How Much Paint Do I Need?

Updated 2026-01-15

Nothing stalls a painting project like running dry at 4 p.m. on a Sunday with one wall half-rolled. Estimating paint is genuinely easy once you know the formula — the tricky part is knowing which assumptions to make about doors, windows, coats, and coverage. This guide gives you the same method the paint counter at the hardware store uses.

The short version: measure your walls, subtract the openings, divide by the coverage rate, and multiply by the number of coats. The sections below walk through each step with the real-world adjustments that keep you from buying too little — or three gallons too much.

The basic formula: perimeter × height

Start with the perimeter of the room. Add the lengths of all the walls together — for a rectangular room, that is 2 × (length + width). A 12 × 15 foot bedroom has a perimeter of 54 feet.

Multiply the perimeter by the wall height. Standard American ceilings are 8 feet, so that bedroom has 54 × 8 = 432 square feet of wall area. If you are painting the ceiling too, calculate it separately as length × width (12 × 15 = 180 square feet in this example) — ceiling paint and wall paint are usually different products, and ceilings are typically one flat white coat.

Subtract doors and windows

You do not paint the openings, so do not buy paint for them. The standard deductions the pros use:

1. Doors: subtract 21 square feet per standard door (3 ft × 7 ft).
2. Windows: subtract 15 square feet per average window.
3. Large openings like picture windows, sliding patio doors, or pass-throughs: measure them and subtract the actual area.

For the 12 × 15 bedroom with one door and two windows, subtract 21 + 30 = 51 square feet, leaving about 381 square feet of paintable wall. One honest warning: if the room is small or mostly window, do not bother subtracting — the extra margin works in your favor anyway.

How many coats do you really need?

One coat is only realistic when you are repainting the same color (or very close) over walls in good shape. Plan on two coats for almost everything else — it is the default assumption for any color change.

Budget for three coats (or a tinted primer plus two) in the classic trouble scenarios: going from a dark color to a light one, covering red or deep navy walls, painting over fresh unpainted drywall, or using a budget paint with thin coverage. Skimping on coats to save a gallon is a false economy — you will see the old color ghosting through, and the fix costs more than the gallon you saved.

Coverage rates: 350–400 square feet per gallon

A gallon of quality interior paint covers roughly 350 to 400 square feet per coat on smooth, previously painted drywall. Check the can — manufacturers print their rated coverage, and premium paints often hit 400 while economy lines land closer to 300.

Reduce your expectation for rough surfaces: textured walls, bare plaster, brick, and unprimed drywall can drop real-world coverage to 250–300 square feet per gallon. Divide your total paintable area by the coverage rate to get gallons per coat, then multiply by the number of coats. Our bedroom example: 381 sq ft ÷ 375 = about 1.02 gallons per coat, so 2 coats calls for just over 2 gallons — buy 2 gallons and a quart, or 3 gallons if you want touch-up paint left over.

When you need primer

Primer is a separate line item, not a substitute for a coat of paint. You need it when painting bare new drywall, bare wood, patched areas with joint compound, stained or smoke-damaged walls, glossy surfaces, or when making a dramatic dark-to-light color change. Primers cover about 300–400 square feet per gallon, so calculate it the same way as paint, one coat unless the label says otherwise.

Paint-and-primer-in-one products are fine for repaints in decent shape, but they do not replace a dedicated primer on bare drywall or problem stains — the pros at the paint desk will tell you the same thing.

Why it pays to buy a little extra

Always round up, and consider buying 10–15% more than the math says. The main reason is batch variation: paint is tinted at the store, and two cans mixed at different times can differ just enough to be visible on the same wall. Buying everything you need in one trip, from the same batch, avoids that problem entirely.

Leftover paint is not wasted money. Seal the can tightly, label it with the room and date, and stash it — future scuffs, furniture dings, and nail-hole patches will blend invisibly only if the touch-up paint came from the same batch. Once you have your square footage and coat count, run the numbers through the paint calculator to confirm your gallon count before you head to the store.