- Home
- Planning Tools
- Paint Calculator
Paint Calculator
Enter the room size and get the gallons of wall paint, ceiling paint and primer to buy, with every assumption visible and editable.
Your result
A planning estimate. Check it against the product label before you buy.
How the paint calculation works
The calculator measures the walls, removes the openings you will not paint, and divides by how far a gallon goes:
- Wall area = 2 × (length + width) × wall height.
- Minus openings: 21 sq ft for each door (a standard 3 ft × 7 ft opening) and 15 sq ft for each window (a typical 3 ft × 5 ft unit).
- Times the number of coats, divided by the coverage printed on the can.
- Rounded up to the next quart, because paint is sold in quarts and gallons. Where three quarts would be needed the result shows a full gallon instead.
Ceiling paint uses the floor area (length × width) with the same number of coats. Primer is figured as a single coat over everything you are painting.
A worked example
A 14 ft × 12 ft bedroom with 8 ft walls has 2 × (14 + 12) × 8 = 416 sq ft of wall. One door and two windows remove 21 + 30 = 51 sq ft, leaving 365 sq ft. Two coats is 730 sq ft of coverage; at 350 sq ft per gallon that is 2.1 gallons, so you would buy 2 gallons and 1 quart.
When to change the defaults
- Coverage. Use the figure on your can. Paint labels generally state a range between 250 and 400 sq ft per gallon; rough, textured or unprimed surfaces sit at the low end.
- Coats. Two coats is the usual assumption. A strong colour change (dark to light, or the reverse) may need primer or a third coat.
- Openings. If your room has a wide cased opening, French doors or a large picture window, count it as two or three doors or windows so the subtraction is closer to the real opening.
- Trim, doors and cabinets are not included; they are usually painted in a different finish and bought separately.
The result is a planning estimate, not a guarantee. Keep the receipt, and ask the store about returning unopened, untinted cans.
Questions about this calculator
How much paint do I need for a 12 × 12 room?
A 12 ft × 12 ft room with 8 ft walls, one door and two windows has about 333 sq ft of wall to paint. Two coats at 350 sq ft per gallon comes to roughly 1.9 gallons, so two gallons covers the walls.
How many square feet does a gallon of paint cover?
It depends on the product and the surface. Paint labels generally state between 250 and 400 sq ft per gallon. This calculator starts at 350 sq ft and lets you enter the figure printed on your can.
Do I need primer?
Primer is generally recommended over new drywall, patched or stained areas, glossy surfaces and big colour changes. Many wall paints are sold as paint-and-primer products for repainting a sound, previously painted wall. Follow the instructions on the product you buy.
Does the calculator store my measurements?
No. The calculation runs in your browser. Nothing you type is saved or sent anywhere.
One considered layer at a time.
New house plan reviews, before-and-after transformations and room styling notes. No noise, unsubscribe anytime.