Paint Calculator
Work out how much paint you need for a room — or a whole house — before you buy. Enter your room sizes, doors and windows, and the calculator estimates your wall area, paint in gallons and litres, primer, ceiling and trim paint, and the cost if you enter a price. It works in feet and inches (US gallons) or metres (UK litres).
How much paint do I need?
Check the can label — coverage varies by brand. Defaults: smooth 350, textured 250, new drywall 200 sq ft/gal.
Rooms
Leave blank if you do not know the price — no cost estimate will be shown.
Your paint estimate
- Total wall area:
- Wall paint needed:
- Buy:
- Primer needed:
- Ceiling paint:
- Trim paint:
- Estimated cost:
Paint amounts include 10% extra for waste and touch-ups, and are rounded up to what you actually buy. Cost is based on the rounded-up amount you buy, at the price you entered.
Shopping list
How the paint calculation works
The calculator follows the standard method used by paint manufacturers:
- Wall area: room perimeter × ceiling height. Perimeter is 2 × (length + width). For each room, add the wall areas together.
- Subtract doors and windows: about 20 sq ft (~1.9 sq m) for each standard door and about 15 sq ft (~1.4 sq m) for each standard window.
- Divide by coverage: net wall area ÷ coverage per gallon (or per litre). Default coverage is 350 sq ft per gallon (~8.6 sq m per litre) on smooth, previously painted walls. Textured walls cover less (~250 sq ft/gal) and new drywall covers less still (~200 sq ft/gal) because it absorbs more paint. You can edit the coverage to match your can.
- Multiply by coats: most repaints need 2 coats; a similar colour over a sound wall may need only 1, and a big colour change may need 3.
- Add 10% for waste: for roller absorption, cutting in, spills and touch-ups.
- Round up: paint is sold in gallons and quarts (or litres), so the buy amount is rounded up to the nearest quart (or nearest litre in metric) — never round down, or you will run out mid-wall.
Primer is one coat: net wall area ÷ coverage, plus 10% waste. Ceiling paint is one coat: ceiling area (length × width) ÷ coverage, plus 10% waste. Trim paint is estimated at about 1 quart (about 1 litre) per room for one coat on baseboards and door and window frames.
Worked example: a 12 × 12 ft room
Room 12 ft long × 12 ft wide, 8 ft ceiling, 1 door, 1 window, 2 coats, smooth walls at 350 sq ft per gallon:
- Perimeter: 2 × (12 + 12) = 48 ft
- Gross wall area: 48 × 8 = 384 sq ft
- Less 1 door (20 sq ft) and 1 window (15 sq ft): 384 − 35 = 349 sq ft net wall area
- One coat: 349 ÷ 350 = 1.00 gallon
- Two coats: 1.00 × 2 = 2.00 gallons (1.99 before rounding)
- Add 10% waste: 1.99 × 1.10 = 2.19 gallons (about 8.3 litres)
- Buy, rounded up to the nearest quart: 2 gallons + 1 quart (2.25 gallons, about 8.5 litres)
For the same room with one coat only: 1.00 × 1.10 = 1.10 gallons — buy 1 gallon + 1 quart. Primer for this room (one coat): 349 ÷ 350 × 1.10 = 1.10 gallons — buy 1 gallon + 1 quart. Ceiling (144 sq ft, one coat): 144 ÷ 350 × 1.10 = 0.45 gallons — buy 2 quarts. Trim: about 1 quart.
Paint needed by room size
Standard rooms with 8 ft ceilings, 1 door and 1 window, smooth walls at 350 sq ft per gallon, including 10% waste and rounded up to the nearest quart. Use the calculator above for your exact sizes.
| Room size | Net wall area | 1 coat — buy | 2 coats — buy |
|---|---|---|---|
| 10 × 10 ft | 285 sq ft | 1 gal (3.8 L) | 2 gal (7.6 L) |
| 12 × 12 ft | 349 sq ft | 1 gal + 1 qt (4.7 L) | 2 gal + 1 qt (8.5 L) |
| 12 × 15 ft | 397 sq ft | 1 gal + 1 qt (4.7 L) | 2 gal + 2 qt (9.5 L) |
| 15 × 20 ft | 525 sq ft | 1 gal + 3 qt (6.6 L) | 3 gal + 2 qt (13.3 L) |
“Buy” is the amount to purchase, rounded up to the nearest quart. Litres are the equivalent of the buy amount (1 gallon = 3.79 litres).
How much primer, trim and ceiling paint?
Primer
Primer is one coat over the net wall area, plus 10% waste. You usually need it on new drywall or bare plaster, bare wood, patched areas, stained or smoke-damaged walls, and when changing from a dark colour to a light one. Previously painted walls in good condition in a similar colour often do not need primer — tick the primer box off in that case.
Ceiling paint
Ceiling area is simply length × width for each room. Divide by coverage for one coat and add 10% waste. Ceilings are usually painted with flat/matte ceiling paint, which is a different product from wall paint, so buy it separately. A 12 × 12 ft ceiling needs about half a gallon — buy 2 quarts.
Trim paint
Trim — baseboards (skirting), door frames and window frames — is usually painted with a semi-gloss or satin finish so it can be wiped clean. Allow about 1 quart (about 1 litre) per room for one coat. If your trim is being painted a very different colour, allow for a second coat.
What to buy — shopping list
Once you have your numbers, a typical shopping list for one room looks like this:
- Wall paint in your chosen colour and finish — the “Buy” amount from the calculator
- Primer, if needed — one coat over the wall area
- Ceiling paint (flat/matte) — if you are painting the ceiling
- Trim paint (semi-gloss/satin) — about 1 quart per room
- Rollers, roller covers, extension pole, angled brush for cutting in, painter’s tape, drop cloths, paint tray and stir sticks
Buy all your wall paint in one batch if you can, and mix cans of the same colour together (“boxing”) so the colour stays consistent from wall to wall. Keep a little paint aside for touch-ups.
Frequently asked questions
How much paint do I need for a 12x12 room?
For a 12x12 ft room with an 8 ft ceiling, one door and one window, two coats of paint on smooth walls need about 2.19 gallons before rounding up — buy 2 gallons + 1 quart (2.25 gallons, about 8.5 litres). One coat needs about 1.10 gallons, so buy 1 gallon + 1 quart.
How many square feet does a gallon of paint cover?
A gallon of paint covers about 350 square feet (about 8.6 square metres per litre) on a smooth, previously painted wall. Textured walls cover less, around 250 square feet per gallon, and new drywall covers around 200 square feet per gallon because it absorbs more paint. Always check the coverage stated on the paint can label, as it varies by brand and product.
Do I need primer?
You usually need primer on new drywall, bare wood, patched areas, stained walls, or when making a big colour change (for example dark to light). Primer is applied in one coat: divide your net wall area by the coverage to work out how much you need, then add about 10% for waste. Previously painted walls in good condition in a similar colour often do not need primer.
How much paint do I need for the ceiling and trim?
For the ceiling, divide the ceiling area (room length x width) by the paint coverage for one coat, then add about 10% for waste. For trim — baseboards, door frames and window frames — allow about 1 quart (about 1 litre) per room for one coat. Trim paint is usually a different finish from wall paint, so buy it separately.
How do I calculate paint when a room has doors and windows?
Work out the gross wall area (room perimeter x ceiling height), then subtract about 20 square feet (about 1.9 square metres) for each standard door and about 15 square feet (about 1.4 square metres) for each standard window. Divide what is left by the paint coverage, multiply by the number of coats, add about 10% for waste, and round up to the nearest quart.
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Disclaimer: Results are planning estimates, not a guarantee. Actual paint needed varies with surface condition, application method, colour change and the product used — always check the coverage stated on the paint can label and buy a little extra for touch-ups. In older homes, paint may contain lead: do not sand or scrape it without professional advice, and get professional help if you suspect lead paint.