How Much Paint Do I Need? The Short Answer
The formula pros use is simple. Add up the wall area of the room, subtract the area of doors and windows, multiply by the number of coats, and divide by the coverage printed on the can (about 350 square feet per gallon for interior paint):
Gallons = (wall area - doors - windows) x coats / 350
A standard door eats about 20 square feet, a typical window about 15. For quick planning, most bedrooms need 2 gallons, most living rooms need 3 to 4 gallons, and a small bathroom needs just 1 gallon, all for two coats. The calculator above applies the full version of this math, including the ceiling, trim, and primer, so you buy exactly what you need on the first trip.
How to Calculate Paint for a Room: Step by Step
Let us walk through a 12 by 12 foot bedroom with 8-foot ceilings, one door, and one window, painted with two coats:
- Wall area: perimeter x height = (12 + 12 + 12 + 12) x 8 = 384 sq ft
- Subtract openings: 384 - 20 (door) - 15 (window) = 349 sq ft
- Two coats: 349 x 2 = 698 sq ft of coverage needed
- Gallons: 698 / 350 = 1.99, so buy 2 gallons
If you are measuring in meters, convert to feet first with our unit converter, since paint coverage in the US is always stated per gallon in square feet. Want to add a 10 percent waste buffer for touch-ups? Work out the exact bump with our percentage calculator before you round up.
Paint Coverage by Finish: Flat vs Eggshell vs Satin
The sheen you choose changes both the look and the practical coverage. Flatter paints hide wall imperfections but scuff easily; glossier paints wipe clean but highlight every bump in the drywall.
| Finish | Best for | Coverage |
| Flat / matte | Ceilings, low-traffic bedrooms | 350 to 400 sq ft/gal |
| Eggshell | Living rooms, dining rooms | 350 to 400 sq ft/gal |
| Satin | Kids' rooms, hallways | 350 to 400 sq ft/gal |
| Semi-gloss | Trim, doors, kitchens, bathrooms | 350 to 400 sq ft/gal |
Major US brands like Sherwin-Williams, Behr, and Benjamin Moore all print their coverage on the label, and it usually lands in this range on smooth primed drywall. Textured walls, bare drywall, and dramatic color changes can pull real-world coverage down to 250 to 300 square feet per gallon, which is why the calculator lets you adjust the coverage number.
Primer: When You Need It and When You Can Skip It
Primer seals porous surfaces so your topcoat goes on evenly instead of soaking in unevenly. You need it on bare drywall, bare wood, patched or spackled areas, water stains, and any dramatic color change, especially dark to light. One gallon of primer covers about 300 square feet. You can skip separate primer if you are repainting a similar color over a sound, previously painted wall, or if you buy a quality paint-and-primer-in-one for a minor color shift. Big color changes still deserve a tinted primer: it is cheaper than the extra topcoat it replaces.
Common Room Sizes: Quick Gallons Guide
For two coats on walls with standard doors and windows, at 350 sq ft per gallon:
| Room | Wall area | Gallons (2 coats) |
| Small bathroom (8x8) | 200 sq ft | 1 gallon |
| Bedroom (10x10) | 285 sq ft | 2 gallons |
| Bedroom (12x12) | 349 sq ft | 2 gallons |
| Kitchen (12x14) | 400 sq ft | 2 to 3 gallons |
| Living room (16x20) | 800 sq ft | 4 to 5 gallons |
| Open-plan space | 1,600 sq ft | 8 to 9 gallons |
Buying paint is one of the few home projects where five minutes of math saves a real trip back to the store. Enter your exact room above and you will get wall gallons broken into gallons and quarts, separate ceiling and trim paint, primer if you need it, a total cost estimate, and a supply checklist so the rollers and tape come home in the same run. Painting an exterior or a masonry surface too? Estimate the concrete and base material for any prep work with our concrete calculator.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12x12 room with 8-foot ceilings has about 384 square feet of wall area. After subtracting one door (20 sq ft) and one window (15 sq ft), you get roughly 349 sq ft. At 350 sq ft per gallon, that is 1 gallon per coat, so 2 gallons for the standard two coats.
How many square feet does a gallon of paint cover?
One gallon of interior paint covers about 350 to 400 square feet per coat on smooth, primed drywall. Textured walls, bare drywall, or dark-to-light color changes can drop coverage to 250 to 300 square feet per gallon.
Do I need one or two coats of paint?
Plan for two coats. One coat is only enough when painting over a very similar color with quality paint on a well-prepped wall. Two coats give even color, better durability, and the finish the manufacturer intended.
How much paint do I need for a ceiling?
Multiply the room's length by its width to get the ceiling area, then divide by 350 square feet per gallon. A 12x12 room has a 144 sq ft ceiling, which needs about half a gallon per coat, so buy 1 gallon for two coats.
When do I need primer?
Use primer on bare drywall, bare wood, patched or repaired areas, stains, and when making a dramatic color change (especially dark to light). One gallon of primer covers about 300 square feet.
Last updated: 2026 | Coverage figures reflect typical US interior latex paint on smooth drywall. This calculator is for estimation only.