How Much Paint Do You Need? A Room-by-Room Guide

Quick answer

Measure the paintable wall area in each room, subtract 15–20 sq ft for every door and 10–15 sq ft for every window, then divide by the spread rate on the can. Most interior wall paints cover 350–400 sq ft per gallon, so a typical 12x12 bedroom needs about 2 gallons for two coats. Ceilings add roughly the same square footage as the floor, so buy separately for them.

Start with a room-by-room paint calculator

Work one room at a time so a single mistake doesn't throw off your whole order. For each room, measure the length of all four walls and the ceiling height, then multiply length by height for each wall and add the totals. A 12x12 room with 8-ft ceilings has 384 sq ft of wall area before deductions. Write that number at the top of a page for that room, then repeat for every space you plan to paint.

• Measure in feet and round up to the nearest half foot.
• Include closet walls only if you plan to paint inside them.
• Note the ceiling separately; it is measured by floor area, not wall length.

Subtract doors, windows, and trim

Deduct 15–20 sq ft per standard door and 10–15 sq ft per window from your wall total. In the 12x12 example, one door and two windows remove about 45 sq ft, leaving roughly 339 sq ft of paintable wall. If you are painting trim or doors a different color, keep those areas out of the wall calculation entirely so you don't buy wall paint for them.

• Large sliding doors or picture windows can remove 40 sq ft or more.
• Skip deductions only if you are painting the trim the same color and want a small buffer.

Convert square footage to gallons

Divide your paintable square footage by the spread rate printed on the can. Most interior paints list 350–400 sq ft per gallon, which is where the phrase gallons of paint per square foot comes from: one gallon covers about 0.0025 to 0.0029 sq ft per fluid ounce, or simply 350–400 sq ft. For two coats on 339 sq ft, you need about 678 sq ft of coverage, or roughly 2 gallons at 350 sq ft per gallon. Round up to the next whole gallon rather than trying to buy a partial can.

• Two coats are standard for a color change; one coat may work only when refreshing the same color.
• If your walls are textured or porous, assume the lower end of the spread range.

Add ceiling paint coverage separately

Ceiling paint coverage is calculated from the floor area, not the wall area. A 12x12 room has a 144 sq ft ceiling, which is less than half a gallon for one coat. Because ceilings are usually painted in one coat over white, a single gallon often covers a bedroom ceiling with enough left for touch-ups. Keep ceiling paint on its own line in your paint quantity guide so you don't accidentally use wall paint upstairs.

• Measure the floor footprint and use that number for the ceiling.
• Add 10% if the ceiling has beams or heavy texture.

Check your numbers before you order

Before buying, compare your total gallons against the room list. If a room lands at 1.2 gallons, buy 2; if it lands at 2.1, buy 3. Leftover paint is useful for touch-ups, and most interior paints store for years when sealed. For the bedroom example, 2 gallons of wall paint plus 1 gallon of ceiling paint is a realistic order for a standard 12x12 room with two coats on the walls.

• Buy all gallons for one room in the same batch to avoid slight color shifts.
• Keep a paint quantity guide note on your phone so you can reorder the same amount later.

Your step-by-step paint quantity guide

1. Measure each room: Record wall length and ceiling height for every room, then multiply and add for total wall square footage. Checkpoint: write the room name and square footage at the top of each page.
2. Subtract openings: Remove 15–20 sq ft per door and 10–15 sq ft per window from the wall total. Mistake to avoid: forgetting closet doors or large windows, which can overstate your order.
3. Divide by spread rate: Use the can's spread rate, usually 350–400 sq ft per gallon, and multiply by the number of coats. Checkpoint: round up to the next whole gallon for each room.
4. Add ceiling paint: Calculate ceiling area from the floor footprint and buy ceiling paint separately from wall paint. Mistake to avoid: using wall paint on the ceiling and running short on both.
5. Confirm and order: Compare your room-by-room totals against a single store order and buy all gallons for a room in one batch. Next step: keep the final quantities in a note for future touch-ups.

Products that help you hit your number

• Make Waves: A serene greenish blue that shifts with natural light, available in Size and Finish options. Choosing your finish first lets you read the spread rate on the label and confirm how many gallons your room actually needs.
• Drop Cloth: Made of durable, leak-proof plastic to protect floors and furniture from paint spills and drips. A drop cloth keeps your measurement and painting process clean, so spills don't force you to repaint an area you already counted.