Calculator
Paint quantity calculator
Enter the room size, the doors and windows, and the number of coats. The result shows how much paint to buy and how it was worked out. It updates as you type, and nothing leaves your device.
2 gallons to buy
Worked example (shown when scripts are off): a 12 ft by 10 ft room with 8 ft ceilings has 2 x (12 + 10) x 8 = 352 sq ft of wall. One door and two windows take off 20 + 30 = 50 sq ft, leaving 302 sq ft. Two coats at 350 sq ft per gallon is 1.73 gallons, and a 10 percent allowance makes 1.90 gallons, which rounds up to 2 gallons.
Before you decide
Read the coverage figure on the can you plan to buy, and put it in the coverage box. Dark colors over light walls, bare drywall and rough texture usually need more paint or a primer coat first. If the answer lands just above a whole gallon, think about whether you want the leftover for touch-ups.
The formula
The calculator uses the standard room estimate. Add up the wall lengths (twice the length plus twice the width) and multiply by the ceiling height. Subtract 20 square feet for each door and 15 square feet for each window, which are the usual allowances in published estimating guides. If you tick the ceiling box, length times width is added. Multiply by the number of coats and divide by the coverage of one gallon.
The default coverage is 350 square feet per gallon, the cautious end of the 350 to 400 range that paint guides quote for one coat on a smooth, previously painted wall. The calculator then adds a 10 percent allowance for rough patches, a bit of waste and touch-ups, and rounds up to the next quart. Metric inputs are converted to feet first, and the result is also shown in litres.
Worked example with a bigger room
A 14 ft by 20 ft room with 8 ft walls has 544 square feet of wall. Two doors and two windows remove 70, leaving 474. For one coat at 350 square feet per gallon, that is about 1.35 gallons. With the 10 percent allowance it is 1.49 gallons, so you would buy 1 gallon and 2 quarts. A second coat doubles the paint, which comes to 2.98 gallons with the allowance, so 3 gallons.
What the number leaves out
- Trim, baseboards, doors and window frames. They are usually painted in a different product, so estimate them separately.
- Primer. New drywall, stains, patched areas and big color changes often need it, and it is a separate purchase.
- Very porous, rough or textured walls, which soak up more paint than a smooth wall.
- Walls with large alcoves, vaulted ceilings or stairwells. Measure each wall as a rectangle and add them up by hand.
Limits
This is a planning estimate, not a promise. The coverage printed on the can for that product, and the maker's instructions, come first. Buying all the paint for one wall at the same time, from the same batch, helps the color match. If you are unsure, buy a little more than the result and keep the can sealed so you can return it if the store allows.
If the house was built before 1978, the paint you are sanding or scraping may contain lead. Do not dry sand or scrape it. Read the safety checklist on this site before you start.
Where this comes from
The formula, the 350 to 400 square feet per gallon range, and the door and window allowances come from published paint-estimating guides, including the Old Farmer's Almanac room paint guide and a home improvement article on estimating paint. The 10 percent allowance and rounding to a quart are our own planning choices, not a published rule. The lead note follows the Environmental Protection Agency guidance for do-it-yourselfers on pre-1978 homes. This page is general information, not professional advice.