Paint Calculator
Work out how many gallons of paint you need for a room. Enter the room size, doors, windows and number of coats to get wall, ceiling and total paint quantities.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Total paint needed
- 1.91 gal
- Buy 2 gallons for 2 coats
- Wall paint
- 1.91 gal
- 334 sq ft of wall after deducting openings; buy 2 gallons. Primer is a separate purchase
- Paintable area per coat
- 334 sq ft
Paint formula
Wall area = 2 x (Length + Width) x Height - (Doors x 20) - (Windows x 15)
Ceiling area = Length x Width
Gallons = (Area x Coats) / Coverage per gallon (typically 350 to 400 sq ft)Worked example
A 14 x 16 ft bedroom with 9 ft ceilings, two doors and three windows: 2 x 30 x 9 = 540 sq ft, minus 40 for doors and 45 for windows = 455 sq ft of wall. Two coats at 350 sq ft per gallon: 455 x 2 / 350 = 2.6 gallons, so buy 3 gallons.
Adding the 224 sq ft ceiling needs another 1.3 gallons for two coats, so buy 2 gallons of ceiling paint.
Quick reference
| Room size | Wall area | Gallons needed | Gallons to buy |
|---|---|---|---|
| 10 x 10 ft | 270 sq ft | 1.54 | 2 |
| 12 x 12 ft | 334 sq ft | 1.91 | 2 |
| 12 x 15 ft | 382 sq ft | 2.18 | 3 |
| 15 x 20 ft | 510 sq ft | 2.91 | 3 |
| 20 x 20 ft | 590 sq ft | 3.37 | 4 |
How to calculate paint for a room
Add the lengths of all walls to get the perimeter, then multiply by ceiling height for gross wall area. A 12 by 12 room with 8 ft ceilings has 48 ft of wall by 8 ft, or 384 square feet. Subtract about 20 square feet per door and 15 per window.
Divide the paintable area by the coverage on the can (usually 350 to 400 square feet per gallon) and multiply by coats. Textured or knockdown walls run 250 to 300, so set the coverage field to match. Two coats is standard for a color change or new drywall. Paint-and-primer is a marketing term for thicker paint: new drywall still needs PVA primer, and patches need spot priming or they flash.
Tips for buying the right amount
Primer is separate. New drywall gets PVA primer; deep reds, yellows and oranges go over a gray-tinted primer, which turns four coats into two; water stains need a stain-blocking primer. Ceiling paint is a different product, a dead flat that hides roller laps.
Over 3 gallons buy a 5 gallon bucket: one batch, cheaper per gallon. Box separate gallons into one bucket before starting. Custom tint is not returnable, so test a sample quart on a 2 ft square first. Touch-ups on eggshell, satin and semi-gloss flash, so a patch means repainting corner to corner.
What this calculator leaves out
Trim, doors and casings are not counted. Trim takes about a quart per 100 square feet and each door side about 20 square feet. A room with one door, base and casing needs about a quart of trim enamel per coat. Primer, ceiling paint, caulk, spackle and drop cloths are separate line items.
Mistakes that cost money
- Skipping PVA primer on new drywall: paper and mud absorb differently, joints flash through two coats and you buy a third gallon.
- Deep red or yellow straight over white takes four coats; a gray-tinted primer gets it done in two and any paint store will tint it.
- Four single gallons cost more than one 5 gallon bucket and come from different batches; the shade shift shows where cans changed.
- Custom tint mixed before a sample is not returnable; buy a sample quart and test a 2 ft square in daylight first.
- Touching up eggshell, satin or semi-gloss from the leftover quart flashes; a scuffed wall in those sheens gets repainted corner to corner.
- Wall paint on a ceiling shows every roller lap; ceiling paint is a dead flat made to hide them and costs less.
Key facts
- One gallon of interior paint covers about 350 to 400 sq ft per coat on smooth walls.
- A standard interior door is deducted as 20 sq ft and an average window as 15 sq ft.
- Two coats are standard for color changes and new drywall; deep reds and yellows over white need a gray-tinted primer or a third and fourth coat.
- A quart covers about 90 to 100 sq ft; trim takes about a quart per 100 sq ft and each door side about 20 sq ft.
Frequently asked questions
How much paint do I need for a 12x12 room?
For walls only with 8 ft ceilings, one door and two windows, you need 1.91 gallons for two coats, so buy 2 gallons. The ceiling takes another 0.82 gallons, one separate gallon of flat ceiling paint.
How many square feet does a gallon of paint cover?
Roughly 350 to 400 square feet per coat on smooth, previously painted walls. Textured walls run 250 to 300 and unprimed drywall or patches soak up more.
Do I need primer, or is paint-and-primer enough?
Paint-and-primer is a marketing term for a thicker paint. New drywall still needs a PVA primer, patches need spot priming or they flash, and a deep red or yellow over white needs a gray-tinted primer. Over a sound wall in a similar color, two coats with no separate primer is fine.
Sources and references
- How to use the paint calculator: coverage per gallon (Sherwin-Williams)
- How much interior paint to buy (Behr)
- Paint calculator for rooms, doors and windows (Benjamin Moore)
Next steps for this project
The usual order for a paint & walls project. See all paint & walls calculators.
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.