To calculate square footage for painting, add up the room’s perimeter, multiply by the ceiling height, then subtract about 21 square feet per door and 15 square feet per window. Multiply by the number of coats (usually two) and divide by 350 — the safe coverage of one gallon of latex paint on smooth walls.
Paint is the rare project where the math directly saves money: every gallon you don’t need is $30–$70 you keep. The formula below works for any room, takes about five minutes with a tape measure, and ends with an exact gallon count — no guessing at the paint counter.
The Wall Area Formula for Paint
The formula is: (2 × (length + width)) × ceiling height − openings = net paintable wall square footage.
Broken into steps:
- Find the perimeter. Add the room’s length and width, then double it: 2 × (length + width).
- Multiply by the ceiling height. This gives gross wall area — every wall surface, openings included.
- Subtract openings. Subtract about 21 sq ft per standard door (3 ft × 7 ft) and about 15 sq ft per standard window (3 ft × 5 ft).
- Multiply by the number of coats. Two coats is the standard for a durable, even finish over any color change; one coat only works for same-color touch-ups.
- Divide by 350. One gallon of standard latex paint covers roughly 350 sq ft on smooth walls. Round up to whole gallons.
Why 21 and 15? A standard interior door is 3 × 7 ft = 21 sq ft. A typical window is about 3 × 5 ft = 15 sq ft. These are averages — if your windows are floor-to-ceiling, measure them individually — but for a normal bedroom or living room they’re accurate within a few percent, and paint is rounded up to whole gallons anyway.
Need the numbers before you start? Calcrio’s square footage calculator gives you the room’s base dimensions in seconds; the formula above takes it from there.
Worked Example: A 12×14 Bedroom
Take a bedroom that is 12 ft long, 14 ft wide, with 8-ft ceilings, one standard door, and one standard window. Two coats.
- Perimeter: 2 × (12 + 14) = 52 ft
- Gross wall area: 52 × 8 = 416 sq ft
- Subtract openings: 21 (door) + 15 (window) = 36 sq ft
- Net paintable walls: 416 − 36 = 380 sq ft
- Two coats: 380 × 2 = 760 sq ft of painting
- Gallons: 760 ÷ 350 = 2.17 → buy 3 gallons
So a typical 12×14 bedroom needs 3 gallons for two coats. You can check the arithmetic with a square footage calculator — enter the room dimensions and openings, and compare against your own figures before heading to the store.
Quick Gallons Reference by Room Size
The table below assumes 8-ft ceilings, two coats, one door, one window, and 350 sq ft per gallon. Find your room size for an instant estimate:
| Room size | Perimeter | Net wall sq ft (1 door, 1 window) | Two coats | Gallons to buy |
|---|---|---|---|---|
| 10×10 ft | 40 ft | 284 sq ft | 568 | 2 |
| 12×12 ft | 48 ft | 348 sq ft | 696 | 2 |
| 12×14 ft | 52 ft | 380 sq ft | 760 | 3 |
| 14×16 ft | 60 ft | 444 sq ft | 888 | 3 |
| 16×20 ft | 72 ft | 540 sq ft | 1,080 | 4 |
Math check: 10×10 → 40×8 = 320 − 36 = 284; 284×2 = 568; 568÷350 = 1.62 → 2 gallons.
Do You Count the Ceiling in Paint Square Footage?
Not in the walls number — ceilings are a separate calculation, and many rooms don’t need the ceiling painted at all. If you are painting the ceiling, its area is simply length × width (the room’s floor footprint). A 12×14 room has a 168-sq-ft ceiling, which at one coat ÷ 350 coverage needs one gallon.
Ceiling paint is usually a flat white bought separately — wall paint with any sheen looks wrong overhead, and most painters keep the two jobs in separate buckets.
How Wall Texture Changes Paint Coverage
The 350-sq-ft-per-gallon figure assumes smooth drywall. Texture drinks paint, so adjust the coverage divisor:
| Wall texture | Coverage per gallon | Notes |
|---|---|---|
| Smooth drywall | ~350 sq ft | Standard reference |
| Light knockdown / orange peel | ~300 sq ft | Add roughly 15% more paint |
| Heavy stucco or brick | ~250 sq ft | Add roughly 40% more paint |
| Unprimed / bare drywall | Prime first | Bare drywall absorbs unevenly; a primer coat fixes the coverage math |
For textured walls, the worked example above would change at the final step: 760 ÷ 300 = 2.53 → still 3 gallons. But on a larger room or with heavier texture, the divisor change can tip you into a fourth gallon — which is exactly why professionals measure instead of guessing.
One Coat or Two?
Plan for two coats. One coat is enough only for same-color refreshes over paint in good condition. Any color change, any patching or spackle work, or any primer situation needs two coats for uniform color and sheen. Two gallons of paint at $40 each is cheaper than redoing a one-coat job that flashes through.
Primer counts separately. If you’re painting over bare drywall, a dark color, or a stained wall, budget one coat of primer at the same 350-sq-ft coverage — for the 12×14 example, 380 ÷ 350 = 1.09 → 2 gallons of primer, or 1 gallon plus the leftover wall paint working as the second coat.
Common Mistakes That Wreck the Math
- Measuring the floor instead of the walls. Floor square footage tells you nothing about paint — walls are perimeter × height. (If you need the floor number for other projects, our guide to calculating square footage of an L-shaped room covers irregular rooms.)
- Forgetting the second coat. The single most common cause of a mid-project paint run.
- Counting windows as full wall area. A room with three windows hides 45 sq ft of non-wall — over a gallon at two coats.
- Using the manufacturer’s maximum coverage. Labels often claim 400 sq ft per gallon under perfect lab conditions. Professionals use 350 for real walls; 300 for texture.
FAQ
What is the formula for wall square footage?
Perimeter times ceiling height, minus openings: 2 × (length + width) × ceiling height − (21 sq ft per door + 15 sq ft per window). A 12×14 room with 8-ft ceilings, one door, and one window: 52 × 8 − 36 = 380 sq ft of paintable wall.
How many square feet does a gallon of paint cover?
About 350 square feet of smooth drywall with one coat of standard latex paint. Use 300 for lightly textured walls and 250 for heavy stucco or brick. Always round up to whole gallons.
How do you subtract doors and windows?
Subtract about 21 square feet per standard door (3 ft × 7 ft) and about 15 square feet per standard window (3 ft × 5 ft) from the gross wall area. Measure individually if your doors or windows are unusually large.
Do you count the ceiling in paint square footage?
No — the walls number excludes the ceiling. If you’re painting the ceiling too, calculate it separately as length × width (the room’s floor footprint) and budget it as one coat of flat ceiling paint, usually white.
How many coats of paint do most rooms need?
Two. One coat works only for same-color refreshes over paint in good condition. Any color change, patched walls, or bare drywall needs two coats for uniform color and sheen — budget primer separately if the wall is bare, dark, or stained.
Does textured wall change paint coverage?
Yes. Light texture (knockdown, orange peel) drops coverage to about 300 sq ft per gallon; heavy stucco or brick drops it to about 250. For the 12×14 example at two coats (760 sq ft), heavy texture means 760 ÷ 250 = 3.04 → 4 gallons instead of 3.
Doing the whole room? Measure once, calculate with a square footage calculator, and buy your gallons in one trip — paint from the same batch matches perfectly, and you’ll never have to pause mid-wall.