How to Calculate Paint Needed

Estimating paint for a room is a matter of wall area and coverage per gallon. This guide walks you through measuring, choosing coats and coverage, and using the calculator so you buy the right amount without over- or under-ordering.

What it's for

Use this when you're painting walls in a room (or multiple rooms). You enter the room’s length, width, and height (or total wall area), number of coats, and coverage (sq ft per gallon). The result is gallons (and liters) needed. Coverage varies by product and surface; the calculator lets you adjust.

Step by step

1. Measure the room — Length, width, and height in feet. The calculator assumes a simple rectangular room and computes wall area (2 × (length + width) × height). For multiple rooms, add areas or run the calculator per room.
2. Choose number of coats — Usually 2 for a solid finish; 1 may be enough for a touch-up or light color change.
3. Set coverage — Most interior paint covers 350–400 sq ft per gallon. Check your product’s label and enter it in the calculator (e.g. 350).
4. Read the result — Gallons (and liters) needed. Round up to the next gallon so you have a little left for touch-ups.

Formula

• Wall area = 2 × (Length + Width) × Height (for four walls).
• Gallons = (Wall area × Coats) ÷ Coverage per gallon. The calculator rounds up.

Example

Room 12 ft × 10 ft, 8 ft ceiling, 2 coats, 350 sq ft/gallon:

• Wall area = 2 × (12 + 10) × 8 = 352 sq ft.
• Gallons = (352 × 2) ÷ 350 ≈ 2.01 → 3 gallons (round up).

Tips

• Doors and windows — Subtract their area if large; for a quick estimate, the calculator’s result is often close enough if you round up.
• Ceilings — Calculate separately (length × width) and add to your total if you’re painting the ceiling too.
• Use the Paint Calculator for your room size and product coverage.