Paint Calculator: How Much Paint to Buy for a Room

There are two ways to end up unhappy at the end of a painting job: run out with one wall left to go and have to buy a fresh gallon that dries a shade different, or buy three gallons for a job that needed one and leave two to skin over in the garage. The paint calculator above keeps you out of both ditches. Give it the room's dimensions, subtract the doors and windows, choose how many coats, and it tells you how many gallons to put in the cart.

This guide walks through the simple area maths behind that number, the coverage rates you can trust, and the real-world adjustments — primer, dark colours, textured walls — that the raw formula does not know about.

How paint coverage works

Estimating paint comes down to two quantities: how much surface you are covering and how far a gallon goes. Get those two right and the rest is division.

The surface is wall area. For a rectangular room you do not measure each wall separately — you measure the room's perimeter and multiply by the ceiling height. Then you subtract the openings you are not painting: the doors and windows.

The coverage is the part people guess at, usually badly. A gallon of interior latex covers a fairly predictable amount of wall, and the manufacturers all quote a similar figure.

The paint formula

Here is the calculation the tool runs, broken into its two steps:

The first line is just the perimeter of the room — twice the length plus twice the width — multiplied by how tall the walls are. The second line takes that paintable area, multiplies by the number of coats, and divides by how far a gallon stretches.

A worked example. A 12 × 12 foot room with 8-foot ceilings, one door and one window, painted in two coats:

• Wall area = 2 × (12 + 12) × 8 = 384 square feet
• Subtract a door (21 sq ft) and a window (15 sq ft) = 384 − 36 = 348 square feet
• Gallons = 348 × 2 coats ÷ 350 = 1.99 → buy 2 gallons

That is the answer the calculator gives, and it matches what the paint-store estimators will tell you. The live formula strip beneath the tool shows your own room's numbers running through the same steps.

Standard sizes the calculator assumes

You could measure every door and window, but for estimating it is faster to use standard sizes — and that is what the tool subtracts per opening unless you tell it otherwise.

If your room has a wall of glass or an oversized opening, the simplest fix is to count it as two openings, or to lower your door/window counts and trust the conservative 350 coverage to absorb the difference. For most rectangular bedrooms, living rooms and offices, one door and one or two windows is all you need to enter.

How far different paints go

The 350 default is right for ordinary interior wall paint on a previously painted, smooth surface. Other situations change the coverage, and you can type a different number into the Coverage field to match.

The pattern is intuitive once you see it: the rougher or thirstier the surface, the less area a gallon covers. Bare drywall, raw wood, textured plaster and stucco all pull more paint out of the can than a smooth, sealed wall. When you are working on any of those, drop the coverage figure and let the calculator buy you a little more.

How to use the calculator

1. Room length, width and height in feet. A quick tape measure read of the floor and one wall is all it takes.
2. Doors and windows. Enter how many of each; the tool subtracts standard sizes.
3. Coats. Two is the realistic default for a quality finish, and for any colour change. One coat only works when you are refreshing the same colour over a clean, sound surface.
4. Coverage. Leave it at 350 for normal walls, or lower it for bare or textured surfaces using the chart above.
5. Ceiling. Tick the box if you are painting the ceiling too — the tool adds the floor-area-equivalent (length × width) to the job.

The big number is the gallons to buy, already rounded up to whole cans because that is how paint is sold. Underneath, you will see the exact gallons needed and the breakdown of wall area, openings and coats.

Real-world adjustments the formula can't see

The maths is honest, but a paint job has variables a calculator cannot measure. Build these into your decision:

A few more habits that keep the estimate accurate:

• Always prime bare or repaired surfaces. Primer is cheaper than topcoat, seals the surface so your colour goes on evenly, and means you actually get the coverage the can promises. New drywall, patched holes and raw wood all need it.
• Buy trim and wall paint separately. Trim and doors usually take a different sheen (semi-gloss) and a small amount goes a long way. Estimate them on their own rather than rolling them into the wall total.
• Keep a labelled jar of leftover paint for touch-ups. A small surplus is genuinely useful; two unopened gallons are just clutter. The calculator's round-up usually leaves you the right amount.
• Box your paint. If you do buy multiple cans of the same colour, pour them together into one larger container and mix before you start. It evens out any tiny batch-to-batch colour difference across the whole room.

Painting is one of the highest-impact DIY jobs for the lowest tool cost — a decent roller, a brush, a tray and some tape. If you are putting together your first kit, our best DIY tool kit for beginners and the broader essential DIY tools guide cover what is worth owning before you start.

Measure your room in two minutes

You do not need to measure all four walls. For the calculator you need just three numbers: the room's length, its width, and the ceiling height. Run a tape along two adjacent walls at floor level for length and width, then measure floor to ceiling once for the height — standard ceilings are 8 feet, though older homes and new builds often run 9 or 10. Round to the nearest foot; paint estimating does not reward fractions of an inch, and the conservative 350 coverage figure already builds in a little slack.

For an irregular, L-shaped or open-plan space, split it into rectangles in your head, estimate each as its own "room," and add the gallons together. Vaulted or sloped ceilings are the one case worth measuring carefully, since the extra triangle of wall above the normal ceiling line adds real area — measure to the highest point and treat the gable end as an extra opening's worth of wall rather than guessing low.

Match the finish to the room

The calculator tells you how much, but choosing the right sheen matters just as much to how the job looks and lasts. Sheen is how glossy the dried paint is, and it is a genuine trade-off between hiding flaws and surviving scrubbing.

• Flat / matte hides wall imperfections best and is the usual choice for ceilings and low-traffic adult bedrooms. It is the hardest to clean, so keep it away from busy spaces.
• Eggshell and satin are the everyday middle ground — a soft, washable finish that suits living rooms, hallways and most walls. Satin is the slightly tougher of the two.
• Semi-gloss and gloss shrug off moisture and scrubbing, which makes them right for kitchens, bathrooms, trim, doors and anything little hands touch. The shine does highlight every bump, so the surface underneath has to be smooth.

A common, reliable scheme for a whole house: flat on the ceilings, eggshell or satin on the walls, and semi-gloss on the trim and doors. Because trim takes so little paint, estimate it separately — a single quart often does the doors and baseboards of an average room.

Painting a whole house? Estimate room by room

It is tempting to measure a whole floor at once, but the openings and ceiling heights vary too much room to room for one big number to be accurate. Run each room through the calculator on its own, note the gallons, and total them at the end. Then look for paints you can share: every room getting the same wall colour can pool into one larger order — which is usually cheaper per gallon and guarantees a perfect colour match — while bathrooms and kitchens in a moisture-friendly sheen get their own. Buying by the room but pooling by the colour is how professionals keep both the budget and the colour consistent across a big job.

Frequently asked questions

How many gallons of paint do I need for a 12x12 room?

About two gallons for two coats on the walls, after subtracting a door and a window. That covers roughly 348 square feet of wall twice over at 350 sq ft per gallon. Add another gallon if you are also painting the ceiling or using a bold colour.

How much does a gallon of paint cover?

Around 350 to 400 square feet for one coat of interior latex on a smooth, previously painted wall. Primer and paint on rough or bare surfaces cover less — sometimes as little as 150–250 square feet.

Do I subtract doors and windows?

Yes, for a tighter estimate — the calculator removes about 21 square feet per door and 15 per window. If a room has very large openings, count them generously so you do not over-buy.

How many coats of paint do I need?

Two is standard for a durable, even finish and essential for any colour change. One coat suffices only when refreshing the same colour over a clean, sound surface. Dark or vivid colours often need a tinted primer plus two coats.

Should I include the ceiling in the calculation?

Only if you are painting it. Tick the ceiling box and the tool adds the ceiling's area (length × width) to the job. Ceilings usually take flat paint and one coat on a sound surface.

Use this estimate to shop with confidence, but treat it as a well-judged starting point rather than a guarantee — surface texture, colour and application method all move the real figure. When the result lands right on a round number, buying one extra quart for touch-ups is cheap insurance against that last awkward patch.