Paint Calculator
Estimate how many gallons of paint you need for a room, after taking off doors and windows.
Results
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Quick Answer
Multiply the room’s perimeter by the wall height, subtract doors and windows, multiply by your coats, then divide by coverage per gallon. A 12 ft by 12 ft room with 8 ft walls, 1 door and 2 windows has 334 sq ft to paint, so 2 coats at 350 sq ft per gallon takes 1.91 gallons. Buy 2 gallons.
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Table of Contents
How This Paint Calculator Works
This calculator finds the area of your four walls and removes doors and windows. It then multiplies by your number of coats and divides by the paint’s coverage, rounding up to whole gallons.
Quick explanation: Gallons = paintable area × coats ÷ coverage.
- Coverage: the sq ft one gallon covers. 350 is a common starting figure.
- Coats: two coats is a common choice.
- Several rooms: run each room and add the exact amounts together.
Core Formulas
- Wall area: 2 × (length + width) × height
- Paintable area: wall area − (doors × 20) − (windows × 15)
- Gallons: paintable area × coats ÷ coverage, rounded up
Paint makers commonly give about 350 to 400 sq ft per gallon. Some paints differ, and rough or porous surfaces can cover much less.
Worked Example
Your room is 15 ft by 12 ft, with 9 ft walls, 2 doors and 3 windows. You plan 1 coat at 400 sq ft per gallon.
- Wall area = 2 × 27 × 9 = 486 sq ft
- Paintable area = 486 − 40 − 45 = 401 sq ft
- Gallons = 401 ÷ 400 = 1.00, which rounds up to 2 gallons
With a result this close, one gallon may be enough if you are careful. Check the can before you decide.
What Can Change How Much You Need
- Surface: textured or new walls absorb more paint.
- Color change: covering a dark color often needs an extra coat or primer.
- Paint type: always check the coverage on your can.
- Touch-ups: a little left over is useful later.
Frequently Asked Questions
Find the wall area, subtract doors and windows, multiply by your coats, and divide by the coverage on the can.
About 350 to 400 square feet is typical. Coverage varies by product and surface, so check the label.
With 8 ft walls, 1 door, 2 windows and 2 coats, you need about 1.91 gallons, so buy 2 gallons.
Two coats is common. You may need more when changing color or painting a porous surface.
No. It covers walls only. To estimate a ceiling, multiply length by width and divide by coverage.
References & Sources
| Source | Used For | Link |
|---|---|---|
| Sherwin-Williams, Paint calculator | Typical coverage per gallon. | https://www.sherwin-williams.com/en-us/color/color-tools/paint-calculator |
| This Old House, How much paint do I need | How to estimate paint for a room. | https://www.thisoldhouse.com/painting/how-much-paint-do-i-need |