings and reaches 416 sq ft, which matches the same formula.
How many cans do you need for one coat?
Divide the wall area by the coverage printed for the can. Using the 375 to 400 sq ft range for a 3.70 L can:
352 ÷ 375 = 0.94 can 352 ÷ 400 = 0.88 can
Both results are below one can, so a single 3.70 L can should be enough for one coat on the walls. At the lowest coverage figure, about 23 sq ft remain spare, which is a thin margin. Coverage figures assume a typical surface and one coat over a similar colour. Porous or rough walls, and patched areas, can absorb more paint. Treat the estimate as a starting point and check the manufacturer’s technical documentation for the product you choose.
Why might you need two coats?
One coat is usually enough when the new colour is close to the old one. Two coats are often needed for a major colour change, such as going from a deep red to a pale grey, when the surface is damaged or old, or when you want more durability and coverage. The manufacturer guidance advises deciding one room at a time whether it needs one or two coats.
For two coats on the same room, the wall area doubles to 704 sq ft:
704 ÷ 375 = 1.88 cans 704 ÷ 400 = 1.76 cans
Both figures round up to two cans of 3.70 L each. Two cans give enough paint for the full job at either coverage figure, which is why two coats are usually planned as two full cans rather than a can and a bit.
Should you subtract doors and windows?
Yes, if you want an accurate figure. Measure each door and window, multiply its width by its height, and subtract the total from the wall area. The guides consulted do not mention deducting openings, and their worked examples add up the full wall area. Skipping the deduction therefore gives a slight overestimate, which is the safer side when buying paint. For a figure this close to the limit of one can, the deduction can decide whether a single can is enough, so it is worth measuring.
What about the ceiling and the trim?
The ceiling is a separate surface. In a 10 ft by 12 ft room, its area is 120 sq ft (about 11.1 m²). Do not reuse the wall coverage figure for it. The ceiling may be a different product, and its coverage is listed on the label or in the manufacturer’s documentation. The guides consulted do not give coverage figures for ceilings or trim, so check the product documentation for those surfaces before you buy. If the ceiling and walls use the same product, add the ceiling area to the wall total and recalculate.
When should you round up, and what can size should you buy?
Round up to whole cans. The manufacturer’s example works out to 1.1 cans and does not say how to round, so rounding up is the practical choice. Running out halfway through a wall is the main reason to buy the extra fraction.
If the shops near you sell other sizes, work in litres. At the two coverage figures, one coat on the walls needs between about 3.3 and 3.5 L (352 ÷ 400 × 3.70 L and 352 ÷ 375 × 3.70 L). Two coats need between about 6.5 and 6.9 L (704 ÷ 400 × 3.70 L and 704 ÷ 375 × 3.70 L). Choose the closest package size that is at least as large as the higher figure, and keep any unopened or leftover paint for touch-ups.
How do you paint safely and dispose of leftover paint?
Health Canada (2024) advises opening windows when renovating or using products such as paint. Keep windows open while you paint and for as long as the room smells of paint. The official sources consulted do not specify how long to keep the room ventilated after the job is finished. Health Canada also advises choosing low-emission paints, but a product labelled low emission releases fewer volatile organic compounds, not none, so ventilation remains necessary.
If you are unsure what the existing paint contains, especially in an older home, test or check before sanding or scraping, and follow the advice of your provincial or territorial health or environment authority.
Do not pour leftover paint down a drain or flush it down a toilet. Health Canada advises taking leftover household chemical products to a local hazardous waste disposal depot. This advice covers household chemicals in general, so check your municipality’s rules for paint specifically before you dispose of a can.
Frequently asked questions
What if my bedroom ceiling is higher than 8 ft?
Recalculate with your measured height using the same formula. Every extra foot adds wall area, so once the total passes the coverage of one can, plan for a second can for a single coat.
How do I estimate an alcove or a sloped ceiling?
Split the space into simple rectangles, calculate each wall or section separately, and add them together. The guides consulted do not describe sloped ceilings, so check the product documentation for coverage on those surfaces.
Do I need a primer before painting?
The guides consulted explain when two coats may be needed, but they do not specify when to use a primer. Check the product documentation for the surface you are painting and follow its instructions.
