How much paint do I need for a room? Litres, gallons and tins

Guide · Updated 2026-11-10

Measure a room, take off doors and windows, choose coats and coverage, and see how many litres or US gallons of paint, and which tins, to buy.

Open the Paint calculator Free, in your browser, nothing uploaded.

Paint is sold by volume but walls are measured by area, which is why people come home one tin short. The paint calculator closes the gap. You give it the size of a room or each wall, the doors and windows to skip and the number of coats, and it tells you how many litres or US gallons to buy and which mix of tins holds that amount with the least waste. It suits anyone redecorating a room, from a first flat to a landlord between tenants. If a room is getting new boards first, the drywall calculator counts them, and for a feature wall in paper, the wallpaper calculator counts the rolls.

What it is

The idea is one chain of multiplication and division.

First, the wall area. For a box-shaped room:

• Wall area = 2 × (L + W) × H, where L is the room’s length, W its width and H the wall height.

The 2 × (L + W) part is the distance all the way round the room, called the perimeter. Times the height, it gives the area of all four walls.

Next, take off the openings. Each door and each window is a patch you will not paint, so the tool subtracts the number of doors times the area of one door, and the same for windows.

Then add coats and divide by coverage:

• Paint = net area × coats ÷ coverage

Coverage is how much wall one litre (or gallon) covers in one coat. It is printed on the tin, often around 10 to 12 m² per litre.

Last comes the shopping step. The tool looks at the tin sizes (5 L, 2.5 L and 1 L in metric; gallons and quarts in imperial) and picks the smallest number of tins that holds enough. If two choices use the same number of tins, it picks the one with less paint left over.

How to use it

1. Choose Units: metric (metres and litres) or imperial (feet and US gallons). Changing it loads a sample room in those units.
2. Under Measure by, pick Whole room and type length, width and wall height, or pick Wall by wall to type each wall. Add a wall adds a row, and Remove drops one.
3. Type how many doors and windows there are, and the area of one of each. Typical sizes are filled in.
4. Set Coats and Coverage per coat from your tin’s label.
5. Tick Paint the ceiling too (whole room only) or Add one coat of primer if you need them. The primer gets its own coverage box.
6. Read the paint total, the tins to buy and the lines that show each step. Reset to the example starts over.

A worked example

Priya is painting a bedroom 3.6 m long and 3.2 m wide, with walls 2.5 m high. It has one door (1.9 m²) and two windows (1.5 m² each). She wants two coats of a paint that covers 12 m² per litre, and the ceiling in the same paint. The walls are freshly plastered, so she adds primer that covers 8 m² per litre.

• Perimeter: 2 × (3.6 m + 3.2 m) = 13.6 m
• Wall area: 13.6 m × 2.5 m = 34.0 m²
• Openings: 1.9 m² + 2 × 1.5 m² = 4.9 m²
• Walls to paint: 34.0 − 4.9 = 29.1 m²
• Ceiling: 3.6 m × 3.2 m = 11.52 m², so 40.62 m² in all
• Two coats: 40.62 × 2 = 81.24 m²
• Paint: 81.24 ÷ 12 = 6.77 L

Two tins are the fewest that hold 6.77 L. Both 2 × 5 L (10 L) and 5 L + 2.5 L (7.5 L) work, and the second leaves less over, so the tool says 1 × 5 L + 1 × 2.5 L.

The primer is one coat over the same 40.62 m²: 40.62 ÷ 8 = 5.08 L. That just tips over 5 L, so the tool suggests 1 × 5 L + 1 × 1 L.

If Priya left the ceiling white and only painted the walls, she would need 29.1 × 2 ÷ 12 = 4.85 L, which fits in a single 5 L tin.

Where it is used

• Home decorating, to buy the right tins in one trip and keep the batch colour the same.
• Landlords and property managers, who repaint the same rooms between tenants and can keep the numbers for next time.
• Painters’ quotes, where the litres feed straight into a material cost.
• Fences, sheds and garage walls: use Wall by wall with each panel as a “wall”, and set coverage from the exterior paint’s tin.

Common mistakes

• Using the ceiling paint for walls, or the other way round. The ceiling box uses the same coats and coverage as the walls. If the ceiling is a different paint, run it again as its own job (one “wall” of length × width).
• Typing the coverage for two coats. Some tins quote coverage per coat, others for the whole job. The tool wants one coat; check which the label means.
• Ignoring rough or new surfaces. Bare plaster, brick and textured walls soak up more. Lower the coverage figure or add the primer coat.
• Taking off a huge glass door as a small door. Patio doors and bay windows are far bigger than the defaults. Measure them and type their real area.
• Leaving out alcoves and chimney breasts. A box-shaped room formula misses extra wall. Switch to Wall by wall and add each face.

Questions people ask

Why does it suggest a 2.5 L tin and not just two 5 L tins? Both are two tins, so they cost about the same trip, but the smaller mix leaves 0.73 L over instead of 3.23 L. Less waste and less to store.

Does it work for US gallons? Yes. Pick imperial and the answer comes in US gallons, with tins as gallons and quarts (a quart is a quarter gallon).

How high is the wall if there is a picture rail? Measure only the part you are painting in this colour, from the skirting to the rail, and run the area above it separately.