How the coverage is calculated
Paintable area = perimeter × height, minus 21 ft² per standard door and 15 ft² per standard window — most calculators skip this and overstate how much you need. Multiply by the number of coats, then divide by 350 ft² per gallon, the middle of the range most paint labels list (350–400 ft²/gal).
Coats needed by situation
| Situation | Coats | Note |
|---|---|---|
| Same colour repaint | 1 | if the existing colour is close |
| Colour change | 2 | standard for most jobs |
| Light over dark | 2–3 | budget an extra quarter-can |
Related guides
Frequently asked questions
Does this account for primer?
No — primer coverage is similar to paint (roughly 300–400 ft²/gal), so if priming bare drywall, run the calculator again with coats set to 1 for the primer gallons.
What if my room has an odd shape?
Measure each wall separately and add the lengths together for the perimeter figure — the calculator only needs the total.
Estimating paint coverage
Paint quantity depends on surface area, number of coats and the coverage stated by the paint manufacturer. The calculator subtracts common openings such as doors and windows so the estimate is closer to the actual paintable area.
Coverage is product-specific
Paint cans usually state an approximate coverage per gallon or litre. Coverage changes with surface texture, porosity, colour changes and application method, so treat the calculator's quantity as an estimate rather than a guarantee.
Why coats matter
A strong colour change or a porous surface can require additional coats. Enter the number of coats you actually expect to apply. Primer may also be appropriate in some situations and should be considered separately if the product recommends it.
Buying strategy
For a multi-room project, calculate each room and add the quantities rather than relying on a single rough house-wide estimate. Keep the product name and batch information when possible so touch-ups can match later.