About this tool
This paint calculator turns a room's dimensions into a shopping list: the wall area after subtracting doors and windows, the ceiling if you are painting it, the exact gallons for the number of coats you plan, and how that rounds into 1-gallon cans, 5-gallon buckets, and quarts. A primer toggle adds a separate primer quantity at its own coverage rate, and a waste allowance covers touch-ups and roller loss. It is for homeowners pricing a repaint, renters freshening one room, and contractors who want a quick sanity check against a supplier's estimate. Enter a price per can and the page gives a material cost estimate; switch to metric to work in meters and liters.
How it works
Wall area = 2 x (length + width) x height. Each door subtracts 21 sq ft (a 3 x 7 ft slab) and each window 15 sq ft (about 3 x 5 ft); both figures are editable. The ceiling adds length x width. The net area is multiplied by the number of coats and by 1 + waste%, then divided by the coverage rate, 400 sq ft per gallon by default, which is the typical spread rate printed on interior latex cans for smooth, previously painted drywall. Gallons are rounded up to whole cans; the tool also shows a gallons-plus-quarts option when the fraction is small. Primer is figured the same way for one coat at its own coverage (300 sq ft per gallon by default, because primers are thinner and soak into bare surfaces). Metric mode converts meters to feet internally and reports liters, with 1 US gallon = 3.785 L and 1 m2/L = 40.75 sq ft/gal.
Frequently asked questions
How much does one gallon of paint cover?
Most interior latex paints list 350 to 400 sq ft per gallon per coat on a smooth, sealed surface. Textured walls, bare drywall, and rough plaster absorb more and can drop the figure to 250 to 300 sq ft. Dark-to-light color changes usually need a tinted primer or a third coat rather than more paint per coat. Check the can; if it lists a range, use the lower number for the first coat.
Do I subtract doors and windows from the wall area?
Yes, unless the room has only one small window, in which case the difference is negligible. A standard interior door is 21 sq ft (3 x 7 ft) and an average window about 15 sq ft; the calculator uses those defaults and lets you change them. Sliding glass doors are about 40 sq ft each, so enter them as two doors, and note that trim around openings is often painted separately in a different sheen.
Do I need two coats of paint?
Two coats are the norm for an even color and full sheen, and paint warranties usually assume two. One coat can work when repainting the same color with a premium paint-and-primer product. Going from a dark or saturated color to a light one often needs primer plus two coats. The calculator multiplies the area by the coats you enter, so compare one and two coats before buying.
Should I buy a 5-gallon bucket or five 1-gallon cans?
A 5-gallon bucket is usually 10 to 15 percent cheaper per gallon and guarantees one consistent batch color, which matters on large walls. Buy singles when the total is under about four gallons or when you want a gallon left over for touch-ups. The tool shows both the bucket-plus-cans split and the total gallon count so you can price each at your store.