How much paint do I need? Room-by-room paint calculator

Enter the room's length, width and wall height, choose how many coats you plan to apply, and this calculator returns the gallons of paint required including the ceiling-height wall area, minus the space taken by doors and windows.

Room length (ft)

Room width (ft)

Wall height (ft)

Coats 1 coat2 coats3 coats

Doors (count)

A standard door is about 21 sq ft, deducted per coat.

Windows (count)

A standard window is about 15 sq ft, deducted per coat.

Coverage per gallon (sq ft)

Most wall paints state 350-400 sq ft per gallon.

Extra allowance (%)

Paint needed

1.87gallons

Buy (whole gallons) 2 galArea being painted 624 sq ft

Assumes a rectangular room with flat walls. Angled ceilings, stairwells and textured surfaces are not measured, and they typically require 10-20% more paint.

Wall area is the perimeter multiplied by the height: four walls, not the floor. A 12 × 12 ft room with 8 ft ceilings has 64 linear feet of wall and 512 sq ft of wall surface before openings. Deducting a door and two windows removes roughly 50 sq ft per coat, which is not much on a large room but matters in small ones.

The coverage rate is the number that decides the answer, and it is printed on the can. Most interior wall paints claim 350 to 400 sq ft per gallon, but that figure assumes a sealed, previously painted surface and a single coat. New drywall, bare wood and dark colours over light ones all drink more, which is why the second coat is not a mistake and why the extra allowance exists.

How the calculation works

Wall area = 2 × (length + width) × height. Net area = (wall area − doors × 21 − windows × 15) × coats. Gallons = net area × (1 + extra%) ÷ coverage per gallon.

The 21 sq ft door and 15 sq ft window figures are the usual US residential sizes: a 3 ft × 7 ft door, and a 3 ft × 5 ft window.

• Walls are a single flat colour in a rectangle room; a vaulted ceiling, an angled wall or a staircase wall adds area that this does not measure.
• Door and window deductions use standard sizes (21 sq ft and 15 sq ft). Measure unusual openings yourself and adjust the counts.
• The coverage rate defaults to 350 sq ft per gallon, which is conservative for a previously painted wall and realistic for new drywall or a colour change.
• Primer, ceiling paint and trim paint are separate products and are not included.

Sources: Coverage rates: manufacturer-published 350-400 sq ft/gal for interior wall paint; Standard door 3 ft × 7 ft = 21 sq ft; standard window 3 ft × 5 ft = 15 sq ft

Worked examples

Questions

Does one gallon cover a room?

Often, in a small one. A gallon covers 350-400 sq ft of wall at the stated rate; a 12 x 12 ft bedroom has about 312 sq ft per coat after openings, so one coat is under a gallon and two coats needs about two.

Should I deduct doors and windows?

Yes, once per coat. Two standard doors and two windows remove 72 sq ft, which is roughly a fifth of a gallon of wall paint - worth deducting, though not worth obsessing over.

Do I need to add extra for a colour change?

Not if you are using a paint-and-primer product, and yes if you are not. Going from a dark colour to white often needs three coats regardless of what the can claims.

Is ceiling paint included?

No. Ceiling area is length × width — 144 sq ft in a 12 × 12 room — and ceilings usually take a different, flatter product.

Does this work for exterior painting?

The arithmetic does, but exterior surfaces are rougher and often need more coverage: use 250-300 sq ft per gallon rather than 350.

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Written and maintained by Sam Winters. Last updated 2026-11-04. Every figure is computed in your browser; nothing you enter is sent anywhere. Found an error? Tell me — accuracy reports get fixed first.