About the Paint Calculator
This paint calculator works out how much paint a room needs from its length, width and wall height, accounting for the number of coats you plan to apply and deducting standard door and window sizes from the total wall area. It answers the question every DIY painter and decorator faces before a trip to the hardware shop: how many litres or gallons to actually buy.
How to use the Paint Calculator
- 1
Enter the room's length, width and wall height in metres.
- 2
Set the number of coats you plan to apply — two is standard for most walls.
- 3
Enter how many doors and windows the room has; a standard average size is deducted for each.
- 4
Check the coverage rate against your specific tin, then read the litres and gallons needed.
What people use it for
Budgeting for a room repaint
Work out roughly how many tins to buy before starting a project, avoiding an extra trip to the shop mid-job or paying for far more than needed.
Comparing paint brands by real cost
Combine the litres needed here with different brands' prices per litre to compare the actual total cost of a job, not just the price on the tin.
Planning a multi-room project
Run the calculation room by room to build up a full materials list for a whole-house repaint.
How the wall area is calculated
The four walls of a rectangular room have a combined area of twice the room's perimeter — length plus width, doubled — multiplied by the wall height, which is the standard formula used by every paint calculator and most tins' own coverage guidance. A door is deducted at an average of 1.85 square metres and a window at 1.5 square metres, standard sizes close enough for a shopping estimate without needing to measure every opening individually.
Why coats matter more than people expect
Paint needed scales directly with the number of coats, so quoting two coats instead of one doubles the litres required, and going to three for a strong colour change adds another 50% on top of that. Dark colours painted over light ones, or bright colours generally, very often need a third coat or a dedicated primer to avoid patchy, uneven coverage — factoring this in before shopping avoids running out mid-wall.
Tips
- Buy slightly more than the calculated amount — a small margin covers touch-ups and an inaccurate coverage estimate.
- Textured or previously unpainted walls absorb noticeably more paint than a smooth, already-painted surface — check the tin's coverage rate against your actual wall type.
- For a strong colour change, budget for three coats or a grey/tinted primer rather than assuming two will fully cover.