Paint Calculator: How Much Paint Do I Need?
Enter your room measurements and get an instant, brand-neutral estimate of how many gallons of paint and primer to buy. The calculator subtracts doors and windows and factors in the number of coats — no sign-up, no guesswork.
Paint Calculator
Enter your room measurements below. The calculator subtracts doors and windows, accounts for the number of coats, and tells you how many gallons to buy — for any paint brand.
How to use this paint calculator
Measure the length and width of your room in feet, plus the ceiling height (8 ft is standard). Enter how many doors and windows are in the room — the calculator removes their area so you don't overbuy. Choose your number of coats (two is typical), tick the ceiling or primer boxes if they apply, and press calculate. You'll get a clear "buy this much" result you can print and take to the store.
How to measure a room for paint
For a standard rectangular room, length and width are all you need — the calculator works out the wall area as 2 × (length + width) × ceiling height. For an L-shaped or irregular room, split it into rectangular sections, run the calculator once for each section, and add the gallon totals together. Don't worry about small alcoves or closets you're not painting; the touch-up buffer comfortably covers minor variations.
How much paint do I need for one coat vs. two?
One gallon of quality wall paint covers roughly 350–400 square feet per coat. Most repaint jobs need two coats for an even finish and full color coverage, so the calculator defaults to two. You can usually get away with a single coat only when you're repainting the same color over a clean, primed surface. When in doubt, plan for two.
Do I need primer? (and how much)
Primer seals the surface so your finish paint adheres evenly and the true color shows. Use it when painting over bare drywall, spackled patches, glossy or oil-based paint, water stains, or a dramatic color change (especially dark to light). If you're refreshing a similar color over a sound painted wall, you can often skip it. Tick the primer box and the calculator estimates a single coat at about 300 sq ft per gallon.
How much paint for ceilings, trim, and doors
To paint the ceiling, tick "Include the ceiling" — its area (length × width) is added to your total. Trim, baseboards, and doors use a different product (usually a semi-gloss) and a small amount goes a long way: a single quart of trim paint typically covers the trim in an average room. We'll add dedicated trim and door calculators soon — for now, a quart per room is a safe starting point.
Paint coverage rates explained
"Coverage" is how much area one gallon spreads over at the proper film thickness. The 350 sq ft/gallon default is a safe, common figure. Real-world coverage drops on porous surfaces (new drywall, bare wood), textured walls (knockdown, popcorn), and when going light over dark. It rises slightly on smooth, previously painted walls. Your paint can lists its own coverage rate — enter it under the Advanced option for the most accurate result.
Example: painting a 12×12 room
A 12 ft × 12 ft room with 8 ft ceilings has 2 × (12 + 12) × 8 = 384 sq ft of wall. Subtract one door (21 sq ft) and two windows (15 sq ft each) and you're left with about 333 sq ft of paintable wall. Two coats means roughly 666 sq ft of coverage — at 350 sq ft per gallon, that's 2 gallons of paint. Add the ceiling and you'd bump up to about 3 gallons.
Frequently asked questions
How much paint do I need for a 12×12 room?
A 12 ft × 12 ft room with 8 ft ceilings has about 384 sq ft of wall area. After subtracting one door and two windows you have roughly 330 sq ft of paintable wall. For two coats that is about 660 sq ft, so you would buy 2 gallons of paint (at 350 sq ft per gallon).
How many square feet does a gallon of paint cover?
Most interior wall paints cover 350 to 400 square feet per gallon for a single coat. Textured, porous, or dark-to-light surfaces cover less. Always check your specific paint can — the coverage figure is printed on the label — and use the Advanced option in the calculator to enter it.
How much paint do I need for one coat?
Set the "Number of coats" field to 1. As a rule of thumb, one coat of paint covers 350–400 sq ft per gallon, but most jobs need two coats for an even, durable finish — especially over a different color or over fresh primer.
Do I need primer, and how much?
Use primer when you are painting over bare drywall, patched areas, glossy surfaces, stains, or a big color change. Tick the "I also need primer" box and the calculator estimates one coat of primer at about 300 sq ft per gallon — primer is slightly less spreadable than finish paint.
How much paint do I need for a ceiling?
Tick the "Include the ceiling" box. The calculator adds the ceiling area (length × width) to your paintable surface. A 12×12 ceiling is 144 sq ft, so two coats add about 288 sq ft — roughly one extra gallon.
Is it cheaper to buy gallons or quarts?
Per square foot, gallons are almost always cheaper than quarts. The calculator recommends gallons first and only suggests a quart when you need a small amount on top of a whole number of gallons. For touch-ups, a quart is fine.
Should I buy extra paint?
Yes — buy a little more than the estimate. A bit of extra paint covers touch-ups, missed spots, and future repairs, and it guarantees a color match. Paint from a later batch can differ slightly even with the same color code.
More material calculators
Priming first? Use the primer calculator. Painting outside? Try the exterior paint calculator or the deck stain calculator. You'll also find a wallpaper calculator and a flooring calculator. See all tools ›