Paint calculator: how much paint do I need?
Enter the room dimensions, knock out the doors and windows, pick your coats — and get the gallon count plus a rough paint bill.
Paint quantity & cost
Live estimateYou need
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Estimate only. Buy an extra quart for touch-ups and check the coverage printed on the can.
Most paint jobs fail on quantity, not on technique. Buy too little and you risk a visible batch difference between the first and second trip to the store. Buy too much and you are carrying $80 of paint into the garage, where it will sit until it separates.
How this calculator works
The math is deliberately boring, because boring math is what you can check by hand:
- Wall area = perimeter × ceiling height. A 12 × 14 ft room has a 52 ft perimeter; at 8 ft ceilings that is 416 sq ft of wall.
- Ceiling = length × width, added only if you tick the box. Same room: 168 sq ft.
- Openings are subtracted at industry standard sizes: 21 sq ft per interior door, 15 sq ft per window. Measure your own if they are unusually large.
- Paintable area = walls + ceiling − openings. The example room with one door and two windows gives 416 + 168 − 51 = 533 sq ft.
- Total to cover = paintable area × coats. Two coats: 1,066 sq ft.
- Gallons = total ÷ coverage per gallon, rounded up to the next whole gallon. At 350 sq ft per gallon that is 3.05, so 4 gallons — or 3 gallons plus a quart, which is usually cheaper.
Rounding up to whole gallons is convenient, not optimal. If the result lands just above a whole number, ask for a quart instead of a fourth gallon. Most stores mix quarts in the same color at the same price per ounce.
Coverage per gallon: which number to use
Manufacturers print a coverage figure on the label, and it is almost always measured on a smooth, sealed, uniform surface — a lab panel, not your hallway. Real walls eat paint.
| Surface | Realistic coverage |
|---|---|
| Smooth, previously painted drywall | 375–400 sq ft/gal |
| Typical interior drywall | 350 sq ft/gal |
| Light orange-peel or knockdown texture | 300–325 sq ft/gal |
| Heavy texture, brick or stucco | 200–275 sq ft/gal |
| Bare or patched drywall (first coat) | 250–300 sq ft/gal |
Two more adjustments worth knowing: a deep base color (navy, charcoal, saturated red) usually needs a third coat over a light wall, and spraying uses roughly 15–20% more paint than rolling because of overspray and overlap.
Five mistakes that cost people a second trip
- Measuring floor area instead of wall area. Floor square footage tells you nothing about paint. You need perimeter × height.
- Forgetting the ceiling. A ceiling is roughly 40% of the wall area in a normal room — a big miss if you are pricing the whole job.
- Counting trim as wall area. Baseboards, casings and doors take enamel, not wall paint, and are measured in linear feet.
- Buying primer as if it were paint. Primer covers 200–300 sq ft per gallon and is a separate purchase.
- Ignoring the color change. Going from builder-beige to white is two coats; going from white to forest green is often three.
What the paint will cost
Interior wall paint in the US runs about $25–$70 per gallon in 2026: budget lines sit near $25–$35, mid-tier brands $40–$55, and premium or designer lines $60–$80. Ceiling paint is cheaper, usually $20–$35 per gallon.
| Room | Wall area | Paint (2 coats) | Paint cost at $45/gal |
|---|---|---|---|
| Bathroom, 5 × 8 ft | ~160 sq ft | 1 gal | $45 |
| Bedroom, 10 × 12 ft | ~320 sq ft | 2 gal | $90 |
| Living room, 16 × 20 ft | ~530 sq ft | 3–4 gal | $135–$180 |
| Whole 1,500 sq ft interior | ~3,600 sq ft | 20–22 gal | $900–$990 |
These are paint-only figures. Adding primer, tape, rollers, trays and drop cloths typically adds 15–25% to a DIY job. For labour costs, see how much it costs to paint a house.
Frequently asked questions
How many gallons of paint do I need for a 12x12 room?
A 12x12 room with 8 ft ceilings has about 384 sq ft of wall. After one door and two windows you are covering roughly 333 sq ft per coat — about 666 sq ft for two coats. At 350 sq ft per gallon that is 2 gallons. Add a quart if you are changing color dramatically.
Should I count the ceiling and trim?
Count the ceiling only if you are painting it, and only with flat ceiling paint. Trim, baseboards and doors use a different product (semi-gloss or satin enamel) and are measured in linear feet, not wall area — so leave them out of this calculator.
Does primer count as a coat?
No. Primer is a separate product with its own coverage, usually 200–300 sq ft per gallon on bare or patched drywall. Treat primer as an extra line item on your shopping list rather than a paint coat.
Why does the store say I need more paint than this calculator?
Store calculators often assume one coat and then add a flat 10–15% waste factor on top, or they use the manufacturer’s best-case coverage. Textured walls, deep base colors and sprayers all cut real coverage. If you are spraying, add 15–20% to the result.
How much extra paint should I keep for touch-ups?
Keep one quart of each color, labeled with the room and the date. Paint does not keep forever — an opened can is usually good for two to five years if the lid is sealed and it is stored away from freezing temperatures.