You are standing in the paint aisle with a colour you love and no idea how many cans to buy. Buy too little and you make a second trip. Buy too much and you pay for paint that sits in the garage for years.
The good news: you can work out how much paint you need with a tape measure, a calculator and a simple formula.
Quick Answer
Paint needed = (Paintable wall area × Number of coats) ÷ Coverage per gallon (or litre)
Most paints cover about 350 to 400 square feet per gallon (roughly 10 to 12 square metres per litre). A typical 12 ft × 14 ft room with an 8 ft ceiling needs about 2 gallons for two coats on the walls.
Step-by-Step: How to Calculate Paint for a Room
- Measure the room. Note the length, width and ceiling height.
- Calculate the wall area. (Length + Width) × 2 × Height.
- Subtract doors and windows. A standard door is about 21 sq ft (3 ft × 7 ft). A standard window is about 15 sq ft (3 ft × 5 ft).
- Multiply by the number of coats. Two coats is typical.
- Divide by the coverage on the paint can. The label tells you how far one gallon or litre goes.
- Round up, and consider adding about 10% for touch-ups.
Worked Example (Feet and Gallons)
A room is 12 ft long, 14 ft wide and 8 ft high, with one door and one window.
| Step | Calculation | Result |
|---|---|---|
| Perimeter | (12 + 14) × 2 | 52 ft |
| Wall area | 52 × 8 | 416 sq ft |
| Subtract door and window | 416 − 21 − 15 | 380 sq ft |
| Two coats | 380 × 2 | 760 sq ft |
| Paint needed | 760 ÷ 350 | 2.2 gallons |
If your paint covers 400 sq ft per gallon, the answer is 760 ÷ 400 = 1.9 gallons. In practice, buying 2 gallons plus a quart (or 3 gallons if you prefer to have spare) would cover it.
Worked Example (Metres and Litres)
A room is 4 m long, 3.5 m wide and 2.5 m high, with one door (about 1.9 sq m) and one window (about 1.5 sq m).
- Perimeter: (4 + 3.5) × 2 = 15 m
- Wall area: 15 × 2.5 = 37.5 sq m
- Minus door and window: 37.5 − 1.9 − 1.5 = 34.1 sq m
- Two coats: 34.1 × 2 = 68.2 sq m
- At 10 sq m per litre: 68.2 ÷ 10 = 6.8 litres. At 12 sq m per litre, it is about 5.7 litres.
So you would buy about 6 to 7 litres.
Quick Reference Chart
This chart assumes an 8 ft ceiling, one door, one window, two coats and coverage of 350 sq ft per gallon.
| Room size | Approx. gallons for walls |
|---|---|
| 10 ft × 10 ft | 1.6 |
| 12 ft × 12 ft | 2.0 |
| 12 ft × 14 ft | 2.2 |
| 14 ft × 16 ft | 2.5 |
| 16 ft × 20 ft | 3.1 |
What About the Ceiling and Trim?
- Ceiling: Multiply length × width, then by the number of coats. For a 12 ft × 14 ft ceiling with two coats: 168 × 2 = 336 sq ft, or about 1 gallon.
- Trim and baseboards: A quart is often enough for an average room.
- Doors and window frames: Plan for a little extra, since many trims use a different paint.
Factors That Change How Much Paint You Need
- Surface type. Rough, textured or porous walls soak up more paint.
- Colour change. Going from dark to light (or the reverse) may require extra coats.
- Primer. New drywall, stains or bare wood usually need primer first.
- Application method. Sprayers often use more paint than rollers.
- Paint quality. Thicker, higher-quality paints can cover better, but always check the label.
Tips to Avoid Wasting Paint (and Money)
- Read the coverage on the can. It varies by product.
- Buy all the paint at once, and mix cans together in a bigger container (known as “boxing”) so the colour stays consistent.
- Keep a little for touch-ups, and store it sealed with the colour name written on the lid.
- Compare prices per gallon, not just per can, since sizes differ.
- Return policies: Some stores accept unopened cans, though tinted paint is often non-returnable. Ask before you buy.
Common Mistakes to Avoid
- Forgetting the second coat in your calculation.
- Not subtracting doors and windows, which leads to over-buying.
- Skipping the 10% extra for touch-ups and uneven surfaces.
- Mixing units, such as feet for the room and litres for the coverage.
- Assuming every paint covers the same amount.
Frequently Asked Questions
How much paint do I need for a 12 x 12 room? About 2 gallons for two coats on the walls, with an 8 ft ceiling and normal doors and windows.
How many square feet does a gallon of paint cover? Typically 350 to 400 square feet, but check the label.
How many coats of paint do I need? Two coats are standard. One may be enough when repainting a similar colour with good-quality paint.
Do I paint the ceiling the same as the walls? Not necessarily. Ceilings often use a flat finish and a separate can.
How much paint do I need for a bedroom? It depends on the size. A typical bedroom of 12 ft × 12 ft needs about 2 gallons for the walls.
Final Thoughts
To calculate paint for a room, find the wall area, subtract doors and windows, multiply by the number of coats, then divide by the coverage on the can. Round up a little, and you are ready to buy with confidence.
Disclaimer: This article is for general information only. Actual coverage depends on the product, surface and application method. Always follow the paint manufacturer’s instructions and safety guidance.