Paint Calculator
How much paint the room actually needs, with doors, windows and coats accounted for.
The coverage math
Paintable area is total wall length × height, minus about 21 sq ft (1.9 m²) per door and 15 sq ft (1.4 m²) per window. A gallon of interior paint covers roughly 350 sq ft per coat; a liter covers about 10 m². Multiply by coats, and buy the rounded-up amount.
When coverage drops
Bare drywall, plaster and heavily textured surfaces drink paint, and dramatic color changes (especially covering dark walls with light paint) can take three coats. A tinted primer coat is cheaper than an extra coat of finish paint and usually does the job better.
Don't forget the extras
Ceilings are area = room length × width and usually take a dedicated ceiling paint. Trim, doors and baseboards are typically painted separately in a quart or liter of gloss. Keep a little left over for touch-ups; matching a tint years later is a lottery.