In this article
- The real coverage math
- Step-by-step: how to calculate paint for any room
- 1. Measure wall area
- 2. Subtract doors and windows
- 3. Multiply by number of coats
- 4. Divide by coverage and round up
- Coverage by whole-house scenario
- Why coverage in the can doesn’t match the wall
- Primer: when you actually need it
- Frequently asked questions
- Keep reading
Quick answer: One gallon of wall paint covers about 350–400 sq ft on a single coat of smooth drywall. Plan on two coats for almost every job, and order 10–15% extra to cover touch-ups and waste. For a typical 12×14’ bedroom with 8’ ceilings, that’s about 2 gallons.
Reviewed by John Miller
Licensed painter, 15 years in the field
“The #1 reason paint jobs look streaky isn’t bad technique — it’s running out of paint and finishing with a different batch. Always buy all your paint on day one, in one tint run, with 10% extra.”
The real coverage math
Manufacturer labels quote 350–400 sq ft per gallon as a best-case number on smooth, primed, same-color walls with premium paint. In the real world, coverage varies significantly:
| Surface | Sq ft per gallon (1 coat) |
|---|---|
| Smooth drywall, same color | 400 |
| Smooth drywall, color change | 350 |
| Orange-peel / light texture | 300–325 |
| Knockdown / medium texture | 250–275 |
| Heavy stucco (exterior) | 150–200 |
| Bare wood / new drywall (primer) | 200–250 |
| Trim / doors (semi-gloss) | 250–300 |
Source: Sherwin-Williams and Benjamin Moore technical data sheets, cross-checked with field measurements on 200+ residential jobs.
Step-by-step: how to calculate paint for any room
1. Measure wall area
Multiply room perimeter by ceiling height.
Example: 12’×14’ room = 52’ perimeter. 52 × 8’ = 416 sq ft of wall.
2. Subtract doors and windows
Standard door = 21 sq ft. Standard window = 15 sq ft. One door + two windows = 51 sq ft to subtract.
416 − 51 = 365 sq ft net wall.
3. Multiply by number of coats
Default is two coats.
365 × 2 = 730 sq ft of paint needed.
4. Divide by coverage and round up
730 ÷ 350 = 2.08 gallons. Round up to 2 gallons, plus a quart for touch-ups.
Coverage by whole-house scenario
Quick estimates for complete interiors (walls only, two coats, smooth drywall):
- 1 bedroom (12×14’) — 2 gallons
- 4-room apartment (~900 sq ft) — 6–7 gallons
- 1,500 sq ft house interior — 10–14 gallons
- 2,000 sq ft house interior — 14–18 gallons
- 2,500 sq ft house interior — 18–22 gallons
- 2,000 sq ft house exterior — 12–18 gallons (smooth siding), 20–28 gallons (stucco)
Add trim paint separately: budget 1 gallon of trim paint per 3–4 rooms for typical baseboard, door, and window work.
Why coverage in the can doesn’t match the wall
Three reasons the label number fails in practice:
- Texture eats paint. Heavy stucco has up to 2× the surface area of smooth drywall because of the peaks and valleys.
- Color change demands full coverage. Dark-over-light or white-over-red often needs a tinted primer plus two finish coats — effectively three coats of product.
- Spray applies thicker. A sprayer puts down 25–40% more mils than a roller. Exteriors sprayed-and-back-rolled use more paint than the label suggests.
Primer: when you actually need it
- New drywall — always. Unprimed drywall drinks paint at 1.5× the rate.
- Dark to light — always. Tinted gray primer is a labor-saver.
- Stains (water, smoke, grease) — always, with a stain-blocking primer (Kilz, Zinsser BIN).
- Glossy surfaces — if you can’t de-gloss with sanding, prime with a bonding primer.
- Repaint, same color, clean walls — no primer needed.
Want a number for your project?
The PaintPricing calculator handles coverage, coats, texture, and primer automatically — plus labor if you want the full quote.
Frequently asked questions
How many square feet does 1 gallon of paint cover?
350–400 sq ft on smooth, same-color drywall with one coat. 250–300 sq ft on textured surfaces. Plan for two coats on almost all jobs.
How many gallons of paint for a 12×12 room?
About 1.5–2 gallons for two coats of wall paint, plus a quart for trim. Buy 2 gallons if you’re close — leftover paint is useful for touch-ups.
How much paint to paint a whole house interior?
10–14 gallons for 1,500 sq ft, 14–18 for 2,000 sq ft, 18–22 for 2,500 sq ft. Add 1 gallon of trim paint per 3–4 rooms.
Do I need primer before painting?
Yes for new drywall, dark-to-light color changes, water or smoke stains, and glossy surfaces. No for same-color repaints on clean walls, especially with self-priming paint.
Is two coats always necessary?
Almost always. One coat works only on same-color refreshes with premium self-priming paint on clean, unfaded walls. If you see any roller marks or color unevenness after one coat, you need the second.
How much extra paint should I buy?
10–15% over the calculated amount. This covers waste, touch-ups, and finishing a wall with the same tint batch. Leftover paint stored sealed lasts 2–3 years.
Does paint coverage include the ceiling?
No — ceilings are measured separately (length × width of the room). Ceiling paint is usually a different product (flat, dedicated ceiling paint) and is calculated with the same 350–400 sq ft/gallon assumption.
Keep reading
Cost to paint a house →
What painters actually charge in 2026 for interiors and exteriors.
Interior painting cost by room →
Bedroom, kitchen, bathroom, living room — real price ranges.
Free painting calculator →
Enter your rooms, get a coverage plan and instant quote.
How we source this data
Prices reflect 2026 U.S. averages. We combine contractor-reported rates, manufacturer spec sheets, and federal wage data, then cross-check against John Miller’s 15 years of field experience pricing residential and commercial jobs. Numbers are updated quarterly.
Primary sources:
- U.S. Bureau of Labor Statistics, Occupational Employment Statistics: Painters, Construction and Maintenance (2024)
- Sherwin-Williams product data sheets (Emerald, SuperPaint, Duration)
- Benjamin Moore technical data sheets (Aura, Regal Select, Ben)
- HomeAdvisor / Angi national cost reporting (2025 survey data)
- PaintPricing field data from licensed contractor John Miller (2010–2026)
