How to Calculate How Much Paint You Need for a Room

Why Calculate Before You Buy?

Buying too little paint means a mid-project trip to the store, and if the store mixes a new batch, the color may not match exactly - even with the same formula. Buying too much wastes money and leaves you with gallons to store or dispose of. A quick calculation before you go takes about five minutes and saves both problems.

Step 1: Measure Your Wall Area

The standard method is to calculate the perimeter of the room and multiply by the ceiling height:

This gives you the total square footage of all four walls. Do not worry about subtracting every baseboard or outlet - the small deductions are not worth the effort, and the rounding up gives you a buffer.

Step 2: Subtract Doors and Windows

Doors and windows are not painted with wall paint, so subtract them from your total:

• Standard door: approximately 20 square feet (2.5 ft × 8 ft)
• Standard window: approximately 15 square feet (3 ft × 5 ft)

If your room has unusually large windows or doors, measure them directly and use those numbers instead.

Step 3: Divide by Coverage Rate

Most interior paints cover 350 to 400 square feet per gallon when applied in a single coat. Higher-sheen paints and premium brands tend toward the higher end. Flat and matte paints on textured surfaces cover less.

Using 350 rather than 400 builds in a small buffer for corners, rolling overlap, and uneven surfaces.

Step 4: Multiply by Number of Coats

Most rooms need two coats for a clean, even finish - especially when changing colors or painting over a darker shade. Multiply your single-coat gallon total by the number of coats:

Worked Example: 12 × 15 Room

Here is a full calculation for a bedroom that is 12 feet wide, 15 feet long, with 9-foot ceilings, two standard doors, and two windows:

Wall area:

Subtract openings:

Gallons per coat:

Two coats:

You would buy 3 gallons - two gallons covers most of the job, and the third covers the remainder plus any touch-ups. Alternatively, buy two gallons and one quart if you want to spend less upfront (a quart covers about 87 square feet).

For fast results on any room size, use our paint calculator - enter your room dimensions and it handles the math instantly.

Ceiling Paint

Ceiling paint is calculated separately. Multiply room length by room width to get the ceiling area:

For the example above: 12 × 15 = 180 sq ft. At 350 sq ft per gallon, one gallon covers the ceiling in a single coat. Ceilings usually only need one coat of ceiling paint since they typically stay white and do not see color changes often.

Trim and Baseboards

Trim paint (doors, windows, baseboards, crown molding) is almost always sold and calculated separately. Trim paint is usually a semi-gloss or gloss finish and has different coverage than wall paint. A rough estimate: plan on one quart per room for a standard amount of trim. For rooms with lots of detailed millwork or wainscoting, one gallon may be more appropriate.

Tips for Getting It Right

• Buy all cans from the same batch. When the store mixes your paint, ask them to mix all cans from the same batch number. Even identical formulas can have slight variation between batches, which shows as a subtle color difference on your walls.
• Primer changes the math. If you are painting over a dramatically different color (dark to light, or light to dark), or painting new drywall, you need primer first. Primer is calculated the same way but only requires one coat. Primer-and-paint-in-one products work well for small color changes but are less effective for dramatic shifts.
• Textured surfaces need more paint. Heavily textured walls (orange peel, knockdown, popcorn) have more surface area than they appear. Reduce your estimated coverage rate to 300 sq ft per gallon for moderately textured surfaces, or 250 sq ft for heavy texture.
• Keep a quart for touch-ups. After the job is done, pour the remaining paint into a labeled quart jar and store it for touch-ups. Walls get scuffed over time, and having the exact color on hand saves you from re-buying and re-mixing.

Frequently Asked Questions