Paint Calculator
Estimate how many gallons of wall paint, ceiling paint, and primer a room needs — with doors and windows subtracted from the paintable area.
How the Paint Calculator works
This paint calculator answers the question every painter starts with: how much paint do I need? Enter the room's length and width in feet, the ceiling height, and the number of doors and windows. Choose whether you are painting the ceiling, how many coats you want, whether to add primer, and the coverage rate of your paint. The tool returns the gallons of wall paint, ceiling paint, and primer to buy.
It starts from the paintable wall area, removes the openings you will not paint, and accounts for the fact that two coats need twice the paint. If you include the ceiling it sizes that separately, since a ceiling is a flat horizontal surface measured differently from the walls.
Every result is rounded up to a whole gallon, because that is how paint is sold and because a little extra is exactly what you want for touch-ups. Use it for a single bedroom or a whole-house repaint — the logic is the same.
How to calculate how much paint you need by hand
Begin with the wall area. Add the room's length and width, double that to get the perimeter, then multiply by the ceiling height. A 12 by 10 ft room with 8 ft ceilings is (12 plus 10) times 2 times 8, or 352 sq ft of wall.
Subtract the openings you will not paint: about 21 sq ft for each door and 15 sq ft for each window. One door and two windows removes 21 plus 30, leaving 301 sq ft of paintable wall.
Multiply the paintable area by the number of coats, then divide by the coverage rate — 350 sq ft per gallon is a typical default. For two coats that is 301 times 2 divided by 350, which rounds up to 2 gallons.
For the ceiling, use length times width for the area, figure two coats, and divide by the same coverage rate. Primer is estimated as a single coat over the full paintable area. Round each result up to the next whole gallon.
Tips for an accurate paint estimate
Coverage numbers are averages — the surface and the color you are working with can move the real figure noticeably.
- Textured walls, raw drywall, and porous surfaces drink paint, so drop your coverage rate below 350 sq ft per gallon.
- Big color changes — dark to light especially — often need a third coat or a tinted primer to hide the old color.
- Measure ceiling height precisely; an extra foot across four walls adds up to real square footage and another can of paint.
- Trim, doors, and casings are painted with separate enamel and are not part of this wall estimate — quote them on their own.
- Buy slightly more than the calculator shows and keep the leftover for touch-ups; re-matching a color later from a fresh can is rarely exact.
From gallons to a priced estimate
Knowing the gallons is the start; the client wants the cost. GreenlitBid takes a paint take-off like this and turns it into a priced, client-ready line item — paint, primer, sundries, and prep and painting labor combined with your rates so the repaint shows up as a clean figure in a professional estimate.
Questions
How many square feet does a gallon of paint cover?+
A gallon of interior wall paint covers roughly 350 square feet in one coat. Coverage varies with the paint, the surface texture, and the application method — flat paint on smooth drywall covers further than a thick paint on a textured wall. This calculator uses 350 sq ft/gallon by default and lets you change it.
How many coats of paint do I need?+
Two coats is standard for a durable, even finish and is what most manufacturers recommend. One coat may work when repainting a similar color over a sound surface. Use three coats for big color changes, deep or bright colors, or when going from dark to light.
Do I really need primer?+
Prime when covering bare drywall, patched repairs, stains, or a drastic color change. Primer seals the surface so the topcoat covers evenly and adheres well. This tool estimates one coat of primer over the full paintable area; on a clean, previously painted wall you can often skip it.
How much do doors and windows reduce the paint I need?+
This calculator subtracts 21 square feet for each door and 15 square feet for each window from the wall area, since you do not paint those openings. Trim and casings are painted separately with trim or enamel paint and are not included here.
Should I buy extra paint?+
Yes — buy a little more than the estimate. Touch-ups, a second go at thin spots, and future repairs all need paint, and matching a color later from a fresh can can be tricky. Rounding up to the next whole gallon usually gives you enough margin.
Tired of doing this math by hand?
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