Paint calculators
Interior Paint Calculator
You are standing in the paint aisle with a cart, and the only question that matters is: how many gallons do I actually need? Buy too little and you are back at the store mid-project — possibly with a slightly different tint batch. Buy too much and you have paid $45+ per gallon for paint that will sit in the garage for five years.
This calculator answers that question with real math. Enter your room dimensions, the number of doors and windows, and how many coats you plan to apply. It subtracts the standard allowances for doors and windows, divides by a realistic coverage rate, and rounds up to the nearest quart — the smallest unit a paint store will sell you. If your walls are new drywall, check the primer box and it will add a primer coat too.
- Paintable wall area
- 334 sq ft
- Paint needed
- 2 gallons2.00 gal · 2 coats at 350 sq ft/gal
How we calculate this (with your numbers)
Wall area = 2 × 8 ft × (12 + 12 ft) − 1 × 20 (doors) − 2 × 15 (windows) = 334 sq ft
Paint = 334 × 2 coats ÷ 350 sq ft/gal = 1.91 gal → rounded up to the nearest quart: 2 gallons
How the math works
Worked example
A 12 × 14 ft bedroom with 8 ft ceilings, 1 door, 2 windows, 2 coats:
- Wall area = 2 × 8 × (12 + 14) = 416 sq ft
- Minus openings: 416 − (1 × 20) − (2 × 15) = 366 sq ft paintable
- Paint = 366 × 2 ÷ 350 = 2.09 gallons
- Rounded up to the nearest quart: 2 gallons + 1 quart
Enter these exact numbers into the calculator above and you will get the same result — that is the point. No black box.
Limitations
Frequently asked questions
Most interior latex paints cover 350–400 square feet per gallon per coat, per Sherwin-Williams’ Duration Home product data sheet. This calculator defaults to 350 — the conservative end — so you are less likely to run short. Porous surfaces, heavy texture, and drastic color changes all reduce real-world coverage.
Yes. The standard estimating practice (Hamilton & Hamilton estimating guide) subtracts 20 square feet per door and 15 square feet per average window. Some pros use 21 and 16 — the difference is small, and this calculator uses 20/15.
Double the one-coat number. Two coats is the standard for most repaints: the first coat builds color, the second evens it out. One coat is only realistic for same-color refreshes with a quality paint.
On new, unpainted drywall — yes, always. Drywall and joint compound are porous and will drink finish paint unevenly. Check the "new drywall" box and the calculator adds one coat of PVA primer at about 300 square feet per gallon (KILZ PVA drywall primer technical data sheet: 300–400). On previously painted walls in good condition, primer is usually unnecessary.
Yes — this calculator rounds up to the nearest quart automatically. Paint stores cannot mix a fraction of a quart, and a little extra covers touch-ups later. Running out mid-wall risks a visible color mismatch between batches.
Related calculators & guides
- Primer calculator — primer by surface type, with or without room dimensions
- Paint cost calculator — turn gallons into a full project budget with supplies
- Ceiling paint calculator — do not forget the fifth wall
- Exterior paint calculator — same math for the outside of the house
- How much paint for a 12×12 room — quick answers for common room sizes
- Paint coverage per gallon by type — why 350 is not always 350