How much paint for a room? Quick answers
Gallons for the walls of one room with 8 ft ceilings, one door and two windows, two coats at 350 sq ft per gallon. Ceilings not included.
| Room | Wall area (sq ft) | Gallons (2 coats) | Buy |
|---|---|---|---|
| 10 × 10 ft | 270 | 1.5 | 2 |
| 10 × 12 ft | 302 | 1.7 | 2 |
| 12 × 12 ft | 334 | 1.9 | 2 |
| 12 × 14 ft | 366 | 2.1 | 3 |
| 14 × 16 ft | 430 | 2.5 | 3 |
| 16 × 20 ft | 526 | 3.0 | 4 |
How the paint calculator works
For each room the calculator adds up the four walls, takes off the doors and windows, adds the ceiling if you ticked it, and multiplies by the number of coats. Dividing by the coverage per gallon gives the gallons of paint. It then finds the cheapest combination of 5-gallon buckets and 1-gallon cans that covers that amount.
Wall area = 2 × (length + width) × height − doors × 20 − windows × 15
Ceiling area = length × width
Gallons = total area × coats ÷ coverage (sq ft per gallon)
Primer gallons = total area ÷ 300
Example
A 12 × 12 ft room with 8 ft ceilings: 2 × 24 × 8 = 384 sq ft, minus one door and two windows = 334 sq ft. Two coats at 350 sq ft per gallon = 1.91 gallons, so you buy 2 gallons.
Why a gallon rarely covers 400 sq ft
Coverage figures on the can are measured on smooth, sealed surfaces at the recommended film thickness — the Sherwin-Williams SuperPaint data sheet, for instance, rates 350–400 sq ft per gallon at 4 mils wet. Textured walls, fresh drywall, thirsty old paint and a heavy roller all use more. Planning at 350 sq ft per gallon leaves a little in the can for touch-ups instead of a trip back to the store halfway through the second coat. Paint coverage explained walks through the math.
Doors and windows
A standard interior door is about 20 sq ft and a typical window about 15 sq ft; Inch Calculator uses 12 sq ft for a window, which gives slightly more paint. Very large openings such as sliding doors or picture windows are worth measuring and subtracting as their real size — enter them as extra doors.
Frequently asked questions
How much paint do I need for a 12×12 room?
A 12 × 12 ft room with 8 ft ceilings has 384 sq ft of wall. Take off one door (20 sq ft) and two windows (15 sq ft each) and you have 334 sq ft. Two coats at 350 sq ft per gallon is 1.91 gallons, so buy 2 gallons for the walls. Add about 0.41 gallon per coat if you are also painting the ceiling.
How many square feet does a gallon of paint cover?
Most interior wall paints are rated at about 350–400 sq ft per gallon on a smooth, primed surface. Sherwin-Williams SuperPaint lists 350–400 sq ft and Behr Premium Plus 250–400 sq ft. Rough, porous or unprimed surfaces soak up more, which is why this calculator defaults to 350 sq ft per gallon.
Do I need two coats of paint?
Usually, yes. A second coat evens out the color and sheen, hides roller marks and is needed for most color changes. One coat can be enough when you are repainting the same color over a clean, sound finish. Dark colors over light walls, or light over dark, may need a primer plus two coats.
Is a 5-gallon bucket cheaper than 1-gallon cans?
Per gallon, yes. At the average prices on this page a 5-gallon bucket works out to $30.80 per gallon versus $35.98 for a single gallon, so once you need about 4.3 gallons or more the bucket is the cheaper buy. The calculator picks the cheapest mix of buckets and cans for you.
Do I need primer before painting?
Use primer on new drywall, patched areas, bare wood, stains, glossy surfaces and big color changes. Many wall paints are sold as “paint and primer”, which is often fine for repainting a wall in good condition. Primer spreads less far than paint; this calculator assumes 300 sq ft per gallon for one coat.
Sources
Prices are national shelf prices and cost-guide ranges read on September 17, 2026. The calculator uses the typical price (the median of the shelf prices we saw) and shows the low–high range; type your own local price to update the total.
All sources for this page (21)
Coverage, yield and building specs
- Sherwin-Williams SuperPaint Interior data sheet accessed 2026-09-17
- BEHR PREMIUM PLUS Interior — product page accessed 2026-09-16
- Inch Calculator — Paint calculator accessed 2026-09-16
- KILZ 2 All-Purpose primer — product page accessed 2026-09-16
- Lowe's — Paint calculator accessed 2026-09-16
- Dummies — Estimating how much paint to buy accessed 2026-09-16
- McCormick Paints — How to calculate how much paint accessed 2026-09-16
Prices
- The Home Depot — BEHR PREMIUM PLUS Flat 1 gal accessed 2026-09-16
- The Home Depot — BEHR PREMIUM PLUS Eggshell 1 gal accessed 2026-09-16
- The Home Depot — Glidden Diamond Flat 1 gal accessed 2026-09-16
- The Home Depot — BEHR PREMIUM PLUS Flat 5 gal accessed 2026-09-16
- The Home Depot — BEHR PREMIUM PLUS Eggshell 5 gal accessed 2026-09-16
- The Home Depot — Glidden Premium Flat 5 gal accessed 2026-09-16
- The Home Depot — BEHR ULTRA Eggshell 1 gal accessed 2026-09-16
- The Home Depot — BEHR DYNASTY Eggshell 1 gal accessed 2026-09-16
- Paint Strategies — Sherwin-Williams paint cost accessed 2026-09-16 · page dated 2025-06-14
- The Home Depot — KILZ primers category accessed 2026-09-16
- Walmart — KILZ 2 All-Purpose Primer accessed 2026-09-16
- The Home Depot — Zinsser Bulls Eye 1-2-3 1 gal accessed 2026-09-16
- Paul B. Hardware — Zinsser Bulls Eye 1-2-3 accessed 2026-09-16
- The Home Depot — Zinsser Bulls Eye 1-2-3 5 gal accessed 2026-09-16
Estimates for planning purposes only, not professional construction advice. Check local building codes and your product's instructions before you build. Prices are national averages collected from public retail listings and cost guides and will differ in your area. BuildHowMuch is independent and not affiliated with, endorsed by or paid by any retailer or manufacturer mentioned on this page. Nothing you enter is stored or sent anywhere.