Paint Calculator

Paint is estimated by area and bought by the gallon: measure the walls, take out the doors and windows, multiply by your coats and divide by the coverage rate. Work out a whole room or a single wall in gallons and litres, get the paintable area, the whole cans to buy and the cost — with a standard 350 sq ft per gallon built in so you order enough.

$
Paint needed — gallons
Litres — L
Paintable area — sq ft
Cans to buy — whole gallons
Estimated cost — at price / gallon

How to Calculate Paint

Pick the mode, measure, and read the gallons.

1

Choose the mode

Use Room to enter length, width and wall height — it paints the perimeter walls for you. Use Wall area when you already know the square footage of a single wall or feature and just want the paint for it.

2

Enter the details

Set the number of coats, then count the doors and windows so they can be subtracted — about 21 sq ft per door and 15 sq ft per window. Adjust the coverage rate to match your paint can if it differs from the 350 sq ft default.

3

Read gallons, cans and cost

You get the paint in gallons and litres, the paintable area, and the whole cans to buy after rounding up. Add a price per gallon and it shows the cost, so you know exactly what to pick up at the store.

Frequently Asked Questions

How much paint does a gallon cover?
A gallon of interior wall paint covers roughly 350 square feet in a single coat on primed, smooth drywall, which is the default this calculator uses. Coverage varies with the surface and the paint: a flat finish on new drywall may stretch toward 400 square feet, while a rough, porous or dark wall that needs heavier application can drop to 250 to 300 square feet per gallon. The number printed on the can is your best guide, so if your paint lists a different spread rate, type it into the coverage field and the estimate updates. Remember that this figure is per coat — two coats of the same paint cover half the wall area per gallon, which is why the coat count matters as much as the square footage.
How many coats of paint do I need?
Two coats is the standard for most repaints and the default here, because a single coat rarely gives an even colour and full hide, especially over a different shade. Use one coat only when you are refreshing the exact same colour on a wall in good condition, or applying a tinted primer plus a single topcoat as a two-step system. Go to three coats when you are covering a dark colour with a light one, painting over a bold accent wall, or using a thin, low-hide paint where the wall keeps showing through. Set the coat count to match your situation and the gallons scale directly — three coats needs fifty percent more paint than two.
Do I subtract doors and windows?
Yes — doors and windows are surfaces you do not cover with wall paint, so subtracting them keeps you from buying more than you need. This calculator removes about 21 square feet for each standard door and about 15 square feet for each average window, then paints only what is left. Those are typical sizes: a standard interior door is roughly 3 by 7 feet and an average window around 3 by 5 feet, so adjust the counts if your openings are unusually large or small. For a small room with one door and one window the difference is minor, but in a space with a wall of glass or several large openings, skipping this step can push you into buying an extra gallon you will never open.
How much paint for a 12 by 12 room?
A 12 by 12 foot room with 8 foot ceilings has a wall perimeter of 48 feet, so the wall area is 48 × 8 = 384 square feet before openings. Take out one door at about 21 square feet and one window at about 15 square feet and you are left with roughly 348 square feet of paintable wall. At 350 square feet per gallon that is almost exactly one gallon for a single coat, so two coats needs close to two gallons — round up to two cans. Enter your own length, width and height in Room mode and the calculator does this for you, including any extra doors and windows and your chosen number of coats.
Do I need primer as well as paint?
Primer is a separate product from your topcoat, so this calculator estimates paint only — add primer as its own line when you shop. You need it when you are painting bare drywall, patched or repaired areas, glossy or previously oil-based surfaces, or making a big colour change such as light over dark. Primer roughly follows the same coverage as wall paint, around 200 to 400 square feet per gallon, so you can reuse this tool with the coverage rate from the primer can and one coat to size it. If your walls are already painted, in good condition and you are staying close to the current colour, a quality self-priming paint in two coats will usually skip the separate primer step.

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