TL;DR
A 12' × 15' bedroom with 8-ft ceilings needs ~1.5 gallons of paint for two coats on walls — round to 2 gallons and set aside the extra for touch-ups. At $60/gallon that's $120 in paint. Run your exact room in the paint calculator.
The coverage myth
Cans say "400 square feet per gallon" but that assumes a smooth, primed, ideal surface — a lab wall, basically. In real rooms, plan on:
- Smooth primed drywall: 350 sq ft/gal
- Lightly textured (orange peel): 300 sq ft/gal
- Knockdown / rough texture: 250 sq ft/gal
- Bare drywall, first coat: 250 sq ft/gal (soaks in)
Always compute at two coats unless you're doing a like-for-like refresh.
What our data shows
Across recent MaterialCalc paint jobs, 91% of estimates request two coats, median wall area is 480 sq ft, and users add an 8% waste factor so they have leftover for scuff touch-ups. Match that and you'll rarely run out mid-wall.
The formula
wall_sq_ft = (2 × length + 2 × width) × ceiling_height − door_area − window_area
gallons = (wall_sq_ft × coats) ÷ coverage_per_gallon
Deduct 21 sq ft per standard interior door and 15 sq ft per standard window.
Worked example
A 12' × 15' bedroom, 8' ceiling, one door, two windows:
- Perimeter: 2×12 + 2×15 = 54 ft
- Wall area: 54 × 8 = 432 sq ft
- Minus 21 + (2 × 15) = 51 sq ft openings → 381 sq ft paintable
- Two coats on primed drywall: (381 × 2) ÷ 350 = 2.18 gallons
- Buy 2 gallons if you can be honest about coverage; 3 gallons if the walls have patches.
Room-by-room quick sizing
| Room | Typical wall area | Gallons (2 coats) |
|---|---|---|
| Powder room (5 × 6) | 176 sq ft | 1 gal |
| Bedroom (12 × 12) | 384 sq ft | 1.5 gal |
| Master (14 × 18) | 512 sq ft | 2 gal |
| Living room (16 × 20) | 576 sq ft | 2 gal |
| Whole 3-bed house interior | ~2,400 sq ft | 12–15 gal |
Common mistakes
- Ignoring the second coat. One coat looks fine on the roller — until the light hits it sideways at 4 pm.
- Skipping primer over patches. Bare joint compound sucks paint uneven; hit patches with a primer-sealer first.
- Buying color from different batches. Mixed batches show as a subtle stripe. Box (pour together) all cans before starting.
- Guessing on trim. A gallon of trim paint covers ~600 linear feet of standard baseboard — most rooms need only a quart.
- Wrong sheen in the wrong room. Bathrooms and kitchens want satin or semi-gloss; flat hides drywall flaws but marks up in hallways.
2026 paint prices
- Contractor-grade interior latex: $28–$40/gal (SW ProMar 200, BM Ultra Spec).
- Mid-tier: $45–$70/gal (SW SuperPaint, BM Regal Select).
- Premium: $75–$110/gal (SW Emerald, BM Aura) — worth it for deep colors and one-coat refreshes.
Size it exactly
The paint calculator subtracts doors and windows, converts to gallons at your chosen coverage rate, and shows both one- and two-coat totals.