Multi-Room Paint Calculator
Estimate the gallons of paint needed for a whole project across several rooms. Enter the number of rooms, average paintable wall area, the percentage taken up by doors and windows, how many coats you plan, and your paint's coverage to get a precise gallon count.
Last updated: May 2026
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About this calculator
This calculator scales a single-room paint estimate up to a whole project. The formula is: Gallons = ⌈ (Rooms × AvgWallArea × (1 − Openings ÷ 100) × Coats) ÷ CoveragePerGallon ⌉. Multiplying the number of rooms by the average paintable wall area per room gives the gross surface area. The factor (1 − Openings ÷ 100) subtracts the share of that area occupied by doors and windows, which do not get painted — 10–20% is typical for a normal room. Multiplying by the number of coats accounts for the fact that most colors need two coats for full coverage, and a third over bare or drastically different surfaces. Dividing by the paint's spread rate (the manufacturer's coverage in square feet per gallon, which drops on rough or porous surfaces) converts area into gallons, and the ceiling function rounds up because paint is sold in whole cans. Buying one consistent batch avoids slight color shifts between dye lots.
How to use
Suppose you are repainting 3 bedrooms that each have about 350 ft² of wall area, with doors and windows taking up 15% of each wall, applying 2 coats of a smooth-wall paint rated at 400 ft²/gal. Step 1 — Gross area: 3 × 350 = 1,050 ft². Step 2 — Subtract openings: 1,050 × (1 − 15 ÷ 100) = 1,050 × 0.85 = 892.5 ft². Step 3 — Apply coats: 892.5 × 2 = 1,785 ft² of total coverage. Step 4 — Divide by spread rate: 1,785 ÷ 400 = 4.46 gallons. Step 5 — Round up: buy 5 gallons. Dropping to 1 coat would need only 3 gallons.
Frequently asked questions
How many gallons of paint do I need for multiple rooms with two coats?
Start with the total paintable wall area across all rooms, subtract 10–20% for doors and windows, multiply by the number of coats, and divide by your paint's coverage rate (usually 350–400 square feet per gallon on smooth walls). For example, three bedrooms with 350 square feet of wall each, minus 15% for openings, at two coats works out to about 1,785 square feet of coverage, or roughly 5 gallons. Always round up to the next full gallon and buy it all in one batch to avoid color variation.
Should I subtract doors and windows when calculating paint?
Yes, for an accurate estimate you should subtract the area of doors and windows because those surfaces are not painted with wall paint. A standard interior door is about 20 square feet and an average window 12–15 square feet, so in a typical room they remove 10–20% of the gross wall area. Skipping this deduction overestimates paint by up to a fifth, which wastes money — though a small surplus is useful for touch-ups, so many painters deduct openings and then round generously.
Why do walls usually need two coats of paint?
Two coats are standard because a single coat rarely delivers uniform color and full hiding power, especially when changing colors, painting over a patched or porous surface, or using lighter or more vivid shades. The first coat seals and establishes the color base while the second evens out coverage and depth. Deep accent colors or bare drywall can even need a primer plus two finish coats. This calculator multiplies your coverage by the coat count so the gallon estimate reflects the real number of passes.