Paint Calculator – How Much Paint Do You Need?
Our free paint calculator helps you estimate exactly how many gallons of paint you need for any room, saving you from costly over-buying or frustrating mid-project run-outs. Simply enter your room's dimensions, the number of doors and windows, and your preferred paint type to get an instant, accurate estimate. Whether you're tackling a single accent wall or repainting an entire house, this tool takes the guesswork out of your next painting project.
Enter the length of the room in feet.
Enter the width of the room in feet.
Enter the height from floor to ceiling in feet.
Each standard door is assumed to be 20 sq ft and will be subtracted from the paintable area.
Each standard window is assumed to be 15 sq ft and will be subtracted from the paintable area.
Most rooms require 2 coats for full, even coverage.
Coverage varies by paint quality. Check your paint can label for the exact rate.
Select yes if you also want to paint the ceiling.
Optional: Enter the price per gallon to estimate total paint cost.
Your results will appear here
How to Use This Calculator
1. Enter your room's length and width in feet. 2. Input the ceiling height of your room. 3. Enter the number of standard doors and windows in the room — these will be subtracted from the total paintable area. 4. Select how many coats of paint you plan to apply (most projects require 2 coats). 5. Choose your paint's coverage rate from the dropdown, or check your paint can label for the exact number. 6. Select whether you also want to paint the ceiling. 7. Optionally, enter the price per gallon to get an estimated project cost. 8. Review your results, including total paintable area, gallons needed, and how many full gallons to purchase.
How the Paint Calculator Works
This calculator uses the standard formula for estimating paint coverage based on the surface area of your walls, minus any doors and windows. It accounts for the number of coats you plan to apply and divides by the coverage rate of your chosen paint.
Wall Area Formula
The total wall area is calculated by finding the perimeter of the room and multiplying it by the ceiling height:
- Perimeter = 2 × (Length + Width)
- Wall Area = Perimeter × Ceiling Height
Subtracting Doors and Windows
Standard door openings average about 20 square feet (roughly 3 ft × 6.7 ft) and standard windows average about 15 square feet (roughly 3 ft × 5 ft). These areas are subtracted from the total wall area to give you the paintable area.
- Paintable Area = Wall Area − (Doors × 20 sq ft) − (Windows × 15 sq ft)
Calculating Gallons Needed
The gallons of paint needed is determined by dividing the total paintable area (multiplied by the number of coats) by the coverage rate of the paint:
- Gallons Needed = (Paintable Area × Number of Coats) ÷ Coverage Rate
Coverage rates typically range from 300 to 400 square feet per gallon depending on paint quality and surface texture. Always check your paint can label for the manufacturer's stated coverage.
Ceiling Paint
If you choose to paint the ceiling, the calculator adds the ceiling area (Length × Width) multiplied by the number of coats, then divided by the coverage rate, to your total paint requirement.
Rounding Up
Paint is sold in fixed quantities — typically quarts (1/4 gallon) and full gallons. The calculator rounds up to the nearest quart and nearest full gallon so you always have enough paint to finish the job without running short.
Tips for a Successful Paint Job
- Always buy a little extra paint — at least 10% more — to account for touch-ups and mistakes.
- Porous or unprimed surfaces may absorb more paint and reduce effective coverage.
- Darker colors often require an extra coat when painting over light colors, and vice versa.
- Use a primer coat before applying your finish color when making dramatic color changes.
- Store leftover paint in an airtight container for future touch-ups.