DIY Home CalcsPaint Calculator
Find out how many gallons of paint you need for a room. Enter the room size, count the doors and windows, choose the number of coats, and the calculator rounds the result to the cans you can actually buy.
How the paint calculator works
Paint is sold by coverage, so the job is to work out the surface area you will roll and divide it by how far one gallon goes. The calculator does it in four steps:
- Wall area. The perimeter of the room, 2 × (length + width), times the wall height.
- Openings. It subtracts 20 sq ft for each door (a typical 3 ft × 6 ft 8 in door) and 15 sq ft for each window (about 3 ft × 5 ft).
- Coats. The remaining area is multiplied by the number of coats.
- Coverage. The total is divided by the coverage figure on your paint can, then rounded up to whole gallons plus quarts.
Worked example
A 12 ft × 12 ft bedroom with 8 ft walls, one door and one window:
- Perimeter: 2 × (12 + 12) = 48 ft, so the wall area is 48 × 8 = 384 sq ft.
- Minus openings: 384 − 20 − 15 = 349 sq ft.
- Two coats: 349 × 2 = 698 sq ft.
- At 350 sq ft per gallon: 698 ÷ 350 ≈ 1.99 gallons, so buy 2 gallons.
If you add the 144 sq ft ceiling with two coats, that is another 288 sq ft, or about 0.82 gallon. Ceilings are usually a different flat white paint, so buy that separately: one gallon covers it.
What changes your coverage
The number printed on the can assumes a smooth, sealed surface and a paint applied at the recommended thickness. Real rooms vary, so adjust the coverage field before you calculate:
| Surface | Typical coverage per gallon | What to enter |
|---|---|---|
| Previously painted, smooth drywall | 350–400 sq ft | 350–400 |
| New drywall (after primer) | 300–350 sq ft | 300 |
| Textured walls, orange peel or knockdown | 250–300 sq ft | 275 |
| Bare plaster, brick or block | 200–250 sq ft | 225 |
| Primer on new drywall | 200–300 sq ft | 250 |
Ranges are typical values printed by major paint brands; always check the technical data sheet for your exact product.
Tips for buying the right amount
- Big color changes need more paint. Going from dark red to pale gray may take a tinted primer plus two coats. Add a primer coat to your plan instead of a third finish coat; primer is cheaper and hides better.
- Buy enough of one batch. Tinted paint can vary slightly between mixes. If the estimate is close to the next gallon, round up, or "box" (mix together) all your cans before you start.
- Trim is separate. Baseboards, door casings and window trim are usually painted in semi-gloss. A quart covers the trim of an average bedroom.
- Keep a little back. Save a labeled jar of leftover paint for touch-ups. Paint keeps for years if the lid is sealed and it never freezes.
Planning the walls and floor together? Use the flooring calculator for the floor, or read how to measure a room's square footage if the room is not a simple rectangle.
Frequently asked questions
How much paint do I need for a 12x12 room?
With 8 ft walls, one door and one window, a 12x12 room has about 349 sq ft of wall. Two coats at 350 sq ft per gallon need about 2 gallons. Add roughly 0.4 gallon per coat if you also paint the ceiling.
How many square feet does a gallon of paint cover?
Most interior paint labels list 350 to 400 sq ft per gallon on smooth, primed walls. Rough, porous or unprimed surfaces such as new drywall, bare plaster or textured walls can drop that to 250 to 300 sq ft.
Do I really need two coats?
Usually yes. A second coat evens out sheen and hides roller marks. One coat can be enough when you repaint the same or a very similar color with a good paint-and-primer product.
Should I subtract doors and windows?
Yes for a tighter estimate. This calculator subtracts 20 sq ft per standard door and 15 sq ft per average window. If you have a large picture window or patio door, add extra windows to account for it.
Is it better to buy gallons or a 5-gallon bucket?
If the estimate is 4 gallons or more, a 5-gallon bucket is often cheaper per gallon and keeps the color perfectly consistent. Leftover paint is useful for touch-ups.