This paint calculator estimates how many gallons and quarts you need to paint a room, based on its size, the number of doors and windows, how many coats you plan, and the coverage listed on the can. It also gives a primer estimate so you can buy everything in one trip.
How to Use the Paint Calculator
- Measure the length and width of the room in feet at floor level.
- Measure the wall height from floor to ceiling. Most rooms are 8, 9 or 10 feet.
- Count the doors and windows in the walls you are painting.
- Choose the number of coats. Two coats is the normal choice for a color change or fresh drywall.
- Enter the coverage per gallon from the paint label. If you do not have it yet, leave the default of 350 square feet.
- Tick “Include the ceiling” if you are painting the ceiling with the same product, then read the results.
How the Calculation Works
The calculator first finds the total wall area by adding the room’s length and width, doubling that to get the perimeter, and multiplying by the wall height. It then subtracts 20 square feet for each door and 15 square feet for each window. These are standard allowances for a typical interior door and an average window.
If you include the ceiling, the ceiling area is simply length times width. Each area is multiplied by the number of coats and divided by the coverage per gallon to get gallons of paint. The wall and ceiling amounts are added together for the total.
The total is then turned into something you can actually buy. The whole number becomes gallons, and the leftover fraction is rounded up to the next quarter gallon, which is one quart. If the leftover is less than 0.05 gallon it is ignored. If the leftover would need three or more quarts, the calculator rounds up to one more gallon instead, since a gallon usually costs about the same as three quarts. Primer is estimated as one coat at 300 square feet per gallon over the walls (and ceiling, if included), rounded up to a whole gallon.
Worked example with the default values: a 12 x 12 foot room with 8 foot walls, 1 door, 2 windows, 2 coats and 350 square feet per gallon.
- Perimeter: 2 x (12 + 12) = 48 feet
- Gross wall area: 48 x 8 = 384 square feet
- Openings: 1 door x 20 + 2 windows x 15 = 50 square feet
- Paintable wall area: 384 – 50 = 334 square feet
- Paint needed: 334 x 2 coats / 350 = 1.91 gallons
- Rounding: 1 gallon plus 0.91 left over, which would need 4 quarts, so the result is 2 gallons
- Primer: 334 / 300 = 1.11, rounded up to 2 gallons
If you tick the ceiling box, the 144 square foot ceiling adds 144 x 2 / 350 = 0.82 gallon, for a total of 2.73 gallons. That rounds to 3 gallons.
Typical Paint Coverage and Coats by Surface
Coverage on the label assumes a smooth, primed surface. Rough or porous surfaces soak up more paint, so lower the coverage figure in the calculator when your walls fit one of the situations below.
| Surface or situation | Typical coverage per gallon | Typical coats |
|---|---|---|
| Previously painted smooth drywall, similar color | 350 to 400 sq ft | 1 to 2 |
| Previously painted drywall, noticeable color change | 350 to 400 sq ft | 2 |
| New drywall (after primer) | 350 to 400 sq ft | Primer plus 2 |
| Light texture (orange peel, knockdown) | 300 to 350 sq ft | 2 |
| Heavy texture or popcorn ceiling | 200 to 300 sq ft | 2 |
| Bare plaster or masonry | 200 to 300 sq ft | Primer plus 2 |
| Dark to light color, or light to deep color | 350 sq ft | Tinted primer plus 2, sometimes 3 |
| Ceiling paint on smooth ceiling | 350 to 400 sq ft | 1 to 2 |
Tips and Common Mistakes
- Trim and doors are not included. The calculator covers flat wall and ceiling surfaces. Trim, baseboards and doors are usually a different sheen and a small amount, so a quart is often enough for a single room’s trim.
- Use the coverage on your actual can. Coverage ranges from roughly 250 to 400 square feet per gallon depending on the product. Thicker paints and paint and primer blends often list lower numbers.
- Do not skip the second coat to save paint. One coat almost always looks patchy over a color change, and you end up buying more paint later anyway.
- Keep some leftover. A small amount of matching paint is useful for touch ups after furniture is moved back. Write the room and date on the lid.
- Buy all your paint at once. Tinted paint from separate batches can vary slightly. If you need more than one gallon, mixing the cans together in a larger bucket (called boxing) keeps the color even.
- Large open areas count once. For a wide archway or pass through, subtract it as a door or two instead of leaving it in the wall area.
Frequently Asked Questions
How much paint do I need for a 12 x 12 room?
With 8 foot walls, one door, two windows and two coats at 350 square feet per gallon, you need about 1.9 gallons for the walls, so buy 2 gallons. Add roughly another gallon if you are also painting the ceiling with two coats.
Do I need primer?
Primer is recommended on new drywall, bare wood, patched areas, stained surfaces, and when making a large color change. On clean walls that are already painted a similar color, a good quality paint usually goes on without separate primer.
Why does the calculator suggest a gallon instead of three quarts?
Three quarts is 0.75 gallon, and a single gallon is often priced close to three quarts. Rounding up gives you a little extra for touch ups at about the same cost.
Is a gallon of paint enough for one coat on an average room?
For a small to medium bedroom, one gallon often covers the walls with a single coat. Most projects need two coats for even color, so plan on two gallons for a typical bedroom.