From wall area to what you actually buy
Paint estimation has two separate steps. First calculate the coated area and divide by the product coverage rate. Then round that theoretical quantity into the tin or bucket sizes that are actually sold. AmountHub keeps those steps separate so you can see both the maths and the practical purchase quantity.
The calculator subtracts standard door and window allowances for speed. For rooms dominated by glazing, large doors or unusual openings, measuring the opening area directly will be more accurate.
Worked example
An 18 m room perimeter with 2.5 m walls gives 45 m² of gross wall area. After one standard door and two standard windows, the paintable area is about 40.1 m². Two finish coats at 10 m²/L require about 8.0 litres before any touch-up reserve.
The planning options above can add a small reserve, estimate one primer coat and round the result into the container size you intend to buy.
Coverage is the most important assumption
| Condition | Why coverage changes | Planning response |
|---|---|---|
| Smooth previously painted wall | Low absorption | Manufacturer headline coverage may be realistic. |
| Bare drywall or plaster | Porous substrate | Use the correct primer or sealer first. |
| Strong colour change | Opacity becomes limiting | Allow more coats or a suitable tinted primer. |
| Heavy texture | More true surface area | Use a more conservative coverage rate. |
Primer should be calculated separately
Primer is a different product and often has a different coverage rate from the finish coat. If the project needs primer, calculate it as its own coating rather than pretending it is simply another finish coat. The optional primer planner above uses the same paintable wall area but lets you enter a separate primer coverage rate.
Walls, ceilings and several rooms
This tool focuses on walls. A ceiling is normally calculated from room length × room width and should be kept separate if it uses a different paint. For several rooms with the same finish, add the paintable areas before applying the final coat count. For different colours or sheens, calculate each group separately.
Before buying paint
- Use the exact coverage stated on the product can or technical sheet.
- Check whether the quoted coverage is per coat.
- Round up to real container sizes rather than buying the exact decimal result.
- Keep a modest amount for touch-ups if colour matching later would be difficult.
- Do not assume primer and finish paint share the same coverage.