How much paint do I need per square foot of wall?

Take the total wall area, subtract the doors and windows you are not painting, multiply by the number of coats, divide by the coverage on the tin, and round up to the next whole tin. For example, 480 sq ft of wall minus 50 sq ft of openings is 430 sq ft; at two coats that is 860 sq ft, and at 350 sq ft per gallon you buy 3 gallons.

Does one gallon of paint really cover 350 to 400 square feet?

That range is a realistic starting point for one US gallon of interior paint on smooth, primed drywall in a single coat. Rough or porous surfaces and high-sheen finishes cover less in practice, so check the exact rate printed on the tin you are buying and treat the figure as an upper bound rather than a guarantee.

How do I count doors and windows when working out paint per square foot?

Add up the area of every opening you are not painting and subtract the total from your wall area. A standard interior door works out to roughly 20 sq ft and an average window around 12 to 15 sq ft. You can leave the openings blank to keep a slightly higher estimate — the excess simply becomes spare paint for future touch-ups.

What if my paint tin quotes coverage in square metres per litre?

Switch the calculator to metric units and measure the room in square metres instead of square feet. One litre of emulsion typically covers about 10 to 12 square metres per coat on smooth, primed walls, and the rest of the formula — openings subtracted, coats chosen, result rounded up — stays the same.

Paint Per Square Foot: How to Calculate Coverage

Paint coverage per square foot is the rate at which one gallon of paint covers a wall in a single coat — for most interior paints on smooth, primed drywall, that rate is roughly 350 to 400 square feet per US gallon, or 10 to 12 square metres per litre. The phrase "paint per square foot" gets used in two opposite directions, and clarifying which one you mean is the first step to a useful number. Sometimes it means "how many square feet does a gallon cover" — that is the tin's stated coverage, and it grows as paint quality and surface smoothness improve. Other times it means "how many gallons do I need per square foot of wall" — that is its reciprocal, the small number used to scale an area into a shop-ready quantity. Most DIY jobs actually want the second form: a wall area in square feet multiplied by the per-square-foot paint demand at a chosen number of coats. The same idea works in metric with square metres and litres, and the two pairs always travel together.

how to calculate paint per square foot — Paint Per Square Foot: How to Calculate Coverage

Paint Per Square Foot vs Paint Per Square Metre: The Two Unit Pairs

The "per square foot" idea only behaves if you keep the area unit and the paint unit matched. A US gallon of paint does not naturally pair with square metres; it pairs with square feet. A litre pairs with square metres. Mixing them either gives nonsense numbers or forces a hidden conversion step most people forget. The table below shows the working pairs that every paint estimate should be checked against before you buy.

Unit system	Measure the wall in	Enter the paint coverage in	Buy paint in
Imperial (US)	square feet (sq ft)	sq ft per gallon	US gallons
Metric	square metres (m²)	m² per litre	litres (L)

The Paint Calculator handles this pairing for you automatically. As soon as you pick imperial or metric, the coverage field and the result switch together, so you cannot accidentally divide square feet by a coverage rate quoted in square metres per litre.

The Formula Behind Paint Per Square Foot

Once the unit pair is fixed, the math is one short formula:

Paint needed = (wall area − openings) × number of coats ÷ coverage per coat

Each piece of that formula is a familiar real-world measurement:

Wall area is the length of every wall, multiplied by the ceiling height, then added together.
Openings are the doors and windows you are not painting, totalled up and subtracted.
Coats is how many layers you intend to roll on — most repaints default to two.
Coverage is the number on the tin: how many square feet one gallon covers in a single coat, or how many square metres one litre covers.

The result is a paint quantity in the matching unit — gallons when you measured square feet, litres when you measured square metres. Because no shop sells a third of a gallon or a fraction of a litre, the final answer is rounded up to the next whole tin.

How to Calculate Paint Per Square Foot Step by Step

Pick your units. Decide whether you are working in square feet with US gallons or square metres with litres, and keep that pair all the way through the calculation and at the shop.
Measure and total the wall area. Measure the length of each wall, multiply by the ceiling height, and add the walls together. A rectangular room with 60 linear feet of wall and an 8 ft ceiling comes to 60 × 8 = 480 square feet of wall.
Subtract doors and windows. Walk the room and add up every opening you are not painting. A standard interior door is around 20 sq ft and an average window around 12 to 15 sq ft. Total them and enter the figure — the calculator will not let the openings equal or exceed the wall area, since then there is nothing left to paint — or leave it blank to keep a small over-estimate as spare paint.
Set the number of coats. Two is typical for an even, durable finish. One coat can work for a fresh colour over matching primer; three is common when a light colour goes over a dark wall or over bare, porous drywall.
Pull the coverage figure from the tin. Look for "Coverage per coat" on the label — the 350 to 400 sq ft per US gallon range is a typical starting point for smooth, primed drywall. Type that number directly into the coverage field of the Paint Calculator.
Read the rounded-up result. The calculator divides the total area by your coverage and rounds up to whole gallons or whole litres, so the figure you see is exactly the number of tins to put in the trolley.

A Worked Example in Square Feet

To make the formula concrete, run a 480 sq ft rectangular room with one interior door (about 20 sq ft) and two standard windows (about 15 sq ft each), at two coats and a coverage of 350 sq ft per gallon:

Wall area: 480 sq ft
Openings: 20 + 15 + 15 = 50 sq ft
Net wall area: 480 − 50 = 430 sq ft
Two coats: 430 × 2 = 860 sq ft of paintable surface
Gallons needed: 860 ÷ 350 = 2.46 gallons
Rounded up: 3 gallons

That is the whole calculation in plain numbers. Use the calculator for any room where the dimensions, openings, or coverage differ and you want the figure refreshed live as you type.

What Changes the Per-Square-Foot Rate

Coverage on the tin is an ideal-condition number. Real walls move that figure around, sometimes by enough to push you into an extra tin. The shifts come from a handful of predictable causes:

Surface texture. Smooth, primed drywall is the easy case and sits near the top of the tin's range. Light orange-peel, heavy orange-peel, or bare porous drywall soaks up more paint and pulls the rate down.
Primer or no primer. Skipping primer on bare drywall, or going over a repaired patch without sealing it, is one of the fastest ways to use more paint than the tin predicts.
Finish type. Higher-sheen paints (semi-gloss and gloss) carry less solids and spread thinner, so they cover noticeably less per gallon than flat or eggshell finishes.
Paint quality. Premium paints with more titanium dioxide hide better, which usually means fewer coats rather than more paint. A cheaper paint of the same colour family may need a third coat to look even.
Application method. Rollers and airless sprayers tend to land near the rated coverage; cheap rollers, very thick nap rollers, or brushing only on rough walls tend to land below it.

Field experience tends to put real-world coverage somewhere between 80 and 90 percent of the tin's stated rate on textured or unprimed walls. A simple way to bake that into your estimate is to enter a slightly lower coverage number — for example, 300 instead of 350 sq ft per gallon — and let the calculator round up accordingly.

Beyond the Walls: What This Estimate Does Not Cover

The per-square-foot figure produced above is a clean wall estimate. Several adjoining jobs belong to separate calculations and should be added on their own rather than folded into the wall number:

Ceilings are usually a flat surface in their own right and are measured as their own area, often at a different coverage rate than the walls below them.
Trim, baseboards and door frames are measured in linear feet, not square feet, and are usually painted with a smaller brush from a separate tin.
Doors themselves, if painted on both sides, are flat panels with their own area, separate from the wall deduction you already made.
A separate primer coat is its own calculation when the surface needs it, and the per-coat coverage is often lower than the topcoat's.

A couple of finishing practices are worth keeping for any project, even after the math is done:

Buy one spare tin from the same batch as the rest of your paint and keep it sealed for future touch-ups. Colours drift slightly between production runs, so a matched spare tin is much easier to live with than a fresh tin of "the same" colour.
Recheck the exact coverage on the tin you are actually buying at the store. The 350 to 400 sq ft per gallon figure is a starting point, not a guarantee, and the label on the specific product always wins.

Everything here is an estimate to help you buy with confidence. The arithmetic is small, the room is what it is, and the Paint Calculator can run the numbers live as you type so you never have to lean on a formula in the aisle.