The Formula the Tool Runs
Our paint calculator treats the room as a rectangle and computes wall area with one line of code.
area = 2 x (width + length) x height - doors x 1.8 - windows x 1.5
The first term is the room perimeter multiplied by wall height. Each door subtracts 1.8 square meters and each window subtracts 1.5 square meters. The door and window fields default to 1, so set them to 0 when your wall has none.
Coverage uses a fixed constant of 11 square meters per liter. The code comment records the intent behind that constant, an average of the 10 to 12 range the results panel displays. Liters are simply area divided by 11, rounded to two decimals. Area is also rounded to two decimals before display, while can counts are whole numbers by construction.
A Worked Example, Verified
This example uses round numbers and is an illustration, not a specification for your room. A room 4 meters wide, 5 meters long, and 2.5 meters high, with 1 door and 2 windows.
The perimeter of the 4 by 5 room is 18 meters, and 18 times the 2.5 meter height gives 45 square meters of gross wall area before deductions.
area = 2 x (4 + 5) x 2.5 - 1 x 1.8 - 2 x 1.5
area = 45 - 1.8 - 3 = 40.2 square meters
liters = 40.2 / 11 = 3.65 L
The tool then converts liters to cans. Cans of 2.5 liters equal the liter figure divided by 2.5, rounded up, which gives 2 cans. Cans of 5 liters give 1 can.
A second verified example with the default openings. A room 3 by 4 by 2.5 meters with 1 door and 1 window yields 31.7 square meters, 2.88 liters, 2 cans of 2.5 liters, or 1 can of 5 liters. Enter 0 doors and 0 windows for the first example room and the gross 45 square meters stands, which gives 4.09 liters.
Can Counts Always Round Up
The tool applies round up logic to cans, never round to nearest. One 2.5 liter can covers 2.5 liters of need, so 3.65 liters forces a second can even though part of it stays in the tin.
Check the purchase totals for the worked example. Two 2.5 liter cans buy exactly 5 liters, and one 5 liter can buys the same 5 liters, so the leftover is identical at 1.35 liters either way. The can size choice is a price question, and the calculator does not know prices. Compare both counts against shelf prices per liter before buying.
What the Tool Ignores on Purpose
The formula covers walls only. A ceiling would add width times length, which is 20 square meters in the worked example, and you would enter it as a separate rectangular room mentally. There is no ceiling field in the tool.
The liter figure covers one coat. Two coats double the worked example from 3.65 liters to 7.3 liters. Rerun the can math on the doubled figure, which gives 3 cans of 2.5 liters or 2 cans of 5 liters.
The room must be rectangular. For an L shaped room, split it into rectangles, compute each separately, and add the wall areas.
There is no trim or baseboard field either. Trim needs its own estimate from linear meters times coat count, and the tool has no input for it.
Steps to Estimate Any Room
1. Measure room width, length, and wall height in meters.
2. Count doors and windows on the walls you will paint.
3. Enter all five values, using 0 for openings that do not exist.
4. Read the liter figure, which assumes one coat.
5. Multiply liters by your coat count.
6. Round the adjusted liters up to whole cans yourself, using the can size you intend to buy.
Honest Downsides
The coverage constant is fixed at 11. If your paint spreads at 9 or 13 square meters per liter, divide your area by the figure printed on your can instead of trusting the tool liters. The door and window deductions are fixed sizes, so an oversized patio door or a floor to ceiling window is subtracted too little and the estimate runs high. The tool is metric only, with inputs in meters and output in liters. It estimates wall paint and does not price primer, trim, or tools.
Checklist Before You Buy
- [ ] All five inputs set, with 0 for openings that do not exist.
- [ ] Liters multiplied by your coat count.
- [ ] Can math redone on the adjusted liter figure.
- [ ] Coverage figure on your paint can checked against the 10 to 12 range.
- [ ] Non rectangular walls split into rectangles and computed separately.
If the tool estimate missed your real usage on a completed room, send us the room dimensions, the paint used, and the coats applied. Real consumption data against the 11 constant is the only way to tune it honestly. Run your own numbers on the [paint calculator](/en/paint-calculator).
