Paint Coverage Calculator
Enter room size, ceiling height, openings and number of coats to get total area, litres needed and how many cans to buy.
Inputs
Result
Formula
| Wall area | A_wall = 2 × (L + W) × H − openings |
| Ceiling | A_ceiling = L × W |
| Total area | A = A_wall + A_ceiling |
| Primer | V_primer = A ÷ coverage |
| Top coat | V_top = A × coats ÷ coverage |
| Cans | N = ceil(total volume ÷ can size) |
A 5×4 m room at 2.8 m with 4 m² of openings: 46.4 m² of wall plus 20 m² ceiling = 66.4 m². That is 6.6 L primer and 13.3 L of top coat, about 20 L total, so two 18 L cans.
FAQ
Why not just multiply floor area by 3?
Floor area × 3 is a rough rule of thumb. The real ratio depends on ceiling height and room shape: about 2.8-3.2 for a squarish room at 2.8 m, over 3.8 at 3.3 m, and more than 5 for a long corridor. Measuring the actual dimensions is accurate to within about 5%.
Is one primer plus two top coats necessary?
For new or patched walls, yes. Primer seals the substrate and evens out absorption, which saves top coat and prevents patchy colour. On sound existing walls a combined primer-and-topcoat product works. Going from dark to light needs an extra coat or a white primer first.
Why does the calculated volume come up short?
Three reasons: highly absorbent substrates such as new plaster or unsealed drywall use 20-30% more; dark colours cover about 20% less area per litre; and roller application uses less paint but often needs an extra coat. Add 10% on top of the result rather than ordering exactly.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.