Paint Calculator

How to estimate paint for a room

The whole job is one division problem. You are working out how much surface you have, multiplying it by the number of coats, then dividing by how far a gallon goes. Everything else is bookkeeping.

gallons = (wall area − doors − windows) × coats ÷ coverage per gallonWall area = total wall length × wall height. For a rectangular room, total wall length = 2 × (length + width).

Getting the wall area right

Measure the walls, not the floor. A 12 by 14 foot room has 52 feet of wall around it, because you go around all four sides. At an 8 foot ceiling that is 416 square feet before you take anything out.

If the room is not a plain rectangle, switch the tool to wall length and add up the walls yourself. An L-shaped room, a stairwell, or a room where you are only painting three walls all work better that way.

Where the coverage number comes from

This is the only number that really moves the answer, so it is worth knowing what is behind it. Three manufacturers state it like this:

All three quote their figure at roughly the same film thickness, about 4 mils wet, so the ranges are directly comparable. They overlap at 350 to 400, and that is why this calculator starts at 350. It sits inside every one of those ranges and it errs toward having enough.

Notice that each product's own range is as wide as the gap between products. Benjamin Moore's sheet explains why in one line: coverage depends "on surface texture and porosity". Valspar puts it plainly, "less on rough or porous surfaces". A textured wall, fresh drywall, or a wall that has never been primed will pull you toward the low end. Smooth drywall that has been painted before sits at the high end.

So read your can and type that number in. Picking a different brand changes the answer less than picking a different wall.

Why the default is two coats

That default comes from the manufacturers, not from us. Benjamin Moore's data sheet says two coats are recommended on bare substrates and that previously painted surfaces can be finished with one or two. Sherwin-Williams specifies two coats over drywall in its stated system, either self-primed or over a primer coat.

One coat is a fair choice when you are going over a sound, similar color. Anything else, budget two. Doubling the coats doubles the paint, which is why this is the second biggest lever on the page.

Doors and windows matter less than you think

The tool takes out 20 square feet per door and 15 per window. The door figure is real: 36 by 80 inches is a stock slab size, and that is exactly 20 square feet. Windows are not standardized the same way, so 15 is a working convention rather than a published spec.

Here is the useful part. We ran the same 12 by 14 foot room through two independent calculators that use different conventions. Inch Calculator deducts 20 per door and 12 per window. CalculatorSoup deducts 21 and 15. Including ours, the three net wall areas came out at 372, 365, and 366 square feet. That is a spread of under 2 percent.

Meanwhile that same room needs 3 gallons at 350 sq ft per gallon and 2 gallons at 400. The deduction convention is noise. The coverage number decides your answer.

Rounding, and when a quart is the better buy

Always round up. Coming up short means a second trip, and the second can is mixed in a different batch, which can shade a little differently from the first. That difference shows up on a wall.

Rounding up does not always mean another full gallon. If the raw math says 2.09 gallons, you need a hair over 2, and 2 gallons plus a quart covers it for less than 3 gallons costs. The tool shows that option whenever it applies.

A quick cost check

Say the calculator lands on 3 gallons. At $40 a gallon that is $120 of paint. Bumping your coverage figure from 350 to 400, which you would only do on smooth previously painted walls, drops it to 2 gallons and $80. Adding a third coat pushes it to 5 gallons and $200. Those are the three numbers worth thinking about before you check out.

Common estimating mistakes

• Using floor area instead of wall area. A 12 by 14 room has 168 square feet of floor and 416 square feet of wall. They are not interchangeable.
• Taking the number on the front of the can. The marketing figure is the best case. The data sheet range is the honest one.
• Forgetting that the ceiling is a separate job. This tool does walls only. Ceilings usually take a different product, and they are measured as length times width.
• Deducting every small window. Below about 10 square feet, leaving an opening in the math is a free safety margin.
• Treating primer as paint. Primer covers less. The KILZ General Purpose data sheet states 300 to 400 sq ft per gallon, and that figure excludes what you lose to spraying.

When to stop and call someone

Two situations, and neither is about skill.

If your home was built before 1978, the existing paint may contain lead. Sanding or scraping it is regulated work, and the EPA requires certified firms for renovation that disturbs lead paint in pre-1978 housing. Test first. Do not sand blind.

The other one is height. Stairwell walls and vaulted ceilings put you on a ladder at an angle for hours. That is the most common way a paint job turns into an injury, and it is a reasonable thing to hand off.