How to use the paint calculator
- Measure the room: its length, its width and the wall height from floor to ceiling. Type feet and inches, so 8 ft 6 in goes in as 8 and 6.
- Count the doors and windows and check their sizes. The sizes filled in are only starting values, so measure yours and change them.
- Set the coats. Two is filled in. Use the number on your paint's label.
- Type the coverage from the can. It is printed on the label as square feet per gallon. 350 is filled in until you do.
- Tick Paint the ceiling too if you want the ceiling in the same paint.
For one feature wall, a hallway or an odd-shaped room, choose Wall by wall and enter each wall's width and height. Choose Meters to get the answer in liters.
How the amount of paint is worked out
The calculator follows the same steps you would on paper:
- Wall area = 2 x (length + width) x height. That is the distance around the room times the height.
- Take off each door and window: count x width x height.
- Add the ceiling, length x width, if you ticked it.
- Multiply by the number of coats.
- Divide by the coverage per gallon. The answer is the paint you need, before rounding up to whole cans.
Here is a 12 ft by 10 ft bedroom with 8 ft walls, one door of 3 ft by 6 ft 8 in and two windows of 3 ft by 4 ft, painted with two coats at 350 sq ft per gallon. Press Try an example to see it in the calculator.
| Step | Working | Result |
|---|---|---|
| Wall area | 2 x (12 + 10) x 8 | 352 sq ft |
| Door | 1 x 3 x 6.667 | -20 sq ft |
| Windows | 2 x 3 x 4 | -24 sq ft |
| Area to paint | 352 - 20 - 24 | 308 sq ft |
| Two coats | 308 x 2 | 616 sq ft |
| Paint needed | 616 / 350 | 1.76 gallons |
| Buy | rounded up | 2 gallons |
Add the ceiling (12 x 10 = 120 sq ft) and the area becomes 428 sq ft, or 856 sq ft for two coats. That is 2.45 gallons: either 3 gallon cans, or 2 gallons plus 2 quarts.
Where the coverage figure comes from
Coverage is how much surface one gallon covers in one coat. It changes from product to product, so the figure on your own can is the one to use. Sherwin-Williams gives a general figure on its paint calculator page:
"A gallon of paint typically covers about 350-400 square feet"Sherwin-Williams, How to Use Paint Calculator
The calculator starts at 350, the low end of that range, so the estimate leans towards having enough paint. The same page adds that "depending on wall texture and desired coverage, having a little more than a gallon on hand may be best". For primer it gives a lower figure: "A gallon of primer covers about 200-300 square feet". If you prime first, work out the primer on its own with the coverage from the primer can and one coat.
Labels often give a range or an "up to" figure. If you are not sure, use the lower number.
Rounding up to whole cans
Paint is sold by the can, so the calculator rounds up. In the US the common sizes are gallons and quarts. A quart is a quarter of a gallon: NIST's tables give "4 quarts = 1 gallon (gal) = 231 cubic inches". So the calculator shows two ways to buy:
- Whole gallons: 2.45 gallons rounds up to 3 gallons.
- Gallons and quarts: 2 gallons cover the first 2, and the last 0.45 gallon or so is 1.78 quarts, which rounds up to 2 quarts. If it would take 4 quarts, the calculator shows one more gallon instead.
A US gallon is 3.785 liters, which the result also shows.
Measuring in meters (UK and elsewhere)
Choose Meters and the coverage changes to square meters per liter, which is how UK tins state it. Dulux UK gives the same method: Multiply the height and width together to get your square metre measurement. Subtract any windows and doors
, then Divide the number by the square metre per litre listed on your paint tin
.
The figure filled in, 8.6 m² per liter, is the same 350 sq ft per gallon converted, so it is a careful starting point. Many tins claim more. The Dulux Matt product page, for example, lists coverage of "Up to 14m2/litre" and recommended coats of "2".
Example: a room 4 m by 3.5 m with walls 2.4 m high has 2 x 7.5 x 2.4 = 36 m² of wall. Take off a door of 0.8 m by 2 m and a window of 1.2 m by 1.2 m, and 32.96 m² is left. Two coats is 65.92 m².
| Coverage | Paint needed | 2.5 L tins |
|---|---|---|
| 8.6 m² per liter | 65.92 / 8.6 = 7.67 L | 4 tins (10 L) |
| 14 m² per liter | 65.92 / 14 = 4.71 L | 2 tins (5 L) |
That gap is why the figure on your tin matters. Set the tin size you plan to buy and the calculator rounds up to whole tins.
Rooms that are not simple boxes
- Gable ends and sloping walls. A triangle's area is half its base times its height. In Wall by wall, enter it as a wall with the base as the width and half the peak height as the height. That gives the same area.
- Trim, doors and cabinets are not included. The calculator covers walls and the ceiling only.
- Big change of color. If the label suggests an extra coat over a dark color, raise the coats rather than guessing at a lower coverage.
Questions and answers
How much paint do I need for a 12 x 12 room?
With 8 ft walls, the walls are 2 x 24 x 8 = 384 sq ft. Take off a 20 sq ft door and a 12 sq ft window and 352 sq ft is left. Two coats is 704 sq ft. At 350 sq ft per gallon that is 2.01 gallons, so buy 3 gallons, or 2 gallons and 1 quart. Use your own can's coverage for a closer figure.
How many square feet does a gallon of paint cover?
It depends on the paint. Sherwin-Williams says a gallon typically covers about 350 to 400 square feet. The label on your can gives the figure for that product. Textured walls can take more.
Should I subtract doors and windows?
Yes, if you want a close estimate. Work out each door and window as width x height and take it off the wall area. For a room with few openings, leaving them in simply gives you a little spare paint.
How many coats of paint do I need?
Follow the label. The calculator starts at two, which is what the Dulux Matt product page recommends, for example. Some paints are sold as one-coat paints, and covering a dark color can take more.
How many quarts are in a gallon, and how many liters?
There are 4 quarts in a US gallon. A US gallon is 231 cubic inches, which is 3.785 liters, so a quart is about 0.946 liters.
Sources
Checked on 7 October 2026. If a rule has changed, please tell us.
- Sherwin-Williams: How to Use Paint Calculator (gallon and primer coverage)
- Dulux UK: How much paint do I need?
- Dulux UK: Dulux Matt product page (coverage and coats)
- NIST Handbook 44, Appendix C: General tables of units of measurement (gallons, quarts, liters)
- NIST: U.S. Survey Foot (1 foot = 0.3048 meter exactly)