Interior

Paint Calculator: How Much Paint Do I Need?

Enter your room size and get the exact number of gallons to buy — walls, ceiling, and trim, adjusted for coats and openings.

Your measurements

Paint to buy

3gallons

2.1 gal of actual coverage, rounded up to whole cans

Total wall area
416 sq ft
Less doors & windows1 doors, 2 windows
−50 sq ft
Paintable wall area
366 sq ft
Wall paint2 coats
3 gal
Estimated paint costBuilder-grade to premium, per gallon
$90 – $225
  • Buying one 5-gallon bucket is usually cheaper than 5 single gallons, and guarantees a consistent tint batch across the whole room.
  • Going from a dark colour to a light one? Add a coat of tinted primer rather than a third coat of finish paint — it is cheaper and blocks better.
  • Bare drywall, patched areas and raw wood all drink the first coat. Prime them or expect coverage nearer 250 sq ft per gallon.

How this is calculated

  • Wall area is the room perimeter — 2 × (length + width) — multiplied by ceiling height.
  • Each door is deducted at 20 sq ft and each window at 15 sq ft, which matches a standard 3'×6'8" door and a common double-hung window.
  • Paintable area is multiplied by the number of coats, then divided by the coverage rate printed on the can (350 sq ft per gallon by default) and rounded up to whole cans.
  • Ceilings are calculated separately because they are usually a different product — flat ceiling paint rather than the wall sheen — and cannot share a can.

Quantities are rounded up, because materials are sold in whole units and a second trip costs more than a spare box. Cost figures are typical US ranges and are not a quote — always price against your own supplier.

Frequently asked questions

How much paint do I need for a 12x12 room?

A 12×12 room with 8 ft ceilings has 384 sq ft of wall. After deducting one door and two windows you are painting about 334 sq ft. At two coats and 350 sq ft per gallon that is 1.9 gallons — so buy 2 gallons for the walls, plus 1 more if you are doing the ceiling.

Does one gallon really cover 400 square feet?

On a smooth, previously painted, primed wall — yes, roughly. On new drywall, textured or porous surfaces, or when rolling a light colour over a dark one, real-world coverage drops to 250–300 sq ft per gallon. Set the coverage field to match your surface rather than the marketing number.

Should I subtract doors and windows?

For a small room, many painters do not bother — the extra paint becomes useful slack for touch-ups. For a large room or a whole house the deductions add up to real money, so this calculator subtracts them and you can set the counts to zero if you would rather carry the buffer.

How much paint do I need for trim and doors?

One gallon of trim paint covers roughly 600 linear feet of standard baseboard, or about six average doors with two coats. For a single room, a quart is usually enough for the baseboard alone.

Know the quantities. Now see the room.

You have the numbers for the job. Upload a photo of the room and see what it looks like finished, in any style, before you buy a single box.

Try the AI room designer

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