Paint Calculator
Stop guessing at the paint counter. Enter your room dimensions — doors, windows, and coats are subtracted automatically — and this free paint calculator tells you exactly how many gallons of paint (and primer) to buy. Works for walls, ceilings, and whole rooms. Runs entirely in your browser.
Coverage rates are manufacturer averages. Textured, porous, or unpainted surfaces drink more paint — when in doubt, buy one extra gallon; unopened cans are returnable at most stores.
Frequently Asked Questions
How much paint do I need per square foot?
Most interior latex paint covers about 350 sq ft per gallon for one coat. So divide your paintable wall area by 350, then multiply by the number of coats. A standard 12×10 bedroom (8-ft ceilings, 1 door, 2 windows) has roughly 300 sq ft of paintable wall — about 1 gallon per coat, 2 gallons for two coats.
How do I calculate paint for a room?
Add up the lengths of all four walls (that's the room's perimeter), multiply by the ceiling height, then subtract about 20 sq ft for each door and 15 sq ft for each window. Divide the result by your paint's coverage rate (usually 350 sq ft per gallon) and multiply by coats. Always round up to the next whole gallon — or use the calculator above.
Do I need primer?
You need primer for new/unpainted drywall, patched areas, bare wood, or when making a big color jump (dark to light). If you're just refreshing the same color, skip it. One gallon of primer typically covers about 300 sq ft — slightly less than paint.
How many coats of paint are best?
Two coats is the standard for a durable, even finish, especially with color changes. One coat can work for a same-color refresh with quality paint. Three coats is rarely needed except on very porous surfaces or dramatic color shifts.
How much does it cost to paint a room?
For DIY, budget about $30–$70 per gallon of decent interior paint. A 12×10 bedroom needing 2 gallons runs roughly $60–$140 in paint alone, plus $30–$60 for primer if needed, brushes, rollers, and tape.
Does ceiling height change the estimate?
A lot. Going from 8-ft to 9-ft ceilings adds 12.5% more wall area, and 10-ft ceilings add 25%. This calculator uses your actual ceiling height, so vaulted or tall ceilings are handled correctly.