How to use this paint calculator
- Choose Whole room and enter the room’s length, width and wall height, or choose Individual walls to enter each wall separately, such as a single feature wall.
- Tick Include the ceiling if you are painting it with the same paint.
- Enter the number of doors and windows. The calculator subtracts a typical size for each, which you can change.
- Set the number of coats and the coverage printed on your paint can or tin. Add primer if the surface needs it.
- Read the result: total paint, plus a suggested mix of cans or tins so you don’t overbuy.
Paint calculation formula
Paintable area = Wall area − (Doors × door area) − (Windows × window area) [+ Ceiling]
Paint needed = Paintable area × Coats ÷ Coverage
Worked example: a 12 × 14 ft bedroom
- Wall area: 2 × (12 + 14) × 8 ft = 416 sq ft
- Subtract 1 door (≈ 20.5 sq ft) and 2 windows (≈ 15.1 sq ft each) = about 50.6 sq ft
- Paintable area: 416 − 50.6 = 365.4 sq ft
- Two coats at 350 sq ft per gallon: 365.4 × 2 ÷ 350 = 2.09 gallons
- Buy 2 gallons + 1 quart. Adding the 168 sq ft ceiling brings the total to about 3.05 gallons (3 gallons + 1 quart).
In metric: a 4 × 3.5 m room with 2.4 m walls, one door and one window has about 32.7 m² to paint. Two coats at 10 m² per litre need 6.54 litres, so a 5 L tin plus a 2.5 L tin covers it.
How much paint for common room sizes
Walls only, 8 ft (2.44 m) ceilings, one door and one window, two coats at 350 sq ft per gallon.
| Room | Wall area | Paint needed | Buy | Ceiling (2 coats) |
|---|---|---|---|---|
| 10 × 10 ft | 284 sq ft | 1.63 gal | 2 gal | +1 gal |
| 10 × 12 ft | 316 sq ft | 1.81 gal | 2 gal | +1 gal |
| 12 × 12 ft | 348 sq ft | 1.99 gal | 2 gal | +1 gal |
| 12 × 14 ft | 380 sq ft | 2.17 gal | 2 gal + 1 qt | +1 gal |
| 14 × 16 ft | 444 sq ft | 2.54 gal | 3 gal | +1 gal + 2 qt |
| 16 × 20 ft | 540 sq ft | 3.09 gal | 3 gal + 1 qt | +2 gal |
Typical paint coverage
| Surface | Typical coverage per coat |
|---|---|
| Smooth, previously painted walls | 350–400 sq ft/gal (≈ 8.6–9.8 m²/L) |
| New drywall or plaster (primed) | Around the label figure. Unprimed surfaces soak up more. |
| Rough or textured (stucco, brick, heavy texture) | About 200–300 sq ft/gal (≈ 4.9–7.4 m²/L) |
These are typical figures only. Coverage varies by brand, product and colour. Many metric emulsion tins list roughly 10–13 m² per litre, so always use the number printed on your paint.
Tips for buying the right amount
- Use the label’s coverage figure. It is the single biggest factor. Premium paints often cover better, so you may need fewer coats.
- Plan for more on porous or textured surfaces. Fresh plaster, bare drywall and rough walls absorb paint. Prime them first. Hanging new boards before you paint? See the drywall calculator.
- Colour changes need more. Going from a dark colour to a light one often needs primer and an extra coat. A tinted primer can help.
- Buy all your paint at once. Colours can vary slightly between batches. For a large job, mix cans together in a bucket (called “boxing”) for an even colour.
- Keep a little for touch-ups. Rounding up to whole cans usually leaves you enough. Seal it well and label it with the room.
- Trim and doors are separate. Trim, skirting boards and doors usually use a different finish, so estimate them separately.
Frequently asked questions
How much paint do I need for a 12x12 room?
A 12 × 12 ft room with 8 ft walls, one door and one window has about 348 sq ft of wall. Two coats at 350 sq ft per gallon need just under 2 gallons, so buy 2 gallons. Painting the ceiling as well (144 sq ft, two coats) adds about 0.8 gallons, so another gallon.
How many square feet does a gallon of paint cover?
Most interior paints list about 350–400 sq ft per US gallon for one coat on a smooth, primed surface. Rough or textured surfaces such as stucco, brick or heavy texture can drop to about 200–300 sq ft per gallon. Always check the label of the paint you buy.
How many coats of paint do I need?
Two coats is the usual standard and gives even colour and durability. One coat can be enough when refreshing a wall in the same colour. Big colour changes, especially light over dark, may need a primer plus two coats.
Do I need primer?
Use primer on new drywall or plaster, bare wood, patched areas, stains, glossy surfaces, and when making a big colour change. “Paint and primer in one” products can work on previously painted walls, but many manufacturers still recommend a dedicated primer on bare surfaces.
Should I subtract doors and windows?
Yes for an accurate estimate. A standard door is roughly 20 sq ft (about 1.9 m²) and a typical window about 15 sq ft (1.4 m²). For a quick estimate you can skip them, and the extra paint becomes your touch-up supply.
How do I convert paint coverage to litres?
One US gallon is 3.785 litres. A coverage of 350 sq ft per gallon equals about 8.6 m² per litre, and 400 sq ft per gallon is about 9.8 m² per litre.
Last reviewed November 5, 2026.