How to measure a room for paint
You only need three measurements. Use a tape measure along the floor to get the length and width of the room, then measure the wall height from the floor to the ceiling. Round each number up to the next half foot; a little extra paint is better than a second trip to the store.
For an L-shaped room or any room with more than four walls, add up the length of every wall. Enter half of that total as the length and the other half as the width. The calculator only uses the distance around the room, so the result will be the same.
The calculator subtracts 21 square feet for each door (a standard 3 × 7 ft door) and 15 square feet for each window (about 3 × 5 ft). If you have a sliding patio door or a large picture window, count it as two or three windows.
How the calculation works
Wall area = 2 × (length + width) × wall height
Area to paint = wall area − (doors × 21) − (windows × 15) + ceiling, if included
Gallons = area to paint × coats ÷ coverage per gallon
Here is the example already filled in above: a 12 × 14 ft room with 8 ft walls has 416 square feet of wall. Taking away one door and two windows leaves 365 square feet. Two coats means covering 730 square feet, and at 350 square feet per gallon that is 2.1 gallons. Paint is sold in gallons and quarts (four quarts make a gallon), so you would buy 2 gallons and 1 quart.
How far does a gallon of paint go?
Most interior wall paints list a coverage of 350 to 400 square feet per gallon on the label. That figure assumes a smooth, previously painted surface. Use a lower number when the paint has to work harder:
| Surface | Coverage to enter |
|---|---|
| Smooth, previously painted drywall | 350 to 400 sq ft |
| Light texture (orange peel, knockdown) | 300 to 350 sq ft |
| Heavy texture, brick, rough plaster | 250 to 300 sq ft |
| New, unprimed drywall | Prime first, then use the paint's label value |
These ranges are rules of thumb. The can's label is always the better guide, because thick, high-hiding paints and thin budget paints can differ quite a bit.
How many coats do you need?
- One coat can be enough when you repaint in the same or a very similar color over walls that are in good shape.
- Two coats is the safe default for most color changes and gives an even finish without lap marks.
- Three coats, or a primer plus two coats, is common when you go from a dark color to a light one, or when you paint over patched or stained walls.
Primer usually covers less area than paint, so check its label and work it out separately. A tinted primer can save you a coat when the color change is dramatic.
Tips before you buy
- Buy all the paint for one room at the same time, so it comes from the same batch. If you end up with several cans, mix them together in a larger bucket (painters call this boxing) to avoid slight color differences between walls.
- Keep a little paint for touch-ups. Write the room name and the date on the lid.
- For large jobs, a five-gallon bucket usually costs less per gallon than buying single gallons.
- Ceilings are usually painted with a flat white ceiling paint. If you choose a different paint for the ceiling, run the calculator twice: once for the walls and once with only the ceiling.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and two windows, a 12 × 12 ft room has about 333 square feet to paint. Two coats at 350 square feet per gallon comes to 1.9 gallons, so 2 gallons is enough for the walls.
How many square feet does a gallon of paint cover?
Between 350 and 400 square feet per coat on smooth walls, according to most manufacturers. Textured or porous surfaces need more paint, so plan on 250 to 300 square feet per gallon for those.
Should I subtract doors and windows?
Yes for an accurate number, and the calculator does it for you. If you are painting the door and window trim with the same paint, set doors and windows to zero and you will have enough left over for the trim.
Is one gallon of paint enough for a small bathroom?
Often, yes. A 5 × 8 ft bathroom with 8 ft walls has about 208 square feet of wall before you take away the door and any window. Two coats would need a little over a gallon, so one gallon plus a quart is the safe choice.
This calculator gives an estimate based on the numbers you enter and on typical door, window and coverage sizes. Always check the coverage printed on the paint you choose.