To calculate how much paint you need for a room, multiply the total wall area by the number of coats, divide by the coverage rate printed on the tin, and round up to the nearest whole gallon or litre. The four inputs that drive every paint estimate are wall area, the doors and windows you are skipping, the number of coats you plan to apply, and the coverage figure on the paint can. Get those four numbers right and the rest of the job is arithmetic. This article walks through each input, shows the formula behind the answer, and points you to a free Paint Calculator that runs the math live as you type. You can work in square feet with US gallons or in square metres with litres, but every measurement in a single estimate has to stay in the same unit system so the coverage rate and the final paint quantity stay paired correctly. The whole calculation runs in your browser, so nothing about your room gets uploaded.
The four numbers behind every paint estimate
Paint quantity is one formula with four inputs, and once you name them the job stops feeling mysterious. The inputs are:
- Total wall area, in square feet or square metres, found by measuring every wall and multiplying by ceiling height.
- Openings you are not painting, in the same square units — doors and windows.
- Number of coats, almost always two for a repaint, sometimes one or three.
- Coverage, the area one tin covers in a single coat, printed on the label.
Stack those together and the calculation reads: (wall area − openings) × coats ÷ coverage per tin, with the answer rounded up because paint stores do not sell fractions of a tin. The same formula works in any unit system as long as the wall area and the coverage rate use a matching pair — square feet with US gallons, or square metres with litres.
| Unit system | Wall area input | Paint quantity output |
|---|---|---|
| Imperial | Square feet (sq ft) | US gallons |
| Metric | Square metres (m²) | Litres |
How to measure the total wall area
Walk the room with a tape measure and a notepad. For each wall, measure the horizontal length in feet or metres, then multiply by the ceiling height. Add the four walls together. A room with 60 linear feet of wall and an 8-foot ceiling, for instance, works out to 60 × 8 = 480 square feet of wall surface. That single number is the total wall area.
Two habits keep this measurement honest. First, keep every dimension in the same unit — don't mix feet and metres partway through, because square feet and square metres are not the same area and the coverage rates that go with them are different. Second, measure to the nearest inch or centimetre rather than rounding to the nearest foot; a wall that is 14 feet 9 inches long is 14.75, not 15, and over four walls the slack adds up.
For rooms with vaulted ceilings, dormers or bay windows, treat each flat plane as its own wall: measure the length of that plane at the height you intend to paint, and add it to the total. Odd shapes are easier to break into rectangles than to estimate by eye, and a sketch with each wall's number written next to it tends to beat a memory list when you reach the calculator.
Doors, windows and how many coats
You don't paint glass, and you don't paint the face of a closed door, so the next step is to subtract those openings from the wall area. A standard interior door works out to roughly 20 square feet, and an average window lands between 12 and 15 square feet. Add up the openings on the walls you are painting and enter the total.
If you would rather skip that step, leaving the openings field blank simply rounds your estimate up a touch, which becomes spare paint rather than an error. The calculator will refuse to let the openings equal or exceed the wall area, because past that point there is no wall left to paint.
For coats, two is the standard for an even, durable finish, which is why the calculator defaults to two. A fresh colour matched to the existing wall over primer can manage one coat, while a light colour going over a dark wall, or bare and patchy drywall, often needs three. Set the number of coats to match the actual job, because every extra coat multiplies the paint you have to buy.
Reading the coverage rate on the tin
Coverage is the area one gallon or one litre covers in a single coat, and it is the figure most often misread. A typical US gallon of interior emulsion covers about 350 to 400 square feet per coat on smooth, primed walls. A litre of the same paint covers about 10 to 12 square metres per coat. Those are ideal-condition numbers from the label, and they only hold when the wall is smooth, sealed and already primed.
| Tin size | Coverage per coat (smooth, primed wall) |
|---|---|
| 1 US gallon | ~350–400 sq ft |
| 1 litre | ~10–12 m² |
Real walls are usually rougher than the test surface used to derive those figures. Textured finishes, bare drywall, patchy repairs and unprimed plaster all drink more paint, so the effective coverage on the day you roll it on runs 10 to 20 percent below the tin's headline number. Thinner finishes cover less as well — flat and matte emulsions generally spread further per litre than semi-gloss and high-gloss, because the glossier paints lay on a thinner film. Premium paints with more titanium dioxide hide better in fewer coats, which can offset some of that loss.
For a starting estimate, use the number on the tin and treat anything you save as a buffer. If you want a closer read on cost once you have the tin count, the per-square-foot guide in how to calculate paint cost per square foot takes the same wall area and works out the dollar side of the same equation.
Using the Paint Calculator
The Paint Calculator runs the formula live, so you can change any input and watch the tin count move. Work through it in this order:
- Pick your units, then enter the total wall area — the length of every wall times the ceiling height, added together.
- Subtract the doors and windows you are not painting, then set the number of coats (two is typical).
- Enter the coverage printed on the tin to see how much paint to buy, rounded up to whole gallons or litres.
Because every result updates as you type, you can flip between one coat and three coats to see how the tin count moves, or nudge the coverage figure up and down to reflect a rougher wall. The calculator only accepts a sensible combination of inputs — the openings field cannot equal or exceed the wall area, and the output is always a whole number of tins because you cannot buy a partial gallon.
A worked example in imperial units
Take a room with 60 linear feet of wall and an 8-foot ceiling. Wall area = 60 × 8 = 480 square feet. Subtract one door and one window: 480 − 20 − 15 = 445 square feet of paintable wall. At two coats the true area to cover is 445 × 2 = 890 square feet. With a tin rated at 350 square feet per gallon, the division is 890 ÷ 350 = 2.54 gallons, which the calculator rounds up to 3 gallons.
That third gallon is spare, and the spare is the point. Real walls usually cover less than the label suggests, and colours drift slightly between production runs, so the tin you keep back from the same batch becomes the tin you reach for when a scuff shows up next year.
What the calculator doesn't cover
This is a straightforward wall estimate, not a full paint schedule. Ceilings, baseboards, door and window trim, and a separate primer coat on bare drywall each need their own calculation — a ceiling is just another large flat plane, and trim runs on linear feet rather than area. If you are painting a brand-new colour over bare drywall or a dark wall, plan a primer pass first; primer is sold by the same coverage math as paint, but it is a separate tin.
Everything in the estimate depends on the coverage figure you trust. Measure twice, read the tin, and round up — those three habits do more for a clean paint job than any formula, and they are the reason the calculator leans conservative on its rounding.