Drywall
Panels, screws and tape for partition walls before painting.
OpenPaint 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.
How to use this calculator
- Enter the room length and width in meters (the calculator uses the perimeter, so both dimensions are needed).
- Enter the wall height — this is the floor-to-ceiling height, typically 2.4 m (8 ft) in residential.
- Set the number of coats — 2 is standard; use 3 for dark-to-light color changes or over bare plaster.
- Enter the paint coverage in m²/L — check the product datasheet; standard interior paints are 10–12 m²/L per coat.
- Optionally enter total area of openings (doors + windows in m²) to deduct them from the wall area.
- Click "Calculate paint" to see liters, gallons and a cost estimate if you entered a price.
Paint coverage rates and surface types
Coverage varies significantly by surface type and product quality. Use these reference values when you do not have the manufacturer's datasheet:
- Smooth plaster or previously painted wall: 10–12 m²/L per coat for standard emulsion or latex.
- Bare plaster or new drywall: 6–8 m²/L on the first coat (high absorption) — always prime first.
- Rough or textured surfaces: 5–8 m²/L — the texture increases the effective surface area.
- Exterior masonry: 4–7 m²/L for masonry paint on first coat; 8–10 m²/L for subsequent coats.
Premium paints often state higher coverage (12–16 m²/L) because they have more pigment and solids. Budget paints at the same stated coverage often require an extra coat. When in doubt, buy slightly more than calculated — running out mid-wall means color inconsistency.
Priming — when it is mandatory
Primer is not optional in certain situations:
- New drywall (plasterboard): The paper face absorbs paint unevenly without primer, causing "flashing" — dull spots where joints were sanded.
- Fresh plaster or render: Highly alkaline surface attacks latex paints; use an alkali-resistant primer.
- Stain blocking: Water stains, smoke marks or grease spots bleed through standard paint. Use shellac-based or oil-based stain-blocking primer.
- Drastic color change (dark to light): A tinted primer in a shade between old and new color reduces required finish coats from 3–4 to 2.
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.
How many gallons of paint for a 12×12 room?
A 12 ft × 12 ft room with 8 ft ceilings (3.65 m × 3.65 m × 2.44 m) has about 320 sq ft (30 m²) of wall area after deducting one door and one window. At standard coverage of 400 sq ft per gallon (10 m²/L), two coats require about 1.6 gallons (6 liters). Round up to 2 gallons. Enter your exact room dimensions above for a precise result.
Do I need to prime before painting?
Yes, for bare drywall, new plaster, patched areas or a drastic color change. Primer seals the surface, improves adhesion and reduces the number of finish coats needed — typically saving one coat and preventing uneven absorption. Priming is not required if you are repainting a previously painted wall in a similar color.
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.
Technical note
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).
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