How Much Paint Do I Need? A Straight Answer

There are two ways this goes wrong. You buy too little, run out with one wall left, and come back to a can from a different batch that dries a shade off. Or you buy too much and a $60 gallon sits in the garage until it separates.

The maths takes about thirty seconds. Here it is.

The formula

  perimeter × wall height
− doors and windows
× coats
÷ coverage per gallon
= gallons to buy

Four numbers, one division. Walking through each:

Perimeter. Add up the length of every wall. For a rectangular room that's 2 × (length + width). A 12 × 14 room has a 52-foot perimeter.

Wall height. Floor to ceiling. Most modern rooms are 8 feet; older houses run 9 or 10. Measure it rather than assuming — a foot of extra height on that 52-foot perimeter is another 52 square feet, which is a meaningful chunk of a gallon.

Subtract the openings. A standard door is about 21 square feet, a standard window about 15. In a room with one door and two windows that's roughly 50 square feet you don't have to paint.

Coats. Two, unless you have a reason. One coat almost never looks finished, especially in a colour with any depth to it.

Coverage. Printed on the can, usually 350–400 square feet per gallon.

Try it

What the coverage number doesn't tell you

The figure on the can assumes a smooth, primed, mid-coloured wall and someone who rolls efficiently. Three things quietly break that assumption:

Bare or patched drywall drinks paint. New drywall and joint compound are porous — your first coat mostly soaks in. Prime it first, or expect the first coat to cover noticeably less than the can promises.

Texture adds surface area. A knockdown or orange-peel wall has more actual square footage than its flat dimensions suggest. Knock 10–15% off the coverage figure for a textured wall.

Big colour swings need more. Going from navy to cream is a three-coat job however good the paint is. "One-coat coverage" claims are measured over a similar shade, not over the opposite end of the colour wheel.

Buy it all in one trip

Paint is mixed in batches, and two cans of the same colour mixed weeks apart can differ enough to see in daylight. If you need three gallons, buy three gallons today. Better still, buy them mixed as a single order so they come from one batch.

This is also why you round up rather than cutting it fine. Running out three-quarters of the way through a wall is the one mistake that shows in the finished job.

A worked example

A 12 by 14 foot bedroom with 8 foot walls, one door and two windows, two coats.

Wall area first — that's the perimeter times the height:

2 × (12 + 14) = 52 feet of perimeter 52 × 8 = 416 square feet of wall

Take out the openings. A standard door is roughly 21 square feet and an average window about 15:

416 − 21 − (2 × 15) = 365 square feet to paint

Multiply by coats and divide by coverage:

365 × 2 = 730 square feet of coverage 730 ÷ 350 = 2.09 gallons

So 3 gallons, because you round up and paint is sold in whole cans. Those are exactly the figures the calculator above uses, so you can check it against your own room.

How many coats will it actually take?

Two is the honest default, and most of the time it's right. The exceptions are worth knowing before you buy:

• One coat — repainting the same colour, on a sound surface, with a good paint. Rare, and it's usually optimism.
• Two coats — nearly everything. Colour changes within the same family, ordinary refreshes.
• Three coats — dark over light, light over dark, or any strong colour. Reds and deep yellows are notorious; the pigments simply don't cover in two.

Coats multiply everything. Going from two to three doesn't add a bit of paint, it adds half as much again to the whole order.

Do you need primer?

Primer isn't paint and doesn't count toward your paint total — it's a separate calculation at its own coverage rate.

You need it if you're painting bare drywall, wood or plaster, covering stains or smoke, going dramatically lighter over a dark wall, or painting over a glossy surface that fresh paint won't grip.

You can usually skip it when repainting a previously painted wall in a similar colour and reasonable condition.

The genuine shortcut: on a big colour change, having the primer tinted toward your final colour often saves an entire coat of the expensive paint. That's a better use of money than a third coat.

What about ceilings and trim?

The calculator above handles walls, because walls are where estimates go wrong. The other two are easy:

• Ceiling: length × width. A 12 × 14 room is 168 square feet — half a gallon.
• Trim and doors: a quart usually covers a whole room's worth. Trim paint is a different finish anyway, so it's a separate purchase.

The short version

Measure the perimeter, multiply by the height, take off about 21 square feet per door and 15 per window, multiply by two coats, and divide by the number on the can. Round up.

If you're painting bare drywall or making a big colour change, add a gallon. You'll use it on touch-ups eventually, and it costs far less than the trip back.