How many square feet does a gallon of paint really cover?

The can says 350–400 sqft. Plan for 300–350 on smooth, primed drywall and 250–300 on textured, porous, or color-changing walls. The label number assumes one coat on a perfect surface, which almost never describes a real room.

Do I always need two coats?

Almost always for a color change, and always when going light over dark or painting over bare/patched drywall. One coat only works when you're repainting the same color over a sound, previously painted surface. Budget for two coats unless you have a specific reason not to.

How do I account for doors and windows?

Subtract about 21 sqft per door and 15 sqft per average window from your wall area. The Paint Calculator does this automatically — it's a meaningful subtraction in a room with several openings.

Does primer count as a coat?

No — primer is a separate product with a separate purpose (sealing, blocking stains, improving adhesion). On bare drywall, new patches, or dramatic color changes, prime first, then apply your finish coats on top. Paint-and-primer-in-one is a marketing term, not a substitute for real primer on raw surfaces.

Buy one extra quart beyond the calculated amount for touch-ups, and have it shaken from the same batch. Color can drift slightly between batches, so a touch-up from a future can may not match.

How much paint do I need? The square-footage math, coverage truth, and coat reality

A gallon "covers 400 square feet" on the can and 250 in the real world. Here's the actual math for how much paint a room needs, why coverage claims overstate, and when a second coat is non-negotiable.

By Houex Editorial · May 24, 2026

Buying paint is a guessing game most people lose in one direction or the other — either three half-empty gallons in the garage forever, or a frantic second store trip mid-project with the roller still wet. The math is simple once you stop trusting the number on the can.

The actual square-footage formula

Wall area is just the perimeter times the height, minus the openings:

Perimeter = 2 × (length + width)
Wall area = perimeter × ceiling height
Subtract openings = (doors × 21 sqft) + (windows × 15 sqft)
Multiply by coats
Divide by realistic coverage (300, not 400)

For a 12 × 14 room with 9-ft ceilings, one door, two windows, two coats:

Perimeter = 52 ft → wall area = 468 sqft
Openings = 21 + 30 = 51 sqft → paintable = 417 sqft
Two coats = 834 sqft → ÷ 300 = 2.8 gallons (buy 3)

The Paint Calculator runs this exactly, including the ceiling if you're painting it, and rounds up to the nearest quart.

Why "400 sqft per gallon" is a lie of omission

The coverage number on the can is a best case: one coat, smooth sealed drywall, a roller in expert hands. Reality subtracts from it:

Texture (orange peel, knockdown) increases surface area — knock off 15–25%.
Porous surfaces (new drywall, patched areas, flat-finish old paint) drink the first coat.
Color change (especially light over dark, or any deep saturated color) demands a second and sometimes third coat for opacity.
Application — rolling leaves more on the wall than spraying does; cutting in by brush is thinner.

Plan at 300 sqft/gallon and you'll rarely come up short.

When the second coat isn't optional

One coat is enough only when repainting the same color over a sound painted surface. You need two (or prime + two) when:

Changing color at all
Going light over dark
Painting over bare or patched drywall
Using deep or bright saturated colors (reds, yellows, deep blues notoriously need 3)

Prime when the surface demands it

Primer isn't a coat of paint — it's a sealer and adhesion layer. Use real primer on bare drywall, new joint compound, stained areas, and dramatic color changes. Skipping it on raw surfaces is the single most common reason a finish looks blotchy after two coats.

Buy once, plus a quart

Calculate the gallons, then add one quart for touch-ups from the same batch. Fold the paint cost into the room's Renovation Budget, and if you're repainting as part of a furniture refresh, lay the room out first in the Room Planner so you're not painting behind a sofa you're about to move anyway.

Measure once, plan for two coats and 300-sqft coverage, and the only paint left over is the touch-up quart you wanted.

Frequently asked

FAQ

How many square feet does a gallon of paint really cover?: The can says 350–400 sqft. Plan for 300–350 on smooth, primed drywall and 250–300 on textured, porous, or color-changing walls. The label number assumes one coat on a perfect surface, which almost never describes a real room.
Do I always need two coats?: Almost always for a color change, and always when going light over dark or painting over bare/patched drywall. One coat only works when you're repainting the same color over a sound, previously painted surface. Budget for two coats unless you have a specific reason not to.
How do I account for doors and windows?: Subtract about 21 sqft per door and 15 sqft per average window from your wall area. The Paint Calculator does this automatically — it's a meaningful subtraction in a room with several openings.
Does primer count as a coat?: No — primer is a separate product with a separate purpose (sealing, blocking stains, improving adhesion). On bare drywall, new patches, or dramatic color changes, prime first, then apply your finish coats on top. Paint-and-primer-in-one is a marketing term, not a substitute for real primer on raw surfaces.
Should I buy extra?: Buy one extra quart beyond the calculated amount for touch-ups, and have it shaken from the same batch. Color can drift slightly between batches, so a touch-up from a future can may not match.