Work Out How Much Paint A Room Actually Needs
A tin says 'covers up to 12 square metres' and leaves you guessing. This measures your walls, subtracts the doors and windows, applies the number of coats you're actually doing, and divides by a coverage rate you set — then tells you how many tins to buy and how much you'll have left. It also compares tin sizes, because buying the wrong one wastes more than the difference in price. Local only; nothing leaves the tab.
- Slip
- Paintable area, litres, and tin count from your dimensions
- local
- What it replaces
- Guessing in the aisle and buying extra
- $0.00
- Input
- Dimensions, openings, coats, and tin size
- no card
- Output
- A tin count with the leftover shown
- recomputed
What you pay otherwise
| Route | What you get | Cost |
|---|---|---|
| An extra tin bought 'to be safe' | Usually a different batch, rarely used | $30–$60 |
| Running out mid-wall | A second trip and a visible batch line | the finish |
| Contractor's estimate | For a calculation you can do in a minute | $40+ |
| This page | The quantity, to within a fraction of a tin | $0 |
The workflow
Subtract the openings — they are bigger than people think
A standard door is about 1.7 square metres and a window often 1.5 to 2. On a small room, two doors and a window remove a fifth of the wall. Leaving them in is the most common way to over-buy.
Use a coverage rate matched to your surface
Bare plaster soaks up far more than a previously painted wall, and dark-over-light needs more than light-over-light. Ten to twelve square metres a litre is typical for painted walls; use eight to ten on new plaster.
Count the coats you're actually doing
A colour change over a dark wall is usually three coats, not the two on the tin. The calculator multiplies, so getting this wrong scales the whole estimate.
Compare tin sizes, not just the total
Four 2.5-litre tins and two 5-litre tins cover the same area at different prices, and the leftover differs too. The slip prints the waste for the size you enter, which is the number that decides it.
Worked example
Sample: 4.2 × 3.6 m room, 2.4 m ceiling, 1 door, 1 window Wall area 37.44 sq m Less openings 3.30 sq m Paintable 34.14 sq m Two coats at 11 6.21 litres 5-litre tins 2 (10 L bought, 3.79 L spare) Uploaded nothing — computed in the tab
Numbers above are sample data produced by the tool's own pre-loaded example, run in the browser — not averages or measured benchmarks.
What breaks this
Forgetting the openings over-buys by a fifth on a small room. Using a coverage rate for painted walls on bare plaster under-estimates by a third. Assuming two coats over a dark colour scales the whole estimate wrong.
Try it now
A 4.2 × 3.6 metre room needing 6.21 litres is loaded, buying two 5-litre tins. Add two more coats and it needs 12.41 litres — three tins.
Open the calculator →Go deeper: the printable slip
The estimate is free forever. The $4 one-time tier adds:
- Printable slip — the quantities on one sheet to take to the counter — estimate: ~6 min saved re-measuring at the shop.
- Saved rooms shelf — keep up to 10 rooms in this browser — estimate: ~10 min saved across a whole house.
- Cost comparison — the waste and cost for each tin size side by side — estimate: ~8 min saved doing it by hand.