Paint Calculator — How Many Gallons Do I Need?
Calculate exactly how many gallons of paint you need for any room. Accounts for doors, windows, number of coats, and ceilings.
Paint Needed
3 gal
For walls (all rooms)
489 sq ft
Total Wall Area
1 gal
Ceiling Paint
4 gal
Total Gallons
$160
Est. Paint Cost
Buy 3 gallons for walls + 1 gallons for ceilings = 4 total. Always buy 10% extra for touch-ups.
What is Paint Quantity?
Estimating paint is straightforward geometry with one complication that catches almost everyone: coverage figures on the tin are best-case laboratory numbers, achieved on a smooth, sealed, similarly coloured surface by someone not working around a light switch.
Real rooms have texture, corners, cut-in edges and previous colours showing through. The honest planning assumption is that you will get somewhat less coverage than the label promises, and that you will need two coats far more often than one.
Running out halfway through a wall is the specific failure worth avoiding, because a second batch mixed later can differ subtly in shade — and the join will be visible in daylight for as long as you own the room.
The formula — how to calculate Paint Quantity
- Perimeter
- = add the lengths of all walls: 2 × (length + width) for a rectangular room
- Coverage
- = typically 350–400 sq ft per gallon on smooth sealed surfaces; assume less on texture
- Coats
- = two is the realistic default; one only when refreshing the same colour on a sound surface
A standard interior door is roughly 20 sq ft and an average window about 15. Deduct them, but do not deduct small openings — the cutting-in around them consumes the paint you saved.
Step-by-step example
- 01A room 14 ft × 12 ft with 9 ft ceilings, one door and two windows.
- 02Perimeter: 2 × (14 + 12) = 52 ft.
- 03Wall area: 52 × 9 = 468 sq ft.
- 04Deduct openings: 468 − 20 (door) − 30 (two windows) = 418 sq ft.
- 05Two coats: 418 × 2 = 836 sq ft of coverage needed.
- 06At 375 sq ft per gallon: 836 ÷ 375 ≈ 2.23 gallons.
- 07Round up to 3 gallons, or buy 2 gallons plus a quart. The surplus is your touch-up supply, and it is the same batch — which is the whole point.
- 08Adding the ceiling: 14 × 12 = 168 sq ft, two coats = 336 ÷ 375 ≈ 0.9 gallons, so one further gallon of ceiling paint.
Why two coats is the realistic default
One coat works only when you are refreshing the same colour over a sound, previously painted surface. Anything else needs two, and some situations need more.
Colour changes are the obvious case — light over dark especially, where three coats is common without a tinted primer. Bare drywall, patched areas and fresh plaster all absorb the first coat unevenly, which is what primer exists to prevent.
Sheen also matters. Flat and matt paints hide surface imperfections but cover less per coat; gloss and semi-gloss cover better but show every flaw underneath. If you are going light over dark, a tinted primer usually saves a full coat of finish paint and costs less per litre.
Typical coverage by surface
| Surface | Coverage per gallon | Note |
|---|---|---|
| Smooth, previously painted | 375–400 sq ft | Best case |
| Lightly textured drywall | 300–350 sq ft | Texture increases surface area |
| Heavy texture or popcorn | 200–275 sq ft | Can be far lower |
| Bare drywall (unprimed) | 200–250 sq ft | Prime first instead |
| Rough masonry or brick | 150–250 sq ft | Highly variable |
Manufacturer figures assume ideal conditions. Planning at the lower end of a range is the cheaper mistake.
What people forget to buy
- Primer —
- needed on bare surfaces, over patches, and when changing colour dramatically. Tinting it towards the finish colour can eliminate a coat.
- Ceiling paint —
- a different product — flatter, thicker, formulated not to spatter. Do not use wall paint on ceilings if you can avoid it.
- Trim paint —
- usually a harder-wearing enamel in a different sheen, and calculated by linear footage rather than area.
- Touch-up reserve —
- keep the leftover from the same batch. Colour matching later is imperfect even with the formula.
Buy the surplus deliberately
Paint from a later batch can differ subtly even mixed to the same formula, and the difference shows on a wall in daylight. Buying slightly more than you need in one go gives you touch-up paint that will actually match — which matters more than the few pounds saved by cutting it fine.
Key considerations
- Measure the actual perimeter rather than assuming the room is rectangular.
- Plan for two coats unless refreshing the same colour on a sound surface.
- Use the lower end of a coverage range on textured surfaces.
- Prime bare drywall, patches and dramatic colour changes.
- Ceilings need ceiling paint; trim needs trim paint. Budget separately.
- Buy all of one colour in a single trip so the batch matches.
- Homes built before 1978 may contain lead paint — do not sand or scrape without testing first.
Common mistakes to avoid
- Trusting the tin's coverage figure on a textured wall.
- Buying for one coat and discovering the old colour showing through.
- Deducting every small window and door and then running short on cut-in paint.
- Skipping primer over patches, which show as dull spots through the finish.
- Buying exactly enough, leaving no touch-up paint from the matching batch.
- Disturbing pre-1978 paint without testing for lead.
Frequently asked questions
Sources & references
Written and fact-checked by the CalcProLabs Editorial Team. Read our calculation methodology and editorial policy.
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