Paint Calculator
How much paint does a room actually need? Enter the room, the doors and windows, and how many coats you plan. You get litres and the number of standard 3.78 L cans to buy, with primer figured separately. Results update as you type.
Your paint
Coverage assumption: one 3.78 L can covers about 35 m² per coat on smooth walls, the rate most Canadian paint brands print on the can. Rough, unpainted or heavily textured surfaces cover less; see the FAQ.
Worked example
A 4 m by 3.5 m bedroom, 2.4 m ceiling, 1 door, 2 windows
Wall area: the perimeter is 2 × (4 + 3.5) = 15 m, times the height gives 15 × 2.4 = 36.0 m². Deduct the door and windows: 36.0 − 1.9 − (2 × 1.5) = 31.1 m².
Two coats means 31.1 × 2 = 62.2 m² of coverage. Paint needed: 62.2 ÷ 35 × 3.78 = 6.72 L, which is 6.72 ÷ 3.78 = 1.78, so buy 2 cans.
Primer, one coat: 31.1 ÷ 35 × 3.78 = 3.36 L, so buy 1 can. These are the results shown above with the default settings.
How this calculator works
The wall area is the room perimeter times the ceiling height, minus standard openings: 1.9 m² per door (about 0.9 m by 2.1 m) and 1.5 m² per window. If you tick the ceiling box, the ceiling area (length times width) is added at the same number of coats.
Paint volume comes from the coverage rate: litres = total area to cover ÷ 35 × 3.78. One 3.78 L can covers about 35 m² per coat on a smooth, previously painted wall. The cans to buy are the litres divided by 3.78, rounded up, because paint is sold in whole cans. Primer is figured at one coat over the same area and listed separately, since it is a different product.
Painting a single wall instead of a room? Set the room length to your wall width and the width to zero, or measure the wall and divide: a wall of 10 m² at two coats needs 10 × 2 ÷ 35 × 3.78 = 2.16 L.
Common questions
How much does one 3.78 L can of paint cover?
About 35 m² per coat on smooth walls, which is the figure most brands sold in Canada print on the can. In practice, expect closer to 28 to 30 m² on rough or heavily textured walls, and around 25 m² on bare drywall or dark colours going over light ones. If your walls are thirsty, buy for the lower coverage; leftover unopened cans can usually be returned.
Do I really need primer?
On bare drywall, patched areas, stains, or a dramatic colour change, yes: primer seals the surface so the finish coats cover evenly, and it is cheaper than a third coat of paint. On a previously painted wall in good shape, going a similar colour, a paint and primer product in two coats is normally enough.
Is one coat ever enough?
Only for a same-colour refresh on a wall in good condition, and even then missed spots tend to show. Two coats is the standard recommendation for an even finish and accurate colour, which is why it is the default here.
Should I round up or buy exactly what the calculator says?
Buy the rounded-up can count shown, then keep about half a litre back for touch-ups before you seal the can. Running out near the end risks a slight colour difference between cans (called a batch difference), so buy all your paint at once and mix the cans together in a larger bucket for the final coat if colour consistency matters.
Estimates only, verify before you buy materials. Coverage varies by brand, surface and application method; check the coverage rate printed on your paint can and adjust your purchase accordingly.