Paint coverage and coats: how much should you buy?

For a rectangular room, wall area is 2 × (length + width) × height. Deduct doors and windows that will not receive the wall paint. Add ceiling length × width only if the ceiling uses the same product, coat count and coverage; otherwise calculate it separately.

For exterior work, measure each elevation and any gable rather than using interior floor area. Trim and doors may need a different finish. Mixing products into one total can hide different coverage rates and leave you with enough paint overall but too little of a specific color.

A coverage value such as 350 sq ft per U.S. gallon describes area for one coat under the product’s stated conditions. Multiply net area by the number of coats before dividing. One U.S. gallon is about 3.785 liters; do not substitute an imperial gallon or compare a per-liter rate without converting it.

Compare a label-based case with a lower-coverage case when surface texture, porosity or application method makes consumption uncertain. Large color changes may require a different coating plan. The calculator does not determine primer compatibility, the required number of coats or drying time; use the manufacturer’s product instructions.

The room paint tool calculates theoretical volume and a whole-gallon purchase suggestion. It has no separate waste-percentage field. If you decide to keep touch-up stock or account for application loss, apply that allowance separately and record why it is needed; do not quietly alter the measured room dimensions.

Turn the final volume into available can sizes for the exact color and finish. A whole-gallon suggestion is not a requirement to buy only one-gallon cans. Compare packaging price, future touch-up needs and storage instructions, and keep primers, trim coatings and repair materials on their own order lines.

A 12 × 10 ft room with 8 ft walls has 352 sq ft of wall area. Deduct 51 sq ft of openings to leave 301. Two coats at 350 sq ft per U.S. gallon require 602 / 350 = 1.72 gallons, or 2 whole gallons before any separately chosen allowance. At 250 sq ft/gallon, the same walls require 2.41 gallons, or 3 whole gallons.

If a 120 sq ft ceiling receives that same paint at 350 sq ft/gallon and two coats, total coated area is (301 + 120) × 2 = 842 sq ft, needing 2.41 gallons before allowance. If ceiling paint is different, retain the 1.72-gallon wall calculation and calculate the ceiling’s 240 sq ft of two-coat coverage with its own product rate.