How to use the paint calculator
Paint quantities look simple until you start subtracting doors, adding coats and converting between litres and gallons. This calculator does that bookkeeping so you can order the right number of tins without a spreadsheet.
- Choose feet or metres. Coverage switches between ft² per gallon and m² per litre when you change units.
- Enter the walls as room perimeter × height, or as total wall length × height if you are painting only some walls.
- Optionally include the ceiling by entering its length and width.
- Subtract openings with door and window counts, or enter a custom area for each.
- Set coats, coverage and waste. Defaults are two coats, typical interior coverage and 10% waste.
Results show gross wall area, openings deducted, net paintable area (plus ceiling if included), paint needed before and after waste in both litres and gallons, and a short note on how many typical tin sizes that is.
Paint quantity formulas
Gross wall area = Wall length (or perimeter) × Height
Net wall area = Gross − Door areas − Window areas
Total surface = Net wall area + Ceiling area
Paint (before waste) = Total surface × Coats ÷ Coverage
Paint to order = Paint × (1 + Waste % ÷ 100)
1 US gallon ≈ 3.785 litres · 1 litre ≈ 0.264 US gallons
Coverage is the figure on the tin for one coat on a smooth, primed surface. Textured plaster, bare drywall, brick and dark colour changes all use more paint, so lower the coverage number or raise the waste percentage when the surface is rough.
| Surface | Typical coverage | Notes |
|---|---|---|
| Smooth primed drywall | 10–12 m²/L · 350–400 ft²/gal | Standard interior emulsion |
| New plaster / porous | 6–8 m²/L · 250–300 ft²/gal | Use a primer or mist coat first |
| Textured / stipple | 6–9 m²/L · 250–325 ft²/gal | More waste from roller texture |
| Exterior masonry | 4–8 m²/L · 150–300 ft²/gal | Check the product data sheet |
Worked examples
A 12 × 12 ft bedroom
Perimeter is 48 ft, height 8 ft, so gross walls = 384 ft². Two standard doors (40 ft²) and three typical windows (45 ft²) leave 299 ft² net. Two coats at 375 ft²/gal need 299 × 2 ÷ 375 ≈ 1.59 gal before waste, or about 1.75 gal with 10% waste (roughly 6.6 litres). Buy two US gallons, or three 2.5 L tins if you are shopping in metric.
An open-plan living room with a feature wall
Only one 5.4 m long, 2.7 m high wall is getting a deep colour: 14.58 m². No openings on that wall. Two coats at 11 m²/L need 14.58 × 2 ÷ 11 ≈ 2.65 L before waste, about 2.9 L with 10%. One 5 L tin covers it with touch-up left over.
Walls and ceiling of a small office
A 4 × 3.5 m room, 2.6 m high: perimeter 15 m, walls 39 m². One door (1.9 m²) and two windows (2.8 m²) leave 34.3 m². Ceiling is 14 m², so total surface is 48.3 m². Two coats at 11 m²/L need about 8.8 L before waste, roughly 9.7 L with 10% — three 5 L tins of wall paint plus a separate ceiling tin if the finish differs.
Who this calculator helps
- Architects and interior designers writing finish schedules and early cost plans without waiting on a contractor take-off.
- Contractors and painters pricing labour and materials for quotes.
- Architecture students checking material quantities in studio projects.
- Property managers and landlords budgeting repaint cycles across units.
- Homeowners and DIYers buying the right number of tins before a weekend project.
Practical tips for ordering paint
- Buy all the paint for one colour from the same batch so the shade matches across walls.
- Prime new drywall, stains and drastic colour changes — primer is not the same as a finish coat in the calculator.
- Keep a labelled leftover tin for touch-ups; sunlight and cleaning wear paint unevenly over time.
- Measure the room area first with the square footage calculator, and take dimensions off drawings with the architectural scale converter.
Once you know how much paint to buy, try colours on the real walls: upload a photo and preview finishes with AI Wall Paint.
