How to calculate how much paint you need
Paint is sold by coverage, so the math comes down to three numbers: the square footage you are painting, the number of coats, and the coverage printed on the can. The calculator figures wall area from the room's perimeter and wall height, subtracts doors and windows, adds the ceiling if you include it, then rounds the result up to cans you can buy.
Openings = doors × 21 sq ft + windows × 15 sq ft
Gallons = paintable area × number of coats ÷ coverage per gallon
The opening sizes are typical figures: a standard interior door is about 3 × 7 ft (21 sq ft) and an average window is about 3 × 5 ft (15 sq ft). If a room has a sliding patio door, a wide picture window or a double closet door, measure those openings, subtract them yourself and use the total-area mode.
Worked example: a 12 × 12 ft bedroom with 8 ft walls
- Perimeter: 2 × (12 + 12) = 48 ft.
- Gross wall area: 48 × 8 = 384 sq ft.
- Subtract one door and two windows: 21 + (2 × 15) = 51 sq ft, which leaves 333 sq ft.
- Two coats: 333 × 2 = 666 sq ft of coverage.
- Gallons: 666 ÷ 350 = 1.90, so buy 2 gallons.
- Primer, if you need it (one coat at 300 sq ft per gallon): 333 ÷ 300 = 1.11 gallons, so 1 gallon plus 1 quart.
The ceiling adds another 144 sq ft. Two coats at 350 sq ft per gallon is 288 ÷ 350 = 0.82 gallon, so one gallon of ceiling paint covers it.
How much paint for common room sizes
The table assumes 8 ft walls, one door, two windows, two coats and 350 sq ft per gallon. Ceilings are not included.
| Room size | Paintable wall area | Gallons for two coats | What to buy |
|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.54 | 2 gallons |
| 12 × 12 ft | 333 sq ft | 1.90 | 2 gallons |
| 12 × 14 ft | 365 sq ft | 2.09 | 2 gallons + 1 quart |
| 12 × 16 ft | 397 sq ft | 2.27 | 3 gallons |
| 14 × 16 ft | 429 sq ft | 2.45 | 3 gallons |
| 20 × 20 ft | 589 sq ft | 3.37 | 1 five-gallon bucket |
A quart covers roughly a quarter of what a gallon does, about 60–100 sq ft for one coat. When the math leaves you just over a whole gallon, a quart is usually enough to finish; anything more and a full gallon is generally the better buy.
How many coats do you need?
| Situation | Typical approach |
|---|---|
| Same or similar color over clean, sound paint | 1–2 coats |
| New color of similar lightness | 2 coats |
| Dramatic change (dark to light, or a deep color) | Primer (often tinted) + 2 coats |
| New, unpainted drywall or patched areas | Drywall primer + 2 coats |
| Water stains, smoke or marker | Stain-blocking primer + 2 coats |
| Glossy existing paint | Scuff-sand, bonding primer + 2 coats |
"Paint and primer in one" products can save a coat on previously painted walls in good condition. They are not a substitute for a dedicated primer on bare drywall, stains or slick surfaces, and most manufacturers still recommend a separate primer in those cases.
Why coverage varies
The coverage on the label assumes a smooth, previously painted surface. You will use more paint on new drywall, textured walls (orange peel or knockdown), bare wood and masonry. Thick roller covers hold more paint, and spraying uses more than rolling because of overspray. If any of those apply, enter a coverage figure toward the low end of the range on the can, such as 250–300 sq ft per gallon.
Choosing a sheen
- Flat or matte: ceilings and low-traffic rooms. Hides wall flaws best but is harder to scrub.
- Eggshell or satin: living rooms, bedrooms and hallways. A good balance of washability and low shine.
- Semi-gloss: trim, doors, kitchens and bathrooms, where you need to wipe surfaces often and resist moisture.
Buying tips
- Buy it all at once. Tinted paint can vary slightly from batch to batch. If you need several gallons of one color, pour them together into a larger bucket before you start (called boxing) so every wall matches.
- Measure trim separately. Doors, casings and baseboards are usually painted in a glossier finish. A door is about 21 sq ft per side; if the trim total is small, a quart or two is often enough.
- Keep a little extra. Save a labeled quart for touch-ups and write the color formula on the lid.
- Doing more than one room? Add up the paintable area of each room and use the total-area mode, or use our square footage calculator for odd-shaped spaces.
- Hanging new walls first? Start with the drywall calculator for sheets, tape and compound, then plan on primer plus two coats of paint.
Frequently asked questions
How much paint do I need for a 12×12 room?
About 2 gallons for two coats on the walls. With 8 ft walls, one door and two windows, a 12×12 room has about 333 sq ft to paint. Two coats at 350 sq ft per gallon works out to 1.9 gallons. Painting the ceiling adds about one more gallon of ceiling paint.
How many square feet does a gallon of paint cover?
Most interior wall paints list about 250–400 sq ft per gallon for one coat, and 350 sq ft is a common figure. Textured, porous or unprimed surfaces fall at the low end. A quart covers roughly 60–100 sq ft.
Do I really need two coats of paint?
Usually, yes. A second coat evens out color and sheen and hides roller marks. One coat can work when you are repainting the same or a very similar color over clean paint in good condition.
Should I subtract doors and windows when estimating paint?
Yes for an accurate count. A typical door takes about 21 sq ft off the wall area and a typical window about 15 sq ft. In a small room the difference is often less than a quart, so some painters skip the deduction and keep the extra for touch-ups.
How much primer do I need?
Usually one coat over the full area you are painting. Divide that area by the coverage on the primer label, which is often about 200–400 sq ft per gallon. New drywall and bare wood absorb more, so plan toward the low end. The calculator adds this when you turn on the primer option.
Is a 5-gallon bucket of paint cheaper than single gallons?
Often, per gallon. If you need 4 or more gallons of one color, compare the bucket price with four or five single gallons. A single container also guarantees the color is consistent from wall to wall.
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