Paint Calculator
How much paint do you really need? Add your rooms, pick your coats, and get gallons for walls, ceiling, trim and primer, the cheapest mix of quarts, gallons and buckets, and a shopping list.
How this paint calculator works
Walls: for each room, 2 × (length + width) × wall height, minus 20 sq ft per door and 15 sq ft per window. Ceiling: length × width. Trim: total length × trim width ÷ 12.
Gallons: area × number of coats ÷ coverage per gallon. Primer is one coat at its own coverage rate.
Smart rounding: the calculator tries every combination of 5-gallon buckets, gallons and quarts (up to 3 quarts) that covers the amount you need, then picks the cheapest one using the prices you entered. That is why you might see "2 gallons + 1 quart" instead of 3 gallons.
Where the numbers come from
- Coverage: manufacturers publish roughly 350–400 sq ft per gallon for interior paint on smooth, prepared surfaces. See the product data sheets and paint calculators at Sherwin-Williams, Behr and Benjamin Moore, and the overview in Bob Vila's paint guide. Behr publishes a wider 250–400 range depending on the surface. We default to 350, the cautious end. Rough, bare or heavily textured surfaces cover less.
- Door and window deductions: 20 sq ft (a 3 ft × 6 ft 8 in door) and 15 sq ft (a 3 ft × 5 ft window). These are common painter's rules of thumb, not manufacturer figures.
- Primer coverage (300 sq ft/gal): a conservative assumption. Many primers list 300–400 sq ft per gallon, so check your can.
- Prices: typical retail ranges only. Brand, sheen and store all change them.
Tips
- Buy all the paint for one room at the same time, and have it mixed together so the color matches.
- Dark-to-light or light-to-dark color changes often need primer or a third coat.
- Keep a little leftover for touch-ups. Write the room and color name on the lid.
Estimates only. Always read the coverage on the can you buy.
Estimates only — not engineering, legal or code advice. Check requirements with your local building department and a licensed professional before you build.
Common questions
How much paint do I need for a 14 × 12 room with 8-foot walls?
With one door (20 sq ft) and two windows (15 sq ft each) taken out, the walls are about 366 sq ft. Two coats is 732 sq ft, which at 350 sq ft per gallon is about 2.1 gallons, so plan on 2 gallons plus a quart, or 3 gallons if you want a safety margin.
How many square feet does a gallon of paint cover?
Most interior paint labels say 350 to 400 square feet per gallon on smooth, previously painted walls. Rough, textured or bare drywall can soak up more, so this calculator defaults to the conservative 350. Check the label of the paint you choose.
Do I subtract doors and windows?
Yes, but only roughly. This tool takes off 20 sq ft for each door (about 3 ft × 6 ft 8 in) and 15 sq ft for each window (about 3 ft × 5 ft). The numbers are small, so rounding up to the next quart or gallon usually covers any difference.
How much extra does a ceiling add?
A ceiling is just length × width. A 14 × 12 room has 168 sq ft of ceiling, which is about 1 gallon for two coats at 350 sq ft per gallon.
Do I need primer?
Use primer on bare drywall, patched spots, stains, or when you are making a big color change (dark to light). The primer option here adds one coat over the walls (and ceiling if included) at 300 sq ft per gallon, a conservative figure for typical primers.
When is a 5-gallon bucket cheaper?
Usually when you need more than about 4 gallons of one product. The calculator tests every mix of buckets, gallons and quarts using your prices and picks the cheapest one that covers the job. The bucket and quart prices are assumptions you can edit.