How this calculator works
Enter the room's length, width and wall height, the number of doors and windows, and how many coats you plan to apply. The calculator computes the perimeter, multiplies by height for gross wall area, subtracts standard allowances for doors and windows, and converts the remaining paintable area into liters or gallons at a typical coverage rate.
Coverage figures on paint cans vary by brand, surface texture and application method — the 10 m² per liter and 350 ft² per gallon used here are typical manufacturer values for one coat on a reasonably smooth interior wall, not a universal constant. Check your specific product's label for its stated coverage if you have it, and always round up to the next whole can, since a partial can rarely helps and running short mid-job means a color-match trip back to the store.
The formula
Gross wall area = perimeter × height, where perimeter = 2 × (length + width)Paintable area = gross area − (doors × 1.9 m² or 21 ft²) − (windows × 1.4 m² or 15 ft²)Paint needed = paintable area × coats ÷ coverage (10 m²/L or 350 ft²/gal), rounded upThe door and window allowances are standard estimating figures for an average interior door (about 0.9 m × 2.1 m) and a typical window, used so you don't have to measure every opening individually. Trim, ceilings and a second color are not included — this calculator covers wall paint only.
Worked example: a 5 m × 4 m room, 2.4 m walls, two coats
- Perimeter: 2 × (5 + 4) = 18 m. Gross wall area: 18 × 2.4 = 43.2 m².
- Subtract 1 door and 2 windows: 43.2 − 1.9 − (2 × 1.4) = 38.5 m².
- Two coats: 38.5 × 2 = 77 m² of paint coverage needed.
- At 10 m² per liter: 77 ÷ 10 = 7.7 liters, rounded up to 8 liters to buy.
Frequently asked questions
Why round up to a whole can instead of the exact amount?
Paint is sold in fixed can sizes, and mixing a fresh batch mid-project risks a visible color or sheen mismatch. Rounding up to the next whole liter or gallon (and buying enough full cans to cover that) leaves a small, usable amount left over for touch-ups instead of running short.
Does this include the ceiling?
No — this calculator estimates wall paint only. Add the room's floor area (length × width) separately if you are also painting the ceiling, since ceiling paint is a different product with its own coverage rate.
Why subtract a fixed amount for doors and windows instead of asking for their exact size?
A standard door and window are close enough in size across most homes that a fixed allowance is accurate within a small margin, and it is much faster than measuring every opening. For a room with unusually large windows or French doors, measure those openings directly and adjust the door/window counts or the final area accordingly.
How many coats do I actually need?
Two coats is standard for a color change or new drywall; one coat can be enough when repainting the same or a very similar color over sound existing paint. Dark-to-light color changes and stain-prone rooms often need a primer coat in addition to the topcoats counted here.
Does the paint type change the coverage rate?
Yes — textured surfaces, unprimed drywall and darker colors typically need more paint per coat than the typical smooth-wall figures used here. Check the coverage rate printed on your specific can and substitute it if it differs meaningfully from 10 m² per liter or 350 ft² per gallon.