Interior Paint Calculator — Free Online Tool
Running out of paint mid-wall is miserable; buying three extra gallons is expensive. This paint calculator converts your wall area and number of coats into the exact gallons you need — rounded up the way paint stores actually sell it.
What is the Interior Paint Calculator?
Paint coverage is rated in square feet per gallon — most interior latex covers about 350–400 sq ft per gallon per coat on smooth walls. But real rooms aren't lab conditions: textured walls, bold color changes, and porous new drywall all drink more paint. Two coats is the standard for a professional finish, and dramatic dark-to-light changes sometimes need three.
The math is simple but easy to botch: total paintable area times coats, divided by coverage. The classic mistakes are measuring the room's floor area instead of wall area, and forgetting to subtract doors and windows (about 20 sq ft per door, 15 per window). This paint calculator does the arithmetic and rounds up to the nearest quarter gallon — because running short mid-project means a visible color mismatch between batches.
How to use this calculator
- Measure each wall's length × height and add them up for total wall area.
- Subtract roughly 20 sq ft per door and 15 sq ft per window (optional but recommended).
- Enter the number of coats (2 is standard).
- Click Calculate Paint and buy the rounded-up amount.
Formula
Frequently asked questions
How much area does one gallon of paint cover?
About 350–400 sq ft per coat on smooth interior walls — which is why this paint calculator defaults to 350. Textured or unprimed surfaces cover less.
How many coats of paint do I need?
Two coats is the professional standard for even color and durability. One coat rarely covers fully; three is for dramatic color changes.
Should I subtract doors and windows from the wall area?
Yes — roughly 20 sq ft per door and 15 per window. It saves about a quart per room on average.
Does this paint calculator include the ceiling?
No — measure ceilings separately (length × width) and run the calculation again with flat ceiling paint.