Primer Calculator

Why prime before you paint?

Primer seals porous drywall, blocks old stains, and gives topcoat paint a uniform surface to grip. That means truer color and fewer coats of your finish paint. This calculator tells you how many gallons of primer to buy so you are not stuck mid-project or overpaying for a can you will never open.

How the math works

We use the same wall-area math as a paint calculator. Your total wall length times wall height gives the gross area. We subtract a standard 21 sq ft for each door and 15 sq ft for each window, then multiply the paintable area by your number of coats and divide by coverage. Most primers cover about 300 sq ft per gallon, and one coat is typical, so those are the defaults. Gallons are rounded up because paint stores sell whole cans.

How to use it

1. Add up the length of every wall you are priming and enter it as total wall length.
2. Enter your wall height (8 ft is standard for most rooms).
3. Set how many doors and windows are on those walls so their area is removed.
4. Leave coats at 1 for fresh drywall, or bump it up for stain-blocking jobs.
5. Read the gallons-to-buy card and pick up that many cans.

FAQ

How much does a gallon of primer cover?

Around 300 sq ft per coat on smooth drywall. Rough, textured, or very porous surfaces soak up more, so drop the coverage number to 250 for a safer estimate.

Do I really need a second coat of primer?

Usually not. One coat is standard. Go to two coats only when covering dark colors, heavy stains, or bare wood that keeps drinking it in.

Should I round up my gallons?

Yes. The tool rounds up automatically so you always have enough. Keeping a little extra also helps with touch-ups.

Is this the same as a paint calculator?

The area math is identical. Primer just tends to be a one-coat job, while finish paint often needs two, so adjust the coats field to match.

These figures are estimates to help you plan, not professional guidance. Check the coverage printed on your specific primer can.