Use this paint coverage calculator to find how many gallons you need for a room. Enter the room length, width, and ceiling height, subtract doors and windows, then set coats and coverage per gallon. It returns wall area, net paintable area, exact gallons, and gallons to buy, so you purchase enough paint without waste.
Paint is easy to underbuy. One gallon too few means a second trip mid-coat, and one too many means money stored in the garage. The error usually comes from estimating from floor size alone. Paint covers walls, not floors, so height and openings matter more than most shoppers expect.
How the Paint Coverage Calculator Works
The paint coverage calculator follows the steps a professional estimator uses. First, it finds the distance around the room. Then it multiplies that distance by ceiling height for total wall area. Next, it subtracts a standard allowance for each door and window. Finally, it multiplies the remaining area by coats and divides by coverage per gallon.
- Perimeter equals 2 times length plus width.
- Wall area equals perimeter times ceiling height.
- Net paintable area equals wall area minus doors times 21 square feet minus windows times 15 square feet.
- Exact gallons equals net paintable area times coats divided by coverage per gallon.
- Gallons to buy equals exact gallons rounded up to the next whole gallon.
Rounding up matters. Paint is sold in whole gallons, so 1.72 gallons still means buying 2 gallons. The exact figure helps you compare products, but the buy figure is the number to take to the store.
How to Use the Paint Coverage Calculator
You need only a tape measure and a count of doors and windows. Measure walls in feet, using decimals for inches: 10 feet 6 inches is 10.5 feet. Measure ceiling height from floor to ceiling.
- Enter the room length in feet.
- Enter the room width in feet.
- Enter the ceiling height in feet.
- Enter the number of doors in painted walls.
- Enter the number of windows in painted walls.
- Set the number of coats. Two coats is standard.
- Set coverage per gallon from the can label. The default is 350 square feet.
- Read wall area, net paintable area, exact gallons, and gallons to buy, then buy the rounded-up amount.
Paint Coverage Calculator Examples
Both examples below use an 8 foot ceiling, 1 door, 2 windows, 2 coats, and 350 square feet per gallon. Only room size changes, yet the exact gallons still shift.
| Room | Perimeter | Wall area | Net area | Exact gallons | Buy |
|---|---|---|---|---|---|
| 12 by 10 ft, 8 ft ceiling, 1 door, 2 windows, 2 coats, 350 sq ft per gallon | 44 ft | 352 sq ft | 301 sq ft | 1.72 gallons | 2 gallons |
| 12 by 12 ft, 8 ft ceiling, 1 door, 2 windows, 2 coats, 350 sq ft per gallon | 48 ft | 384 sq ft | 333 sq ft | 1.90 gallons | 2 gallons |
Check the first row by hand. Perimeter is 2 times 12 plus 10, or 44 feet. Wall area is 44 times 8, or 352 square feet. Subtract 21 for the door and 30 for two windows to get 301 square feet. Multiply by 2 coats and divide by 350 to get 1.72 gallons, which rounds up to 2 gallons.
What Affects How Much Paint You Need
Label coverage is a guide, not a promise. Smooth, primed drywall usually comes close to the label figure. Rough plaster, textured walls, brick, and bare wood soak up more paint and can push real coverage well below the label. A dark color over a light wall, or the reverse, may need an extra coat for full hiding.
Primer also matters for bare drywall, patched spots, and strong color changes, so measure it separately. For outdoor work, a concrete calculator for a new patio slab can help you plan the base before painting nearby trim, and a mulch calculator for garden beds helps finish beds beside a painted fence. A gravel calculator for a driveway keeps that project on budget, while a topsoil calculator for raised beds helps you fill beds near a painted wall to the right depth.
Tips to Avoid Buying Too Much or Too Little
Measure twice, especially in older homes where opposite walls may differ. Use the larger length when walls differ, so the estimate stays safe. Buy all gallons for one room at the same time so color stays consistent, and save leftovers for touch-ups with the color name and finish written on the lid. Do not stretch a coat thin to make paint last, since thin coats show marks and often force a third coat.
Frequently Asked Questions About Paint Coverage
How many gallons of paint do I need for a 12 by 12 room?
For a 12 by 12 foot room with an 8 foot ceiling, 1 door, 2 windows, and 2 coats at 350 square feet per gallon, you need 1.90 gallons, so buy 2 gallons.
How do you calculate paint coverage per gallon?
Divide net paintable area by coverage per gallon, then multiply by coats. For example, 301 square feet times 2 coats divided by 350 gives 1.72 gallons.
Does paint coverage include doors and windows?
No. Subtract 21 square feet per door and 15 square feet per window, since you do not paint those openings. Measure larger openings separately if they differ greatly from standard size.
How many coats of paint do I need?
Two coats is standard for most interior walls. One coat may look patchy over primer or a color change, and a third coat is common when covering a dark color with a light one.
What is the average coverage of one gallon of paint?
Most interior paint covers about 350 to 400 square feet per gallon on smooth, primed drywall. Rough or unprimed surfaces cover less, sometimes 250 to 300 square feet per gallon, so check the label.
Should I round up when buying paint?
Yes. Paint is sold by the gallon, and running short mid-coat can leave a visible line where fresh paint meets dried paint. The extra paint also covers later touch-ups.
Learn More
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