How this calculator works
It measures the four walls as one strip, subtracts doors (21 sq ft) and windows (15 sq ft), and adds the ceiling if you prime it too. It multiplies by coats, divides by the primer’s coverage, adds waste and rounds up to gallons and quarts.
The default coverage is 300 sq ft per gallon. Interior primers typically list 200 to 400 sq ft per gallon; porous surfaces such as new drywall, raw wood and masonry sit at the low end, so check the can.
walls = 2 × (length + width) × height − 21 ft² per door − 15 ft² per window
area = walls + ceiling (length × width, if primed)
gallons = area × coats ÷ coverage × (1 + waste)Worked example
A 12′ × 12′ room with 8′ walls has 2 × 24 × 8 = 384 sq ft of wall. Take off one door (21) and two windows (30) and add the 144 sq ft ceiling: 477 sq ft to prime. One coat at 300 sq ft per gallon is 1.59 gallons; with 10% waste that is 1.75 gallons, which the list rounds up to 2 gallons.
Tips before you buy
- Ask the store to tint the primer toward your finish color. It helps the topcoat cover in fewer coats, especially with deep colors.
- Use a stain-blocking primer over water stains, smoke, marker and knots in wood; ordinary primer lets them bleed through.
- Spot-prime patches and repairs even on walls that do not need a full coat, or the patches will show as dull spots.
- On new drywall, sand the joint compound smooth and dust the walls before priming; primer shows every ridge.
Questions
- How much area does a gallon of primer cover?
- Most interior primers list 200 to 400 square feet per gallon. New drywall, bare wood and masonry absorb more, so this calculator starts at a conservative 300 sq ft and lets you change it to match the can.
- When do I need primer?
- Prime new drywall, bare wood, patched areas, stains and glossy surfaces, and when you change from a dark color to a much lighter one. A sound, clean wall being repainted in a similar color usually does not need it.
- Do I need one coat of primer or two?
- One coat is enough in most cases. Use two on very porous surfaces, over heavy stains that still show after the first coat, or when covering a very dark color with a light one.
- Can paint-and-primer replace a separate primer?
- On walls that are already painted and in good shape, a paint-and-primer product can skip the primer step. On raw drywall, bare wood or stains, a dedicated primer bonds and seals better.