Paint Calculator
Work out how much paint a room needs. Enter the room or the walls, subtract doors and windows, choose the number of coats and the coverage printed on the can, and see the liters or gallons you need, how many cans to buy and the cheapest mix of can sizes.
1. Room
2. Doors and windows to subtract
Doors
Windows
Set the number to 0 if there are none. Openings are taken off the wall area only, never off the ceiling.
3. Paint
Coverage is a typical value for wall paint on a smooth, primed surface. Read the number on your can: it varies by product and by surface.
4. Cans and prices (optional)
Enter the can sizes in liters (0.75, 2.5, 5, 10) and leave a row empty if you do not want to use it. Prices are per can, in any currency. Add a price for every can size to unlock the cheapest option.
| Can size (l) | Paint price | Primer price |
|---|
Result
Calculations run locally in your browser. Nothing you enter is sent to a server.
How to use the paint calculator
- Choose metric (meters, liters) or imperial (feet, US gallons). Switching converts your lengths and areas.
- Pick Room size if the room is a simple box: enter length, width and wall height, and tick the ceiling box if you want to paint it too. Pick Wall by wall for odd shapes: add one row per wall (up to 20) and use the extra-area field for a ceiling or anything else.
- Enter how many doors and windows there are and their size. They are taken off the wall area. Enter 0 where there are none.
- Set the number of coats, the coverage printed on your can and the waste allowance. Tick “Add primer” if you need a separate primer coat.
- Optionally list the can sizes the shop sells, with a price for each. The tool then shows the fewest cans you need, or the cheapest combination when every size has a price. You can switch between fewest cans, least leftover and cheapest.
- Use “Copy summary” to paste the result into a shopping list or message.
How it works
The calculation has three steps: find the area, convert it to a volume, then fit cans to that volume.
1. Paintable area. For a box-shaped room the four walls have a combined area of the perimeter times the height. Openings are subtracted, and the ceiling is added if you chose it:
A = 2 × (L + W) × H − (nd × wd × hd) − (nw × ww × hw) + L × W
Here L, W and H are the length, width and wall height. nd, wd and hd are the number, width and height of doors, and nw, ww and hw are the same for windows. The ceiling term applies only if you tick the box. In wall-by-wall mode the wall term is the sum of width times height for every wall you list, plus any extra area.
2. Volume of paint. Coverage tells you how much area one liter or gallon covers in a single coat:
V = A × coats ÷ coverage × (1 + waste ÷ 100)
A is the paintable area, coats the number of coats, coverage the area per liter or gallon, and waste the extra percentage for spills, rolling and touch-ups. The result is rounded up to 0.01 liter or gallon. Primer uses the same formula.
3. Cans. The tool searches every combination of your can sizes that holds at least V. You choose the rule. Fewest cans (the default without prices) takes the smallest number of cans, and among those the least leftover paint. Least leftover takes the closest match to V, then the fewest cans. Cheapest takes the lowest total price, then the least leftover, then the fewest cans. With a price for every can size the tool uses cheapest automatically until you pick another rule. It also lists single-size options. The search is exact, not a rule of thumb.
Examples
Example 1: the default room. A 5 m × 4 m room, 2.5 m high, with one door of 0.9 × 2 m and two windows of 1.2 × 1.2 m. Walls: 2 × (5 + 4) × 2.5 = 45 m². Openings: 1.8 + 2 × 1.44 = 4.68 m². Area: 40.32 m². Two coats at 10 m² per liter with 10% waste: 40.32 × 2 ÷ 10 × 1.1 = 8.8704, shown as 8.88 l. With cans of 0.75, 2.5, 5 and 10 l and no prices, the fewest cans is a single 10 l can (1.12 l left over). Choosing Least leftover instead gives 9 l: one 5 l, one 2.5 l and two 0.75 l cans (4 cans, 0.12 l left).
Example 2: imperial units. A 16 ft × 13 ft room, 8 ft high, one door of 3 × 6.7 ft and two windows of 4 × 4 ft. Walls: 2 × 29 × 8 = 464 ft². Openings: 20.1 + 32 = 52.1 ft². Paintable area: 411.9 ft². Two coats at 350 ft² per gallon with 10% waste give 411.9 × 2 ÷ 350 × 1.1 = 2.589, shown as 2.59 gal. With cans of 0.25 (a quart), 1 and 5 gal, the fewest cans is a single 5 gal can. Choosing Least leftover gives 2.75 gal: two 1 gal cans and three quarts (5 cans).
Example 3: prices change the answer. Use Example 1 again with can prices of 9 (0.75 l), 22 (2.5 l), 38 (5 l) and 60 (10 l). The 9 l least-leftover mix above costs 9 × 2 + 22 + 38 = 78. A single 10 l can costs 60, so the cheapest combination is one 10 l can for 60.00, even though it leaves 1.12 l over.
Example 4: wall by wall with primer. Four walls of 4.2 × 2.6 m, 3 × 2.6 m, 4.2 × 2.6 m and 3 × 2.6 m add up to 37.44 m². One door of 0.8 × 2 m takes off 1.6 m², no windows, so the area is 35.84 m². Paint: 2 coats at 12 m² per liter with 5% waste gives 6.272, shown as 6.28 l. Primer: 1 coat at 10 m² per liter gives 3.7632, shown as 3.77 l. With 0.75, 2.5 and 5 l cans and the fewest-cans rule: paint 7.5 l (one 5 l and one 2.5 l), primer one 5 l can. Least leftover would give paint 6.5 l (one 5 l, two 0.75 l) and primer 4 l (one 2.5 l, two 0.75 l).
Choosing coverage and number of coats
The coverage on a can is measured under ideal conditions: a smooth, sealed surface and an even application. Interior wall paint commonly lists about 8 to 12 square meters per liter, which is roughly 300 to 400 square feet per US gallon, but the exact figure depends on the product. Rough, bare or textured walls absorb more paint, so real coverage can be well below the label. The tool warns you when your figure is outside the usual range, but it cannot know your surface.
Two coats is the usual recommendation. Going from a dark wall to a light colour may need a primer or a third coat. Follow the instructions on your product.
Common mistakes
- Forgetting the second coat. One coat instead of two halves your estimate.
- Mixing up units. A US gallon is about 3.785 liters; an imperial gallon (about 4.546 liters) is not used here.
- Running short. Paint from different batches can differ slightly in colour, so buy everything for one wall at once.
- Trusting an unusual coverage. Coverage is area per volume (for example m² per liter), not the other way round.
Frequently asked questions
How much extra paint should I allow?
The default is 10%, which covers spills, paint left in the tray and roller, and small touch-ups later.
Should I paint the ceiling with the same paint?
Often not: ceilings usually get a flat ceiling paint. The calculator adds the ceiling to the same volume. To separate them, run the tool once without the ceiling, then enter the ceiling in wall-by-wall mode as one wall (length as width, width as height) with the ceiling paint's coverage.
Why is the cheapest combination not the one with the least paint left over?
Larger cans are usually cheaper per liter, so a single big can may cost less than several smaller ones that add up to a closer volume. Example 3 shows this.
Can I measure irregular rooms?
Yes. Use wall-by-wall mode and enter every wall as a width and a height. For a gable or sloping wall, use the average height.
Is my data sent anywhere?
No. Everything is calculated in your browser. Your entries are saved only in your own browser, and nothing is uploaded.
This tool gives an estimate for planning and shopping. Actual paint use depends on the surface, the product and the way it is applied, so follow the instructions and coverage on your paint can.
Related tools
- Unit Converter — convert feet, meters, liters and gallons before you measure.
- Percentage Calculator — work out waste allowances and price differences.
- Loan Calculator — plan the cost of a bigger renovation project.
- Significant Figures Calculator — round measurements sensibly.