How Much Do I Need?

Paint Calculator

How much paint do I need?

Enter the size of the room and the coverage printed on your paint can. The calculator works out the wall area, takes off doors and windows, multiplies by the number of coats and adds a margin for waste, then rounds up to cans you can actually buy.

Units
Room size

Floor to ceiling.

Doors and windows

A common allowance is 20 ft² (about 1.9 m²) per door.

A common allowance is 15 ft² (about 1.4 m²) per window.

Paint

Printed on the can. 350 ft² per gallon (8.6 m² per litre) is a cautious figure for interior wall paint.

Covers paint left in trays and rollers, spills and touch-ups.

Cost (optional)

Add the price to see an estimated total.

Results update as you type.

Your estimate

Paint to buy

2 gallons

You need 1.90 gal (7.19 L) including 10% extra.

Estimated amount to buy

US cans
2 gallons
Metric tins
1 × 5 L + 1 × 2.5 L (7.5 L)
Needed with 10% extra
1.90 gal / 7.19 L

Calculated quantity (no extra)

Wall area
352 ft²
Doors and windows
− 50 ft²
Area to paint
302 ft²
Paint for 2 coats
1.73 gal / 6.53 L
How this was calculated
  1. Wall area = 2 × (12 ft + 10 ft) × 8 ft = 352 ft²
  2. Doors and windows = 1 × 20 ft² + 2 × 15 ft² = 50 ft²
  3. Area to paint = 352 ft² − 50 ft² = 302 ft²
  4. Paint = 302 ft² × 2 coats ÷ 350 ft² per gallon = 1.73 gal
  5. With 10% extra = 1.73 gal × 1.1 = 1.90 gal
  6. 1.90 gal = 7.6 quarts, rounded up to 8 quarts = 2 gallons
  • Coverage depends on the surface and the paint. Bare plaster or drywall, rough textures and big colour changes can need more paint or a primer coat.

How the paint calculation works

Paint is sold by volume but used by area, so the calculator first works out the area you are painting and then divides by how far the paint spreads:

  1. Wall area = perimeter × wall height = 2 × (length + width) × height.
  2. Subtract openings: number of doors × area per door, plus number of windows × area per window.
  3. Add the ceiling (length × width) if you are painting it.
  4. Paint needed = area to paint × number of coats ÷ coverage per gallon (or per litre).
  5. Add extra for waste and touch-ups: paint needed × (1 + extra ÷ 100).

One US gallon is 3.785 litres. Coverage entered in square feet per gallon is converted exactly, so switching units gives the same answer for the same room.

Rounding rules

  • US cans: the amount including extra is rounded up to the next whole quart (¼ gallon) and shown as gallons plus quarts.
  • Metric tins: the amount including extra is rounded up to the smallest combination of 1, 2.5, 5 and 10 litre tins that covers it. When two combinations hold the same amount, the one with fewer tins is shown.
  • The working shows figures rounded for reading, but every step uses the full, unrounded numbers.

What changes how much paint you need

The result is an estimate. The biggest variable is the coverage figure, so use the one printed on the paint you are buying. Interior wall paints commonly list somewhere around 350–400 ft² per gallon (roughly 8.5–10 m² per litre) per coat, and some list more.

  • Porous or new surfaces such as fresh plaster, bare drywall and unpainted wood soak up paint. A primer or a thinned mist coat is often recommended first.
  • Rough textures like textured ceilings, brick or rough render have more surface than their flat measurements suggest.
  • Big colour changes, especially light over dark, may need an extra coat.
  • Spraying usually uses more paint than rolling because of overspray.

Trim, doors and radiators are not included. If you are painting them in a different finish, estimate them separately.

Worked example

A 12 ft × 10 ft room with 8 ft walls has 2 × (12 + 10) × 8 = 352 ft² of wall. Taking off one door (20 ft²) and two windows (2 × 15 ft²) leaves 302 ft². Two coats at 350 ft² per gallon need 302 × 2 ÷ 350 = 1.73 gallons. With 10% extra that becomes 1.90 gallons, which rounds up to 8 quarts: 2 gallons.