Paint Calculator & Room Measuring Guide
Estimate the gallons you'll need in under a minute — then read the guide so your numbers actually match what you buy.
Buying the right amount of paint is the difference between one relaxing weekend and three trips to the store. Use the calculator below for a fast estimate, then read the guide so you know exactly what to measure, what to subtract, and where homeowners most often come up short.
Paint Calculator
Enter each room's dimensions to estimate the gallons of paint you'll need. Assumes ~350 sq ft coverage per gallon and subtracts standard doors (21 sq ft) and windows (15 sq ft).
Estimated paint needed
Estimates only. Textured walls, deep color changes, and bare drywall can push coverage 15–25% higher — when in doubt, round up so you don't run out mid-wall.
How to measure a room for paint
Grab a tape measure, a notepad, and a phone calculator. You want three numbers per room: length, width, and ceiling height. Round each measurement up to the nearest foot — paint is sold in gallons, not fractions of a square foot.
- Measure the length and width of the room along the floor.
- Measure the ceiling height from floor to ceiling.
- Add up all four wall lengths (the perimeter) and multiply by the height — that's your gross wall area.
- Count the doors and windows in the room.
- Subtract 21 sq ft for each standard door and 15 sq ft for each standard window.
- Divide the result by 350 to get gallons per coat, then multiply by the number of coats.
The paint coverage formula
Every paint calculator on the internet uses some version of this formula. Now you can too:
A gallon of interior latex covers about 350 sq ft in one coat on smooth, primed drywall. Textured walls, freshly patched drywall, and dark-to-light color changes reduce that number, so drop your assumed coverage to 300 sq ft in those cases.
Don't forget the ceiling and trim
Ceiling paint is a separate product from wall paint and is estimated on square footage (length × width). A 12x12 room needs about half a gallon of ceiling paint per coat.
Trim, doors, and baseboards use surprisingly little paint — one gallon of enamel typically covers all the trim in three average rooms with two coats. Buy one gallon per floor of the house as a rule of thumb.
When to buy more than the calculator says
- Dramatic color changes (dark to light, or over deep reds and blues) — plan on a third coat
- Textured walls like orange peel or knockdown — cut coverage by 20%
- Freshly patched or new drywall — the joint compound drinks primer and paint
- Vaulted or two-story ceilings — measure the actual wall height, not 8 feet
- Cabinets and doors — always buy an extra quart for touch-ups
Rather skip the math?
We provide free, same-day estimates on interior painting projects across the Omaha metro — including exact paint quantities, product recommendations, and a fixed total price. No spreadsheets required.
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