Paint Calculator
Find out exactly how much paint your room needs — enter dimensions and coats to get gallons required and cans to buy.
Stop guessing at the paint store. This free paint calculator turns your room dimensions, wall height, and number of coats into gallons needed and whole cans to buy, using the standard 350 sq ft per gallon coverage rate. Works for 1 to 4 walls.
How to use this calculator
- Step 1: Enter how many walls you are painting (1-4) and the room length, width, and wall height in feet.
- Step 2: Choose the number of coats — 2 is standard for a color change, 1 for same-color repaints.
- Step 3: Keep the default 350 sq ft per gallon coverage, or adjust it if your paint label lists a different rate.
- Step 4: Click Calculate to see your total wall area, gallons needed, and the number of whole gallon cans to buy.
- Step 5: Buy the cans listed — paint is sold in whole gallons, so the calculator always rounds up.
The formula
Wall area = 2 x (length + width) / 4 x walls x height. Gallons = wall area x coats / 350.
Worked example
A 12x15 ft room with 8 ft ceilings has 432 sq ft of wall area. Two coats at 350 sq ft per gallon need 2.47 gallons, so you buy 3 gallon cans. Painting just one accent wall (108 sq ft) needs only 0.62 gallons — a single gallon covers it.
Frequently Asked Questions
How much paint do I need per room?
A gallon of interior paint covers about 350 sq ft per coat. Measure your wall area, multiply by the number of coats (usually two), and divide by 350. For a 12x15 room with 8 ft ceilings and two coats, that is 432 x 2 / 350 = about 2.5 gallons, so you buy 3 gallons.
Should I subtract doors and windows from the wall area?
Most painters do not bother for average rooms — the paint you save on openings roughly equals what you lose to cutting in, roller waste, and touch-ups. For rooms with very large windows, sliding doors, or built-ins, subtract their area to avoid overbuying. This calculator gives the unadjusted number, which is the safe side to err on.
Do I need primer, or is paint-and-primer enough?
Use a dedicated primer on new drywall, patched areas, bare wood, stains, or drastic color changes (dark to light). Paint-and-primer-in-one works fine for repainting a similar color over sound existing paint. Primer also seals porous surfaces so your topcoat coverage matches the 350 sq ft per gallon estimate.
How many coats of paint should I apply?
Two coats is the professional standard for full, even color and durability. One coat is only fine for touch-ups or repainting the exact same color. Going from dark to light often needs a primer plus two coats. Cheap paint sometimes needs three coats, which usually costs more than buying better paint in the first place.
Does textured or rough wall surface change how much paint I need?
Yes. Heavy texture, stucco, or rough plaster can add 10-20% to your paint usage because the surface area is larger than the flat measurement. If your walls are heavily textured, buy one extra gallon beyond what the calculator says. Smooth drywall follows the standard 350 sq ft per gallon rate.