Set up the Paint Calculator inputs
Build the estimate from the area you plan to paint, the coats, the waste allowance, and the coverage printed on the paint label. The calculator removes the door and window areas, applies coats and waste, then shows the calculation and the number of containers to buy.
- Enter Gross surface area and choose its unit:
sq ft,sq m, orsq yd. - Set Number of coats and Waste / overage %.
- Enter Number of doors, Average door area, Number of windows, and Average window area. Use zero for a count when that opening area is already outside the gross area.
- Choose Paint unit, then enter the matching Coverage rate and Container size.
- Click Calculate. Use Copy result or Download result (.txt) after the result appears.
Paint quantity formula and example
The calculator converts the area, door area, and window area to the unit selected in Paint unit. It subtracts the openings, multiplies the remaining area by Number of coats, adds Waste / overage %, divides by Coverage rate, and rounds the container count up.
Gross surface area: 400 sq ft
Number of coats: 2
Number of doors: 2 x 21 sq ft
Number of windows: 1 x 15 sq ft
Waste / overage %: 10
Coverage rate: 350 sq ft/gal
Paint needed: 2.16 gal
Containers to buy: 3The net paintable area is 343 sq ft. In the result, you can check each deduction, the area after coats and waste, the exact volume, and the total volume represented by the rounded container count.
Choose gallons or liters and check coverage
Select Gallons (US) for a sq ft/gal rate, or Liters for a sq m/L rate. The defaults are 350 sq ft/gal and 8.6 sq m/L. Changing Paint unit also changes the default Container size to 1 gallon or 4 liters and sets the opening-area units to match.
Read the product label before replacing the default Coverage rate. Gross surface area accepts sq ft, sq m, or sq yd. Average door area and Average window area accept sq ft or sq m, and the calculator converts them before subtracting the openings.
Enter door and window deductions once
For each opening type, the calculator multiplies the count by the average area. It subtracts those two totals from Gross surface area before applying coats and waste. If the gross area already excludes an opening, set that opening's count to zero.
The deductions cannot equal or exceed the entered surface area. The accepted values are surface area 0.1 to 100,000, coats 1 to 10, waste 0 to 50, counts 0 to 200, opening areas 0.1 to 200, coverage 0.1 to 2,000, and container size 0.01 to 100.
Build the gross area from wall measurements
Add each wall's length multiplied by its height, then enter the sum as Gross surface area. Keep doors and windows in that total when you want the calculator to subtract them.
For a metric result, choose Liters, enter the product's sq m/L coverage, and enter the can size in liters. The result then reports paint volume in liters and area in square meters.
Choose a calculator for the material
Use Construction Calculators for a different material, dimension, load, or water-volume estimate. Choose Drywall Calculator for wall or ceiling sheet counts, Flooring Calculator for planks, tiles, boxes, or roll coverage, and Construction Materials Calculator for its other material modes.
Frequently asked questions
How much paint do I need for 400 square feet?
With 2 coats, two 21 sq ft doors, one 15 sq ft window, 10% waste, and 350 sq ft/gal coverage, the calculator returns about 2.16 gallons and 3 one-gallon containers. Replace those values when the openings, coverage label, or Container size differs.
How do I calculate wall area for paint?
Multiply each wall's length by its height and add the wall areas together. Enter that total as Gross surface area, including doors and windows when you want the opening fields to subtract them.
Can I calculate paint in liters?
Choose Liters in Paint unit, enter coverage in sq m/L, and enter Container size in liters. The calculator converts the area and opening measurements to square meters.
Does the paint calculator include primer or trim?
No. It calculates the paint product represented by the entered coats and Coverage rate. Run it separately for primer, trim, or another color when those areas or products need their own estimate.