How Much Paint Do I Need? A Painter's Estimating Method
Updated October 18, 2026 · 5 min read
Most interior paints cover roughly 350-400 sq ft per gallon per coat on smooth, primed walls. Here's how painters turn a room into gallons without running short or overbuying.
Walls
Wall area = room perimeter x wall height. Subtract about 21 sq ft per standard door (3 x 7 ft) and about 15 sq ft per average window.
Ceilings and coats
Ceiling area = length x width. Most repaints need two coats; new drywall needs a coat of primer first. Multiply the area by the number of coats.
Coverage
Use the coverage on the can - typically 350-400 sq ft per gallon. Rough, textured or porous surfaces and big color changes cover less; plan for the low end.
Worked example: a 12 x 14 ft bedroom
Walls: perimeter 52 ft x 8 ft = 416 sq ft, minus one door (21) and two windows (30) = 365 sq ft. Two coats = 730 sq ft. At 350 sq ft per gallon that's 2.1 gallons - buy 2 gallons plus a quart, or 3 gallons if you want touch-up paint.
Ceiling: 12 x 14 = 168 sq ft, two coats = 336 sq ft - 1 gallon.
| Surface | Area | Coats | Gallons (at 350 sq ft) |
|---|---|---|---|
| Walls | 365 sq ft | 2 | 2.1 |
| Ceiling | 168 sq ft | 2 | 1.0 |
Trim and doors
Measure baseboard, casing and crown in linear feet; a quart usually covers the trim in a typical room. Count doors as one or two sides each.
Sentinel Nexus measures every room you draw or scan, then works out gallons by color, prep and crew-days for the estimate.
Estimate a room freeFrequently asked questions
How many square feet does a gallon of paint cover?
Usually 350-400 sq ft per coat on smooth, primed surfaces. Always check the manufacturer's coverage rate.
Do I need primer?
Use primer on new drywall, patches, stains and big color changes. Self-priming paints help, but bare drywall still needs a real primer coat.
Should I subtract windows and doors?
Yes for walls - it keeps you from overbuying on rooms with lots of openings.