How Much Paint Do I Need?
Buy too little paint and the second tin is a different batch; too much and it dries out in the shed. Here is how to work out exactly how many litres or gallons - and tins - you need.
By the Calculate My Reno Team / Published
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Open the Paint Calculator
Estimating paint is wall area times coats, divided by coverage. The details that change the answer are the openings you subtract and the surface you are painting.
The formula
litres = (wall area − openings) × coats ÷ coverage per litre
Wall area is the room perimeter × height - 2 × (length + width) × height. Add the ceiling (the floor area) if you are painting it too. The paint calculator does the whole thing, including doors, windows and tin rounding.
Step 1: Wall area
Measure the room perimeter and the wall height. You do not need each wall separately for a normal rectangular room - perimeter × height gives the total wall area in one go.
Step 2: Subtract doors and windows
Knock off the openings you will not paint:
- Door ≈ 1.8 m² (19 sq ft) each
- Window ≈ 1.2 m² (13 sq ft) each
For a typical room this is about half a tin; for a feature wall with big windows it is a lot more.
Step 3: Multiply by coats
Two coats is standard (see how many coats of paint do I need). Multiply the net wall area by your number of coats.
Step 4: Divide by coverage
Use the coverage on the tin - about 350–400 sq ft per gallon or 10–12 m² per litre per coat on a smooth, sealed wall. Drop it 20–30% for textured, porous or bare surfaces.
Worked example
A 12 ft × 12 ft room, 8 ft walls, two coats, one door and one window:
- Wall area: perimeter 48 ft × 8 ft = 384 sq ft.
- Subtract a door (19) + window (13) = 32 → 352 sq ft.
- Two coats: 352 × 2 = 704 sq ft to cover.
- At 350 sq ft/gallon: 704 ÷ 350 = 2.0 → 2 gallons.
Buy whole tins, and keep the leftover
Round up to whole tins (the calculator does this) and keep the remainder for touch-ups - a later mixed batch may not match exactly. For a big job, one larger tin is usually cheaper per litre than several small ones. Painting outside? Coverage and coats differ - see interior vs exterior paint.
Frequently asked questions
How much paint do I need for a 12x12 room?
A 12 ft × 12 ft room with 8 ft walls has about 384 square feet of wall. After subtracting one door and one window, two coats need roughly 2 gallons (about 7.5 litres) at a typical 350 sq ft per gallon. Enter your exact room, openings and coats in the paint calculator for a precise figure.
What is the coverage of paint?
Most wall paints cover about 10–12 m² per litre, or roughly 350–400 sq ft per US gallon, per coat on a smooth, sealed surface. Textured, porous or bare surfaces soak up more and cover less. The exact figure is printed on the tin - use it for an accurate estimate.
Should I subtract doors and windows?
Yes, for accuracy. A standard door is about 1.8 m² (19 sq ft) and a window about 1.2 m² (13 sq ft). For a normal room this trims roughly half a tin; for a wall of windows or a feature wall the saving is significant. The paint calculator subtracts standard sizes for you.