How to Calculate How Much Paint You Need

Buying paint is a guessing game for most people, and the guess is usually wrong in one of two expensive directions: a second trip to the store mid-project, or three unopened gallons in the garage forever.

Working out how much paint you need is genuinely simple arithmetic — four measurements and a division. What makes it feel hard is that the coverage number on the can assumes conditions your walls probably do not meet.

The basic calculation

Four steps:

1. Measure the perimeter. Add up the lengths of all the walls. A 12 ft × 14 ft room is 12 + 14 + 12 + 14 = 52 feet.

2. Multiply by ceiling height. At 9 feet, that is 52 × 9 = 468 square feet of wall.

3. Subtract openings. Use rough standards rather than measuring each one:

• Standard door: 21 sq ft
• Standard window: 15 sq ft

One door and two windows takes 468 − 21 − 30 = 417 square feet.

4. Divide by coverage, then multiply by coats. At 350 sq ft per gallon: 417 ÷ 350 = 1.19 gallons per coat. Two coats is 2.4 gallons, so buy three — or two gallons plus a quart if you want to be precise about it.

The paint calculator runs this for multiple rooms at once and handles ceilings and trim separately, which is where hand-arithmetic tends to fall apart.

Why the coverage number on the can is optimistic

Most cans claim 350 to 400 square feet per gallon. That figure comes from a laboratory application onto smooth, primed, non-porous drywall. Real surfaces take more:

Surface	Realistic coverage per gallon
Smooth, previously painted drywall	350–400 sq ft
Lightly textured drywall	300–350 sq ft
Heavy texture or knockdown	250–300 sq ft
New, unprimed drywall	200–250 sq ft
Bare wood	200–250 sq ft
Smooth brick or masonry	150–200 sq ft
Rough or split-face masonry	100–150 sq ft

Texture is the variable that surprises people. A textured wall has considerably more actual surface area than its flat dimensions suggest, and the paint has to fill the low spots. Rough masonry can consume three times what the can advertises.

If you are unsure, use 350 for smooth walls and 300 for anything with visible texture. Rounding up costs a few dollars; running out costs an afternoon and risks a visible lap mark where the new can started.

When you need more than two coats

Two coats is the working default for a same-colour refresh. Plan on three, or on priming first, when:

• Going dramatically lighter. Covering a deep red or navy with off-white can take three or four coats without a tinted primer. Prime instead — primer is cheaper than finish paint and made for exactly this.
• The surface is new or patched. Bare drywall and joint compound drink the first coat. Patches will "flash" — dry to a different sheen — unless primed.
• You are using a strong colour. Deep reds and bright yellows have less opaque pigment and often need an extra pass regardless of what is underneath.
• You are switching from gloss to matte, or painting over a glossy surface at all, in which case you need to scuff-sand and prime for adhesion.

Ceilings and trim are separate calculations

Ceilings. Length × width. A 12 × 14 room has a 168 sq ft ceiling, which is about half a gallon per coat. Buy a gallon; ceiling paint is a different product and you cannot use wall paint interchangeably without a sheen mismatch.

Trim. Estimate by length rather than area. A gallon of trim paint covers roughly 500–600 linear feet of standard baseboard or casing at one coat, so most rooms need a quart at most. Trim almost always wants two coats because it is usually a higher sheen, which shows everything.

Doors. About 20 sq ft per side. A quart handles two doors, both sides, two coats, with a little left over.

Mistakes that cost a second trip

Forgetting the second coat entirely. The most common error by a distance. Calculate the area, then double it before you look at the price.

Measuring the floor instead of the walls. Floor area and wall area are unrelated. It is wall perimeter × height that matters — the same distinction that makes flooring a completely separate calculation from painting the same room.

Subtracting every opening precisely. Do not. Deducting a few square feet for a narrow window while ignoring the extra paint lost to cutting in, roller loading and the tray is false precision. Ignore openings smaller than about 20 sq ft.

Buying different batches later. Paint is mixed in batches and colour can shift slightly between them. If a room needs three gallons, buy three at once and have the store box them — mix them together in a larger bucket — so the colour is identical wall to wall.

Ignoring the leftover you already have. Half a gallon from the last project covers a closet or a first coat on patches.

A quick field estimate

For a rough number without measuring, most standard bedrooms with 8–9 ft ceilings land at 2 gallons for two coats. A large living room is 3 gallons. A bathroom or small office is 1 gallon. Add a gallon for the ceiling.

That estimate is good enough for a shopping list, though on anything larger than a single room the measurements are worth the five minutes. If you are pricing a whole project, the paint calculator totals multiple rooms, and for outdoor work the concrete and mulch calculators handle the same measure-then-divide-by-coverage logic for volume rather than area — the method generalises further than you would think.

Manufacturer coverage specifications and surface-preparation guidance are published directly by Sherwin-Williams and Benjamin Moore, and both list per-product spread rates that are more reliable than the generic figures above.