How Much Paint Do I Need? (2026 Calculator Guide)
One gallon of paint covers about 350 to 400 square feet per coat on smooth walls. Most rooms need 2–3 gallons for two coats of wall paint; a whole 1,500 sq ft home interior needs 12–15 gallons. The formula: (wall area ÷ 350) × 2 coats, rounded up. Here’s the math for any project.
What’s the Formula for Calculating Paint?
Gallons = (Paintable wall area in sq ft ÷ 350) × number of coats- 1 gallon ≈ 350–400 sq ft per coat on smooth, previously painted drywall. Use 350 to be safe; textured or porous surfaces drop to 250–300.
- Always calculate for 2 coats (more on this below).
- Round up to the next gallon — running out mid-wall is worse than leftovers.
How Do You Measure Wall Area (Including Windows and Doors)?
- Measure each wall: width × height. A 12-ft wall with 9-ft ceilings = 108 sq ft.
- Add all walls together. For a rectangular room: (length + width) × 2 × ceiling height.
- Subtract openings: a standard door is ~21 sq ft; a standard window is ~15 sq ft. Only subtract them if the room has several — for one door and one window, many pros skip the subtraction and treat it as a built-in safety margin.
- The result is your paintable area. Plug it into the formula.
Worked example — 12×12 room, 9-ft ceilings:
- Walls: (12 + 12) × 2 × 9 = 432 sq ft
- Minus one door (21) and one window (15) = 396 sq ft
- Two coats: (396 ÷ 350) × 2 = 2.26 gallons → buy 3 gallons (or 2 gallons + 1 quart)
How Much Paint Per Room Size?
| Room | Wall Area (8–9 ft ceilings) | Paint for 2 Coats |
|---|---|---|
| Bathroom (5×8) | ~200 sq ft | 1–2 gallons |
| Small bedroom (10×10) | ~320 sq ft | 2 gallons |
| Average bedroom (12×12) | ~400 sq ft | 2–3 gallons |
| Large bedroom / office (14×16) | ~480 sq ft | 3 gallons |
| Living room (15×20) | ~560 sq ft | 3–4 gallons |
| Open-plan kitchen/living (20×25) | ~720 sq ft | 4–5 gallons |
How Much Paint for a Whole House?
| Project | Paint Needed (2 coats) |
|---|---|
| Whole interior, 1,000 sq ft home | 8 – 10 gallons (walls) |
| Whole interior, 1,500 sq ft home | 12 – 15 gallons (walls) |
| Whole interior, 2,500 sq ft home | 18 – 25 gallons (walls) |
| Ceilings, 1,500 sq ft home | 4 – 6 gallons |
| Interior trim and doors, average home | 2 – 4 gallons |
| Exterior, 1,500 sq ft home | 10 – 15 gallons |
| Exterior, 2,500 sq ft home | 15 – 25 gallons |
Exterior needs vary widely with siding texture — rough stucco and cedar can absorb 30–50% more than smooth fiber cement. Calculate ceilings separately (length × width of each room) — see cost to paint a ceiling — and trim by linear footage; a quart typically covers one room’s trim or 1–2 doors (trim and door costs here). For budgeting the full job, see house painting cost and exterior painting cost.
Why Should You Always Plan for Two Coats?
One-coat claims on the can assume ideal conditions: same color over same color on smooth, primed walls. In the real world:
- Color changes (especially dark-to-light or light-to-dark) telegraph through a single coat.
- Coverage is uneven — roller pressure varies, and one coat shows lap marks under daylight.
- Durability doubles — two thin coats outlast one thick coat and scrub better.
- The Painting Contractors Association industry standards define a properly finished surface as uniform in color and sheen — which one coat rarely achieves.
The only common one-coat exception: a fresh maintenance repaint of the exact same color in good condition. Even then, most pros do two.
When Do You Need Primer (and How Much)?
Primer is calculated separately, at roughly the same 350–400 sq ft per gallon, one coat:
- Bare drywall or new skim coat — always prime; raw drywall drinks topcoat.
- Stains (water, smoke, marker, grease) — stain-blocking primer or it bleeds through forever.
- Drastic color change — a tinted primer coat is cheaper than a third coat of premium paint.
- Glossy surfaces — bonding primer so the new coat adheres.
- Patched walls — spot-prime joint compound or patches will “flash” (show dull spots).
If you’re repainting a wall in good condition with a similar color, skip primer and put the money into better topcoat. One safety note: if your home was built before 1978, don’t dry-sand old paint to prep it — old layers may contain lead. Follow the EPA’s lead-safe practices or hire an RRP-certified pro.
How Much Extra Should You Buy for Waste and Touch-Ups?
Plan for about 10% extra:
- Roller covers and trays hold paint you never get back (~half a quart per job).
- Textured patches and corners take more than the math says.
- Touch-ups need the same batch. Paint mixed on different days can vary slightly in tint; leftover paint from the original batch matches perfectly for years of nail holes and scuffs.
Practical rule: if your math says 2.3 gallons, buy 3 — the leftover is your touch-up kit.
Gallon vs. 5-Gallon Bucket: Which Is Cheaper?
A 5-gallon bucket typically costs 10–20% less per gallon than five singles ($150–$250 vs. $175–$300 for mid-range paint). Buy the bucket when:
- You need 4+ gallons of one color (whole interior in a single neutral, large exterior).
- Color consistency matters — one bucket means one batch, zero tint variation between cans.
Stick to single gallons for accent walls and multi-color schemes, since unopened gallons can usually be returned but custom-tinted buckets can’t.
How Do You Store or Dispose of Leftover Paint?
- Storage: seal the lid with a mallet (lay plastic wrap over the rim first), label with room + date, and store in a climate-controlled spot — latex paint ruined by freezing in a garage is the #1 cause of wasted touch-up paint. Properly sealed latex lasts 2–10 years.
- Disposal: never pour paint down a drain. Latex paint can be dried out (with cat litter or hardener) and trashed in many areas, but oil-based paint is household hazardous waste — the EPA’s household hazardous waste guidance explains local collection programs. In the 10+ PaintCare states, PaintCare drop-off sites take leftover paint free at most paint retailers.
- Donate: usable paint is welcomed by Habitat ReStores, schools, and theater groups.
Doing the painting yourself? Compare the full picture in DIY vs. hiring a painter.
Frequently Asked Questions
How much does a gallon of paint cover? About 350–400 square feet per coat on smooth, previously painted walls. Textured, porous, or bare surfaces cover 250–300 sq ft per gallon.
How much paint do I need for a 12×12 room? About 2–3 gallons for two coats on the walls (roughly 400 sq ft of wall area with 9-ft ceilings), plus 1 gallon for the ceiling and a quart for trim if needed.
How many coats of paint do I need? Plan on 2 coats for uniform color and durability. Add a separate primer coat for bare drywall, stains, glossy surfaces, or dramatic color changes.
How much paint for a whole house interior? Roughly 12–15 gallons of wall paint for a 1,500 sq ft home (two coats), plus 4–6 gallons for ceilings and 2–4 gallons for trim.
What do I do with leftover paint? Keep about 10% for touch-ups, sealed and stored away from freezing. Dispose of the rest through PaintCare drop-off sites or your local household hazardous waste program — never down the drain.
Last updated: June 11, 2026. For informational purposes only. Sources: Painting Contractors Association; EPA Lead Renovation, Repair and Painting Program; EPA Household Hazardous Waste; PaintCare.