Paint Calculator
Find how much paint to buy for a room, a ceiling, or the outside of a house. Enter the walls to paint, the number of coats, and the coverage on the can to see the gallons or liters you need and how many cans to buy.
Add a ceiling or more walls, or subtract doors and windows.
- Choose US or Metric. You can still change the unit of any single field.
- The first section is set to Room walls. Enter the room's Length and Width and the wall Height. For a single wall, choose Rectangle and enter the wall's length as Length and its height as Width.
- Use Add section to add a ceiling or more walls. To leave out doors and windows, add a section for each one, measure it, and set it to Subtract. Use Quantity for several of the same size.
- Enter the number of Coats. It starts at 2.
- Check the Coverage. It starts at a typical value, 400 ft² per gallon or 10 m² per liter. Enter the coverage printed on your paint can for a better estimate.
- Optionally, enter a Custom can size and a price per can, gallon, or liter to estimate the cost.
For primer, enter the primer's coverage and 1 coat. For the outside of a
rectangular house, use Room walls with the house's length, width, and wall
height, and subtract the doors and windows. Dimension fields accept whole
numbers, decimals, fractions, and mixed numbers, such as
12, 11.75, 3/4, or
8 1/2.
How much paint you need
The calculator adds up the area to paint, adds the waste allowance, and multiplies by the number of coats before dividing by the coverage:
Total area = net area × (1 + waste % ÷ 100)
Paint = total area × coats ÷ coverage
Cans = paint ÷ can size, rounded up
The walls of a room are the distance around the room times the wall height. Convert every measurement to the same unit before you divide. With several sections, the calculator adds the sections set to Add and subtracts those set to Subtract. The area of each shape is calculated as follows:
| Shape | Area |
|---|---|
| Room walls | 2 × (length + width) × height |
| Rectangle | length × width |
| Circle | π × diameter² ÷ 4 (or π × radius²) |
| Triangle (base and height) | base × height ÷ 2 |
| Triangle (three sides) | Heron's formula: √(s(s − a)(s − b)(s − c)), where s = (a + b + c) ÷ 2 |
| Trapezoid | (parallel side A + parallel side B) ÷ 2 × height |
| Ring | π × (outer diameter² − inner diameter²) ÷ 4 |
| Rectangular border | outer length × outer width − (outer length − 2 × border width) × (outer width − 2 × border width) |
| Known area | the area you enter |
How far a gallon or liter of paint goes
Coverage is the area one gallon or one liter of paint covers in one coat. It depends on the paint and the surface, so the value printed on the can is the best one to use.
- Sherwin-Williams says a gallon of paint typically covers about 350 to 400 square feet, and a quart covers 100 square feet. The calculator starts at 400 ft² per gallon. Gallons here are US gallons (3.785 L).
- The UK retailer Wickes bases its paint calculator on 10 m² per liter. The calculator starts at 10 m² per liter, about the same as 400 ft² per gallon (9.82 m² per liter).
- Some cans show higher values. Dulux (UK) lists up to 14 m² per liter for Dulux Matt, and Dulux (Australia) says its wall paints cover about 16 m² per liter. Dulux notes that the actual coverage depends on the surface.
Rough, porous, or bare surfaces take more paint than the can says. Lower the coverage or raise the waste allowance for them.
Coats and extra paint
Wickes recommends 2 coats for most paints, and the Dulux (Australia) calculator assumes 2 coats, so the calculator starts at 2. Follow the number of coats on your paint can.
Wickes also recommends adding an extra 10% for errors, damage, or obstacles, so the waste allowance starts at 10%. You can change it from 0% to 100%.
Paint can sizes
The calculator shows how many of each common can size you need, so you can compare them or mix sizes.
| Where | Common sizes |
|---|---|
| US | 1 quart, 1 gallon, 5 gallons |
| UK | 2.5 L, 5 L, 10 L |
| Australia | 1 L, 4 L, 10 L, 15 L |
For other sizes, such as the 3.78 L cans sold in Canada, enter the volume on the can as the Custom can size.
Examples
A bedroom
The walls of a room 12 ft × 10 ft with 8 ft walls are 2 × (12 + 10) × 8 = 352 ft². With a 10% waste allowance, the total is 387.2 ft². Two coats at 400 ft² per gallon take 387.2 × 2 ÷ 400 = 1.94 gallons, so buy 2 gallons. Adding the ceiling, 12 × 10 = 120 ft², brings it to 2.60 gallons, or 3 gallons.
Metric dimensions
The walls of a room 4 m × 3.5 m with 2.4 m walls are 36 m², or 39.6 m² with 10% added. Two coats at 10 m² per liter take 7.92 liters, so one 10 L can, or two 4 L cans.
FAQ
With 8 ft walls, the walls are 384 ft². With 10% extra and two coats at 400 ft² per gallon, you need about 2.11 gallons, so buy 2 gallons and 1 quart, or 3 gallons. Add the ceiling if you are painting it too.
With 2.4 m walls, the walls are 38.4 m². With 10% extra and two coats at 10 m² per liter, you need about 8.45 liters, so one 10 L can.
Sherwin-Williams says about 350 to 400 square feet per coat. Check the can, since coverage varies by paint and surface.
Wickes recommends two coats for most paints. Check the paint can for the number of coats the maker recommends.
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