Paint calculator for a room

How the paint calculator works

The calculator finds the wall area first: the distance around the room times the wall height. It then takes away the doors and windows, using a standard size for each. You can change both sizes.

That area times the number of coats, divided by the coverage of your paint, gives the amount you need. The ceiling is worked out the same way from the floor area. It has its own line, because ceiling paint is usually a different product.

The result is rounded up to cans you can buy. In feet, the calculator uses gallons and quarts. In meters, it uses liters and the common can sizes of 1, 2.5, 5 and 10 liters.

How much does a gallon of paint cover?

Most interior paint covers about 350 to 400 square feet per gallon for one coat on a smooth, primed wall. That is about 8.5 to 10 square meters per liter. The calculator starts at 350 square feet, the low end, so you are less likely to run short. Check the label on your paint and change the coverage if it differs.

Rough and porous surfaces, such as new plaster, bare drywall and brick, soak up more paint. Prime them first. A primer seals the surface, so the top coat covers what the label says.

One coat or two?

Plan for two coats. One coat rarely gives an even finish, and a second coat hides roller marks and thin spots. One coat can be enough when you repaint a wall in the same color with a good paint.

When you go from a dark color to a light one, use a primer and then two coats. Deep reds, yellows and blues often need a tinted primer or a third coat.

Keep some paint after the job

Walls get marked, and a touch-up only matches when it comes from the same batch. If your result lands just above a can size, the next size up often costs less than a second small can. Write the color name, the brand and the finish on the lid before you store it.