Paint Calculator
Find out how much paint your room needs
How much paint do I need?
The paint calculator turns the size of your walls into the number of gallons to buy. The math is simple: measure the wall area, subtract the doors and windows you are not painting, multiply by the number of coats, and divide by how far a gallon goes. The care is in the assumptions — get the coverage rate and the number of coats right and the estimate is reliable.
Start with wall area. For a rectangular room it is the perimeter times the wall height: 2 × (length + width) × height. A 3.66 × 4.27 m room with walls about 2.44 m high has 38.6 m² of wall. Then subtract about 1.9 m² for each door and 1.4 m² for each window, since you are not painting those. One gallon of interior latex covers roughly 32.5 to 37.2 m² per coat.
When to buy more, and when to buy less
- Two coats is the norm. Bare drywall, patched repairs, or a dramatic colour change (dark over light, or light over dark) can need a primer coat on top of that.
- Textured, porous, or previously unpainted walls drink paint — expect coverage closer to 27.9 m² per gallon and round up.
- Deep, saturated colours (strong reds, blues) often need an extra coat to look even, regardless of the surface.
- Ceilings and trim are separate jobs with their own areas — this calculator sizes the walls only.
Frequently asked questions
How many square feet does a gallon of paint cover? Around 32.5 to 37.2 m² per coat on a smooth, primed wall. We use 32.5 as the default so you are not left a few square feet short near the end. Rough or porous surfaces cover less.
Should I round up or down? Always up. Running out mid-wall means a trip back to the store and the risk of a slightly different mix from a new batch. We round up to the nearest quart so you buy just enough.
Do I need primer? Primer is worth it on bare drywall, over patches and stains, and when making a big colour change. Paint-and-primer products can skip a separate primer coat on already-painted walls in good condition, but they do not replace a dedicated primer on raw or problem surfaces.
Does this include the ceiling? No. This sizes the walls, which is what most people mean by painting a room. Ceilings have their own area (length × width) and are usually a flat ceiling paint rather than wall paint.
These are planning estimates. For an exact figure, measure each wall, subtract the real openings, and check the spread rate printed on the can you buy. See our painting and decorating guides for prep and technique.