Paint Calculator

What the paint calculator does

Buying paint is a guessing game until you have done the arithmetic, and the cost of guessing wrong is either a half-finished wall or a shelf of leftover cans. This works out how much you need. You give it the surface size, the openings, the number of coats, and how far the paint goes, and it returns the area, the gallons, and a cost if you add a price.

It works for a room inside or a wall outside. Below is how it gets there, and the one figure worth setting carefully before you trust the result.

How to use it

1. Choose internal or external. Internal takes the room width, length, and height and covers the four walls, not the ceiling. External takes a single surface width and height.
2. Enter the window and door details, the window size and how many windows and doors, so their area can be taken off.
3. Set the coats, the coverage, and a price. The coverage is how many square feet a gallon covers, covered in its own section below.
4. Press Calculate for the result, or Reset to clear it.

How the paint amount is worked out

First the calculator works out the area to paint. For a room that is the four walls, the perimeter times the height, and then it subtracts the windows and the doors, since you are not painting those:

Paint area = wall area − (windows + doors)

Then it divides that area by the coverage to get the paint for one coat, multiplies by the number of coats, and rounds to whole gallons. The cost, if you add one, is the gallons times your price.

Setting the coverage figure

This is the number that makes or breaks the estimate, so it is worth getting right. Coverage is how far a gallon goes, and for most interior wall paint that is somewhere around 350 to 400 square feet per coat on a smooth, sealed wall. Your paint can prints its own figure, and that is the one to use, since it is the number that matches what you are actually buying.

A few things eat into it. Rough or textured surfaces drink more paint, so coverage drops, and bare or porous surfaces do the same until they are sealed. Primer covers less ground than paint, nearer 250 to 300 square feet a gallon. So the coverage box is not a fixed thing to leave alone; set it to your can's figure, and lower it a little for a rough or thirsty wall.

Coats, doors, and windows

Most jobs want two coats, not one. A single coat of good paint can do it when you are going over a similar colour, but a colour change, a patchy surface, or bare drywall almost always needs two for an even finish, so the coats box matters as much as the area.

For the openings, the calculator counts a standard door at around 19 to 20 square feet and takes the windows off at the size you enter, times the number of them. Taking these out keeps the estimate honest, since a room with several big windows has a lot less wall to paint than its size suggests.

A worked example: a 12 by 14 room

Say the room is 12 feet by 14 feet with 8 foot walls, two windows of about 3 by 4 feet, one door, two coats, and paint that covers 350 square feet a gallon.

The walls are 2 × (12 + 14) × 8 = 416 square feet. Take off two windows at 12 square feet each and a door at about 19, and roughly 373 square feet is left to paint. At 350 square feet a gallon that is just over a gallon a coat, so for two coats you are looking at about 2 gallons.

Notice how much the coverage figure drives that. Set it far below your can's real number and the calculator will tell you to buy several times the paint you need, so check it first.

Questions people ask

How much paint do I need?

Work out the wall area, subtract the doors and windows, multiply by the coats, and divide by the coverage. A 12 by 14 room in two coats is roughly 2 gallons with normal paint.

How many square feet does a gallon of paint cover?

Usually about 350 to 400 square feet for one coat on a smooth wall, less on rough or bare surfaces. Use the figure on your can, and set the coverage box to match.

How many coats do I need?

Two is normal. One can work over a similar colour, but a colour change or a bare or patchy wall almost always needs two for an even finish.

Should I buy extra paint?

A little, for touch-ups and the odd thin spot. Buying it in one batch also keeps the colour consistent, since separate batches can differ slightly.

References

A quick note on the numbers. The coverage range of roughly 350 to 400 square feet per gallon, and the lower figure for primer, come from the published coverage guidance of major paint manufacturers; always defer to your own can. The area is plain geometry, the wall area less the openings. The unit conversions follow the US National Institute of Standards and Technology guide.

1. Major paint manufacturer coverage guidance (for example, Sherwin-Williams and Behr published coverage figures). https://www.sherwin-williams.com
2. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811