Paint Calculator
Multiply wall perimeter by height for gross wall area, subtract door and window area, multiply by the number of coats, then divide by your paint’s coverage rate — typically 350-400 square feet per gallon for interior wall paint — to get gallons needed.
Quick Answer
Multiply wall perimeter by height for gross wall area, subtract door and window area, multiply by the number of coats, then divide by your paint’s coverage rate — typically 350-400 square feet per gallon for interior wall paint — to get gallons needed.
Paint Calculator
Enter your values below for an instant result, then see the formula, worked example, and common mistakes.
Enter your values and click calculate.
How to Use This Calculator
Measure the length and width of the room and the height of the walls, all in feet.
Two coats is standard for most paint jobs; a single coat rarely gives even, opaque coverage, and dramatic color changes (e.g. dark over light) may need a third coat or a tinted primer.
A standard door is about 20 sq ft and a standard window about 15 sq ft — subtract their combined area so you are not paying for paint you will not use.
Coverage is usually printed on the can, typically 350-400 sq ft per gallon for interior wall paint; the calculator defaults to 375 sq ft/gallon, a reasonable real-world midpoint between label claims and actual surface absorption.
Formula
Gallons needed = [(Perimeter x Height) – Door/Window Area] x Number of Coats / Coverage per Gallon.
Reference Table: Typical Paint Coverage by Type
| Paint type | Typical coverage per gallon |
|---|---|
| Interior wall paint (label claim) | 375-425 sq ft |
| Interior wall paint (real-world, textured/porous drywall) | ~350 sq ft |
| Exterior paint | 250-350 sq ft |
| Primer | 225-275 sq ft |
Common Mistakes to Avoid
- Forgetting to multiply by the number of coats — most rooms need two coats, which doubles the paint volume needed versus a single-coat estimate.
- Not deducting door and window area, leading to buying more paint than necessary.
- Using the can’s label coverage claim (often based on ideal lab conditions) instead of a more realistic real-world figure, especially on textured, porous, or previously unpainted drywall.
- Buying paint in odd fractional amounts instead of rounding up to the next standard can size (quart, gallon) available from the retailer.
When the Estimate May Be Wrong
Textured walls, porous new drywall, dark-to-light color changes, and low-quality (low-solids) paint can all reduce real coverage well below the can’s label claim. When in doubt, round up and buy slightly more than the calculated estimate, or ask your paint retailer to confirm coverage for your specific product and surface.
FAQs
How many square feet does a gallon of paint cover?
Most interior wall paint claims 375-425 sq ft per gallon on the label, though real-world coverage on typical drywall is often closer to 350 sq ft per gallon.
How many coats of paint do I need?
Two coats is standard for most colors and surfaces; dramatic color changes (dark over light, or vice versa) may need a third coat or a tinted primer first.
Should I subtract doors and windows from my paint calculation?
Yes — a standard door is about 20 sq ft and a standard window about 15 sq ft, and subtracting them prevents over-buying paint.
Sources and Methodology
Paint coverage figures (350-425 sq ft/gallon for interior wall paint, 250-350 for exterior, 225-275 for primer) reflect current published manufacturer specifications and paint-industry guidance as of 2026. Confirm exact coverage on your specific product label before final purchase.
