Primer Calculator: How Much Primer Do You Need?

Primer is the coat nobody sees, which makes it easy to skip. On some walls that is fine. On others, skipping it means stains bleeding through, paint peeling off glossy trim, or three coats of expensive paint where one coat of primer and two of paint would have done. The tool at the top tells you how much to buy. The rest of this page helps you decide whether you need it.

How much primer to buy

Primer spreads thinner than paint because it is meant to soak into the surface and seal it. Sherwin-Williams gives a range of 200 to 300 sq ft per gallon for primer, against 350 to 400 for paint. Some products claim more: Kilz rates its Premium and PVA drywall primers at 300 to 400 sq ft per gallon, depending on how porous the surface is.

The calculator defaults to 250 sq ft per gallon, in the middle of the general range. If your can says something different, use that number.

The formula is simple: area to prime × coats ÷ coverage per gallon.

A worked example: a new drywall bedroom

A 12 x 12 ft room with 8 ft walls, one door, two windows and a new drywall ceiling:

Walls: 48 ft perimeter × 8 ft = 384 sq ft, minus 51 sq ft for the door and windows = 333 sq ft
Ceiling: 12 × 12 = 144 sq ft
Total: 477 sq ft
One coat at 250 sq ft per gallon: 477 ÷ 250 = 1.9 gallons, so buy 2 gallons

At $30 a gallon, that is $60 of primer. On fresh drywall, that coat seals the paper and the joint compound so they take paint evenly. Without it, the taped seams often show through as dull stripes, called "flashing."

When you need primer

New drywall. Bare drywall paper and joint compound absorb paint at different rates. A PVA drywall primer seals both. Kilz notes its drywall primer is meant for flat and low-sheen finishes, which is what most walls get anyway.

Patches and repairs. Spot-prime anywhere you used spackle or joint compound. Painting straight over a patch leaves a dull spot that shows in side light.

Stains. Water marks, smoke, marker, crayon and grease bleed through latex paint. A stain-blocking primer stops them. Kilz says its Premium primer blocks most medium to heavy stains from water, tannin, ink, pencil, marker and grease, and recommends testing for bleed-through after priming and adding a second coat if you see any.

Bare wood. New wood soaks up paint unevenly, and woods like cedar and redwood contain tannins that bleed through light paint. Prime first.

Glossy surfaces. Paint has trouble gripping old gloss or semi-gloss trim and cabinets. Scuff-sand them, then use a bonding primer.

Big color changes. Going from dark to light, or light to a deep color, a primer tinted toward the new color can save you a coat of paint. Behr mentions this exact case: covering a dark wall with light beige may require an extra coat or a tinted primer.

Masonry and stucco. New masonry needs to cure first. Kilz says to wait at least 30 days before priming new masonry.

When you can skip it

You can usually go straight to paint when:

The walls are already painted, clean and sound.
The new color is close to the old one.
There are no stains, patches or glossy areas.

Paint-and-primer products help in these cases, but they are not a substitute for a real primer on bare drywall or stains. See how many coats of paint you need for more on when one or two coats will do.

One coat or two of primer?

One coat covers most jobs. Use two when:

Stains still show through after the first coat.
You are covering a very dark color with a very light one.
Bare wood has soaked up the first coat completely.

Drying time before painting

Primer generally dries faster than paint. Kilz lists its Premium primer as dry to the touch in about 30 minutes and ready for topcoat after an hour, and its PVA drywall primer as ready for paint after about 2 hours, at 77°F and 50% humidity. Cold or humid rooms take longer.

Frequently asked questions

How much primer do I need for a 12 x 12 room?

For the walls alone, with one door and two windows, about 1.3 gallons for one coat at 250 sq ft per gallon. Buy 1 gallon and 2 quarts, or 2 gallons if you also prime the ceiling.

Can I use paint instead of primer on new drywall?

You can, but it usually takes an extra coat to look even, and paint costs more than primer. A drywall primer is the cheaper route.

Do I need primer for a darker color?

Not always. Going darker hides more easily than going lighter. For deep reds, navies and blacks, a gray-tinted primer can still save a coat.

Is tinted primer worth it?

For big color changes, yes. Most paint stores will tint primer toward your paint color for free or for a small fee.

Next step

Once you know your primer, the paint calculator gives you the paint for the same room. Hanging new drywall first? The drywall calculator counts sheets, screws and joint compound.

Coverage depends on the product and how porous the surface is. Check the label on the primer you buy.

Primer Calculator: How Much Primer You Need, and When You Need It