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Paint Calculator
One gallon of paint covers less wall than the number on the can suggests. Put in the room and the coats you want, and you'll get the gallons, the primer, and a price range for both brushing it yourself and having it done.
- Gallons for any number of coats
- Primer where you need it
- Paint or painters, priced
Project measurements
Where are you painting? Inside or out
Walls, and the ceiling if you are doing that too.
Measurements Perimeter comes from length and width
Are you doing the ceiling?
What are you painting over? This is what sets coverage
Coats and paint Coats multiply the whole estimate
Allowance
How this number is built
With paint, the amount people expect is usually less than the amount they need.
A gallon covers about 350 sq ft, in theory
That is the published rate for interior latex on smooth primed drywall, and every manufacturer says the same thing about it: it is theoretical coverage at recommended film thickness. Real walls are rougher, so the same can covers less. Benjamin Moore's own sheets give 400–450 for a top-line interior paint; Behr publishes 250–400 and puts the low end on porous or unpainted surfaces.
A 12 × 12 room takes 2 gallons, not 1
The walls are 48 ft around × 8 ft high = 384 sq ft, minus about 50 sq ft for a door and two windows, so 334 sq ft of wall. Two coats over that is 668 sq ft, which at 350 is 1.9 gallons. You cannot buy 0.9 of a can.
Plan on two coats
Coverage rates and warranties are both written on the assumption of two coats. One is a legitimate choice only for a same-color refresh; a color change or a repair needs two plus spot-priming. You can pick one, two, or three here.
The prep is the part that is easy to skip
Fill the nail holes and dings, sand them flush, and spot-prime the patches before the finish coat count starts. Unprimed patch compound dries chalky and shows through a finish coat even after two of them. That is how a careful paint job ends up looking patchy in low sunlight.
Paint is the small part of a painting bill
Paint is only 10–20% of what a painter charges, which runs $2–6 per sq ft of wall. The rest is labor and prep. If you hire it out, most of the price pays for prep.
What a gallon actually covers
| Coverage | 1 coat | 2 coats | Typical surface |
|---|---|---|---|
| 250 sq ft/gal | 250 sq ft | 125 sq ft | Rough, porous, or unprimed |
| 300 sq ft/gal | 300 sq ft | 150 sq ft | Exterior siding |
| 350 sq ft/gal | 350 sq ft | 175 sq ft | Interior, previously painted |
| 400 sq ft/gal | 400 sq ft | 200 sq ft | Smooth, primed |
| 450 sq ft/gal | 450 sq ft | 225 sq ft | Top-line interior paint, ideal surface |
The 2-coats column is the one that matters: at 350 sq ft/gal, a gallon only covers 175 sq ft of finished wall. Multiply your wall area by the number of coats first, then divide by coverage. Do it in that order. Sheen: flat for ceilings and low-traffic walls, eggshell or satin for most walls, semi-gloss for trim, doors, kitchens, and bathrooms.
Painting new drywall? The drywall calculator works out the sheets, mud, and tape for the room you are about to paint, so the finishing coat count and the board count stay in step.
Where these numbers come from
- From the manufacturers' technical data sheets: Benjamin Moore Regal Select Interior Matte 548 (400–450 ft²/gal), Benjamin Moore Regal Select Exterior High Build Flat N400 (250–350), Glidden Essentials Exterior Semi-Gloss GLE-7011N (300–400 smooth nonporous), Behr published guidance (250–400), KILZ Complete Coat, and Zinsser Bullseye primers.
- All of those figures are theoretical, at recommended film thickness, and both Benjamin Moore and Glidden say so on their own sheets. Sherwin-Williams' method (theoretical coverage from percentage volume solids, minus an application loss factor) is the textbook version; this page cites the principle rather than deriving coverage from solids data.
- Two coats is the manufacturer-recommended standard for most products and the consumer-guide consensus; one coat is legitimate only for same-color refreshes.
- 2026 retail paint cost guides. Secondary sources only, and promotional: Sherwin-Williams runs 30 to 40% off list every four to six weeks and contractor accounts discount deeper. Paint has no trade-association price index, the same limitation mulch has.
- Conventional estimating practice rather than a published standard. Behr's own calculator takes doors and windows as inputs, but publishes no canonical area per opening.
The other nine calculators
Every material, same layout, its own crew.
