How many gallons of paint do you need?
Enter your room length, width, ceiling height, doors, windows and number of coats to get an exact gallon count — using the 350 sq ft/gallon rate from Sherwin-Williams, Benjamin Moore and Behr. No sign-up required.
Room Dimensions
Openings & Coats
How the math works
Step 1 — gross wall area
gross_wall = 2 × (length + width) × ceiling_height Step 2 — subtract openings
paintable = gross_wall − (doors × 20) − (windows × 15) Each standard 36×80 in door = 20.0 sq ft. Average window = 15 sq ft (industry convention). Paintable area is clamped to ≥0.
Step 3 — gallons
gallons_to_buy = ⌈ (paintable × coats) ÷ coverage ⌉ Coverage defaults to 350 sq ft/gal — the conservative figure used by Sherwin-Williams, Benjamin Moore and Behr field guides. Always round up to whole gallons; running out mid-job risks a dye-lot mismatch.
More paint calculators
- Ceiling Paint Calculator — length × width, texture +20–30%
- Primer Calculator — new drywall and color-change coverage
- Paint by Square Footage — enter your sq ft directly
- 12×12 Room Paint Guide — worked numbers for the most common room size
- Gallon Coverage Guide — 350 vs 400 sq ft/gal explained
- How to Measure a Room — step-by-step method
Frequently Asked Questions
A 12×12 room with an 8 ft ceiling has 384 sq ft of gross wall area. After subtracting 1 door (20 sq ft) and 2 windows (30 sq ft), the paintable area is 334 sq ft. At 350 sq ft/gallon, one coat needs 0.95 gallons — so you buy 1 gallon per coat. For two coats, you need 1.91 gallons, so buy 2 gallons.
Sherwin-Williams, Benjamin Moore, and Behr all publish 350–400 sq ft per gallon for one coat on smooth, primed drywall. This calculator defaults to 350 sq ft/gal — the conservative realistic figure used in field guides — and lets you switch to 400 (optimal/primed smooth), 300 (light texture/porous), or 250 (heavy texture/unprimed).
Most pros and paint manufacturers recommend 2 coats for best results and even color. One coat can work for a very similar color change over an already-painted surface. Dark-to-light or dramatic color changes often need 3 coats. This calculator lets you choose 1–4 coats and adjusts the gallon count accordingly.
A standard 36×80 inch interior door equals exactly 20 sq ft per side. An average window is treated as 15 sq ft (a 3×5 ft window = 15 sq ft exactly — this is an industry-standard convention). The calculator deducts 20 sq ft per door and 15 sq ft per window from your gross wall area automatically.
Primer is needed for new drywall, patched walls, and dramatic color changes (e.g., dark to light). Primer covers 200–300 sq ft/gal — less than topcoat paint — because bare drywall is porous and absorbs more. Use the primer calculator on this site to estimate primer separately from topcoat paint.
Yes. Most manufacturers and paint stores recommend buying ~10% extra for touch-ups, roller tray waste, and textured areas. The calculator rounds up to whole gallons, which already provides a small buffer, but for large rooms or irregular surfaces it's wise to have one extra quart on hand.