Paint Calculator
Enter your room's dimensions and how many coats you're applying — doors and windows are subtracted automatically so you don't over-buy.
How Paint Calculator Works
This calculator works out how much paint to buy for a room by measuring the total wall (and optionally ceiling) area you actually need to cover, subtracting standard allowances for doors and windows, and dividing by your paint's coverage rate and number of coats.
Formula & Method
The wall area is perimeter × height, where perimeter is 2 × (length + width). From that, the calculator subtracts a fixed area for each door and window you enter (1.86 m² per door and 1.4 m² per window in metric mode, or 20 sqft and 15 sqft in imperial mode) to get the paintable wall area. If you tick "also paint the ceiling," the room's floor area (length × width) is added on top. That total area is multiplied by your number of coats, then divided by the coverage rate (area a unit of paint covers per single coat) to get the volume of paint needed.
Worked Example
For a 4m × 3m room with a 2.5m wall height, 2 coats, 1 door, 2 windows, and a coverage rate of 10 m² per liter (ceiling not included): the perimeter is 14m, so the wall area is 14 × 2.5 = 35 m². Subtracting 1 door (1.86 m²) and 2 windows (2 × 1.4 = 2.8 m²) leaves a paintable area of 30.34 m². Over 2 coats that's 60.68 m² of paint coverage, which at 10 m² per liter works out to about 6.07 liters of paint — so you'd buy at least 7 liters.
Frequently Asked Questions
- Why does the calculator use a fixed area for every door and window instead of letting me measure each one?
- It's a convenience assumption for speed — a standard door is treated as 1.86 m² (20 sqft) and a standard window as 1.4 m² (15 sqft). If your doors or windows are unusually large or small, adjust the door/window count up or down to roughly compensate, or treat the result as an estimate and round up when buying.
- Should I include the ceiling in my paint calculation?
- Only if you're actually painting it. Ceilings are often a different color or left unpainted, so this is an optional checkbox — ticking it adds the room's floor area (length × width) to the total paintable area.
- Why does the "buy at least" figure round up?
- Paint is sold in fixed container sizes, and running short mid-job means an unplanned second trip to match the color. Rounding the raw liters/gallons figure up to the next whole unit gives you a safer minimum to purchase.
- Where do I find the coverage rate for my paint?
- It's printed on the paint can's label, usually as an area-per-volume figure (e.g. "10 m² per liter" or "350 sqft per gallon"). Coverage varies by brand, paint type, and the surface being painted, so use your can's actual number rather than the calculator's default for the most accurate result.
Assumes a standard door is 1.86 m² and a standard window is 1.4 m² — adjust the door/window count to roughly account for unusually large or small openings. Coverage varies by paint brand and surface, so check your paint can's label for a more exact figure.