Paint Primer Coverage Calculator

Why bother with primer

Primer seals a porous surface so the finish coat sits on top of it instead of soaking into it. That is the whole trick. Paint is expensive and primer is cheap, so letting the cheap product do the absorbing usually means one less coat of the expensive one.

The formula

Exact gallons = (surface area × coats) ÷ spread rate With waste: exact gallons × (1 + waste% ÷ 100)

Then round up, because paint stores do not sell 1.47 gallons.

Spread rate is the number that matters

Spread rate is how many square feet one gallon covers. Standard drywall primers land around 250 to 350 sq ft per gallon. Bare wood, fresh drywall, and masonry drink far more, so drop to 200 to 250 for those. The number on the can is measured on a sealed, smooth test panel, and almost nobody paints one of those. Check the label, then take a little off it.

Waste factor

10% is the professional default. Push it to 15 or 20% for a room full of corners, closets, and trim to cut in, where a lot of primer ends up on the brush and the rim of the tray rather than on the wall.

One coat or two

New drywall or a drastic color change wants two. A repaint over smooth, previously painted, similarly colored walls is usually fine with one.

Worked example

A 400 sq ft room, one coat, 300 sq ft per gallon, 10% waste:

• Exact: 400 × 1 ÷ 300 = 1.33 gallons
• With waste: 1.33 × 1.10 = 1.47 gallons
• Round up: 2 gallons

Look at that second-to-last line before you reach for two gallons. You need 1.47, so a gallon plus two quarts gets you 1.5 and leaves half a gallon of primer un-bought. Quarts cost more per ounce, but a quart you use beats half a gallon that skins over in the garage.