Free calculator
Paint Calculator: How Many Gallons Do You Need?
Enter a room's dimensions — doors and windows are subtracted with typical allowances — or enter exterior wall area directly. Pick one or two coats and get your gallon count, always rounded up, with a per-coat breakdown. The math runs entirely in your browser.
By HowMuchBuild Editorial TeamCalculation reviewedUpdated October 2026
feet
feet
feet — 8 ft is standard
≈ 21 sq ft each (typical)
≈ 15 sq ft each (typical)
Coats
Two coats is the usual standard for coverage and durability.
Most cans print 350–400 sq ft per gallon; 350 is the safer default.
Estimated cost
What will it cost?
Enter your local store prices — prices vary by region.
Paint
3 gal
—
Estimate — prices vary by store and region. Check your local store for current pricing.
You need
3gallons
762 sq ft total (381 sq ft × 2 coats) at 350 sq ft/gal — always rounded up
| Coat | Area | Paint (unrounded) |
|---|---|---|
| Coat 1 | 381 sq ft | 1.09 gal |
| Coat 2 | 381 sq ft | 1.09 gal |
| Total to buy | 762 sq ft | 3 gal (rounded up) |
How we calculated it
- Wall area = 2 × (12 + 15) × 8 = 432 sq ft
- Openings (typical allowances) = 1 × 21 + 2 × 15 = 51 sq ft
- Paintable = 432 − 51 = 381 sq ft
- Total for 2 coats = 381 × 2 = 762 sq ft
- Per coat ≈ 381 ÷ 350 = 1.09 gal
- Gallons to buy = ⌈762 ÷ 350⌉ = 3 (always rounded up)
Estimates for planning. Coverage varies with surface texture, color change, and application — a dark-to-light change can take more paint. Always check the coverage printed on your specific can.
Worked example: 12×15 ft room, 8 ft ceiling, 2 coats
Full arithmetic for a standard bedroom repaint:
- Wall area: 2 × (12 + 15) × 8 = 432 sq ft
- Openings (typical): 1 × 21 + 2 × 15 = 51 sq ft
- Paintable: 432 − 51 = 381 sq ft
- Per coat: 381 ÷ 350 = 1.09 gal
- Two coats: 381 × 2 = 762; 762 ÷ 350 = 2.18 → round up = 3 gallons
Gallons chart at 350 sq ft per gallon
Buy quantities, always rounded up:
| Paintable area | 1 coat | 2 coats |
|---|---|---|
| 100 sq ft | 1 gal | 1 gal |
| 250 sq ft | 1 gal | 2 gal |
| 381 sq ft | 2 gal | 3 gal |
| 500 sq ft | 2 gal | 3 gal |
| 750 sq ft | 3 gal | 5 gal |
| 1,000 sq ft | 3 gal | 6 gal |
Quick answers for common rooms (2 coats, 8 ft ceilings)
Assumes 1 door and 2 windows subtracted:
| Room | Paintable | Gallons (2 coats) |
|---|---|---|
| 10×10 ft | 269 sq ft | 2 |
| 12×12 ft | 333 sq ft | 2 |
| 12×15 ft | 381 sq ft | 3 |
| 15×20 ft, 9 ft ceiling | 579 sq ft | 4 |
Do you need primer?
General guidance, not a hard rule: primer earns its keep on bare drywall, patched or repaired spots, stained surfaces, and dramatic color changes. It seals porous areas so the topcoat lays down evenly — without it, bare drywall drinks the first coat and you effectively pay for a third coat of finish paint anyway. Previously painted walls in good condition getting a similar color usually skip primer fine. When in doubt, read the product label: paint-and- primer combos exist, but they are not a substitute on truly bare surfaces.
The formulas
Room mode
Wall area = 2 × (Length + Width) × Height, all in feet. Openings = doors × 21 + windows × 15 (typical allowances). Paintable = wall area − openings.
Exterior mode
Paintable = sum of wall areas, measured and added directly. Subtract large openings, or let the round-up absorb them.
Gallons
Gallons = ⌈ (paintable × coats) ÷ coverage ⌉ — always round up, never down. Default coverage 350 sq ft/gal, the conservative end of the 350–400 range printed on most cans.
Frequently asked questions
How much paint do I need for a 12×12 room?
Walls are 2 × (12 + 12) × 8 = 384 sq ft; minus one door (~21) and two windows (~30) leaves 333 paintable sq ft. That's 1 gallon per coat at 350 sq ft/gal — 2 gallons for the standard two coats.
How much does one gallon of paint cover?
Most interior paints print 350–400 sq ft per gallon on the can. We use 350 as the default because it's the conservative end — textured, porous, or unprimed surfaces land closer to 350, smooth primed surfaces closer to 400.
Should I subtract doors and windows?
Yes. A typical interior door is about 3×7 ft (21 sq ft) and a typical window about 15 sq ft — those are the typical allowances the calculator uses. Trim and casing around them take paint too, which is one reason the result rounds up.
One coat or two?
Two coats is the usual standard for even coverage, true color, and durability. One coat often works over a same-color repaint in good condition, but going dark-to-light or light-to-dark almost always needs two.
Do I need primer?
As general guidance: prime bare drywall, repaired/patchy spots, and dramatic color changes. Primer seals the surface so the topcoat covers evenly — skipping it on bare drywall often means buying a third coat of paint instead. Always check the product label for your situation.
Does ceiling paint use the same math?
The same math, yes — ceiling area is just length × width. But buy actual ceiling paint for it: it's formulated flatter and thicker to hide imperfections, and wall paint drips more overhead.
Should I buy extra paint?
The rounding-up already builds in a margin, which usually leaves enough for touch-ups. If the room has heavy texture or you're changing color drastically, consider one extra quart for the future — cans can be matched later, but batch-to-batch tint can vary slightly.
Before you buy
Coverage on the can assumes ideal conditions, so the 350 default already protects you — don't "save" by picking the 400 end unless the surface is smooth and primed. Buy all gallons for a room at once (tint can vary slightly between batches), and keep a little of the rounded-up extra for touch-ups: a stored, labeled can beats a color-match guess two years later.
Keep estimating