Paint Calculator
Calculate how many liters or gallons of paint you need for a room's walls, accounting for coats and openings.
What this calculates
Calculates the total wall area, liters of paint required and approximate number of gallons for a rectangular room.
Formula used
Perimeter = 2 × (length + width). Wall area = perimeter × height − openings. Liters = (wall area × coats) ÷ coverage per liter.
Worked example
A room of 4 m × 3 m with 2.4 m walls has a perimeter of 14 m. Wall area = 14 × 2.4 = 33.6 m². Two coats at 10 m²/L = 6.72 L ≈ 2 gallons.
Typical paint coverage
Most interior wall paints cover 10–12 m² per liter on smooth surfaces. Rough or unpainted plaster may cover only 6–8 m²/L on the first coat. Check the product datasheet.
When not to use this calculator
This calculator assumes four rectangular walls of equal height. For rooms with vaulted ceilings, gable ends, or complex shapes, measure each surface individually.
Frequently asked questions
How much paint do I need for a room?
Use the perimeter × height to get wall area, subtract doors and windows, then divide by the paint's coverage per liter. Multiply by the number of coats.
How many m² does a liter cover?
Most paints cover 10–12 m²/L on smooth surfaces. Rough surfaces absorb more. Check the product label.
How many coats do I need?
Typically 2 coats. Use 3 when covering a dark color or painting bare plaster.
What can size should I buy?
Divide the total liters by the can size and round up. One gallon (3.785 L) covers about 37–46 m² in 2 coats at standard coverage.
Assumptions
- Wall area = perimeter × height − openings.
- Ceiling paint is not included.
- Coverage is applied uniformly to all coats.
- Gallons = total liters ÷ 3.785, rounded up.
Porous, textured or previously unpainted surfaces absorb significantly more paint on the first coat. A primer coat is recommended before applying the finish color.
Common mistakes
- Not priming bare plaster or new drywall before painting.
- Underestimating coverage on rough or dark-colored walls.
- Buying paint from different batches (color variation).