Paint Coverage Calculator

Background.

The most common painting mistake is buying the wrong amount of paint. Run short and the second trip can bring home a can mixed on a different day, whose slight colour difference shows across the wall in low, raking light. Overbuy and the spare cans sit in the garage for years. The arithmetic behind 'how much paint do I need' is simple in concept and unforgiving in execution: wall area, number of coats, the spread rate on the label and an allowance for waste, turned into gallons, litres, a material cost and a count of 1-gallon and 5-gallon cans.

Measuring the walls is the starting point. For a rectangular room, add the four wall lengths to get the perimeter, then multiply by the ceiling height. A 12-by-14-foot bedroom with 8-foot ceilings has a perimeter of 52 feet and a wall area of 416 square feet before any subtractions. A ceiling is a separate surface of length × width, another 168 square feet here, and usually takes a different paint. A door of about 21 square feet and a window of 12 to 15 barely move the result, but a wall of sliding glass or a great room with several large windows can take off a hundred square feet or more.

The default spread rate of 350 square feet per gallon is a typical label figure for interior latex on smooth, primed drywall. It implies a wet coat about 4.6 thousandths of an inch thick, roughly the thickness of a sheet of copy paper. Real surfaces often fall short of it. Textured drywall, popcorn ceilings, stucco, bare masonry, rough wood and never-painted surfaces absorb more, and a big change of colour, such as a light colour over deep red or navy, often needs a third coat or a tinted primer.

The result is a quantity to the nearest hundredth of a gallon, rounded up, together with both can counts, so you can choose at the paint counter whether a 5-gallon pail, single gallons or a mix covers the job with the least left over.

What is paint coverage calculator?

A paint coverage calculation converts the area you intend to paint into the volume of paint you must buy. It takes wall area in square feet or square metres, the number of coats, the manufacturer's spread rate in square feet per gallon, a waste allowance for overlap, roller absorption and touch-ups, and any window or door area to leave out. Out come the gallons needed, their equivalent in litres, a material cost at your price per gallon, and the number of 1-gallon or 5-gallon cans that would hold that much. Spread rate is the area one gallon covers in one coat at the film thickness the manufacturer recommends; it counts paint that reaches the wall, not paint left in the tray or soaked into the roller cover, which is part of what the waste factor covers. The same arithmetic works for interior latex, exterior acrylic and primer, with only the spread rate changing. US gallons are used throughout: one US gallon is 231 cubic inches, about 3.785 litres.

How to use this calculator.

1. 01Measure the walls: perimeter × ceiling height for a room. Work out a ceiling separately if you are painting it, since it usually takes a different product.
2. 02Enter the area and choose square feet or square metres. The windows-and-doors field is always in square feet, whichever unit you choose, so convert metric openings by multiplying by 10.7639: a 2 m² window is about 21.5 sq ft.
3. 03Set the coats, 1 to 4. Use 2 for a standard repaint and 3 when going light over dark or painting raw drywall without primer.
4. 04Enter the spread rate from the can or the technical data sheet, in square feet per gallon. Reduce it for textured, porous or unprimed surfaces. A label given in m² per litre converts by multiplying by about 40.75, so 10 m²/L is about 407 sq ft per gallon.
5. 05Enter the price per gallon at your retailer and a waste factor from 0 to 20%. Ten percent suits most DIY jobs; rooms with a lot of cutting in around trim may justify more.
6. 06Read the gallons, litres, cost and can counts, then compare 1-gallon and 5-gallon prices before you buy.

The formula.

The calculator works in five steps. First it converts the wall area A to square feet, multiplying by 10.7639 if you entered square metres (1 m² ≈ 10.7639 sq ft, per NIST SP 811). Second it subtracts openings O, entered in square feet, and refuses a subtraction as large as the wall itself. Third it scales by coats c and the waste factor w: effective area E = (A − O) × c × (1 + w ÷ 100). Fourth it divides by the spread rate R and rounds up to the next hundredth of a gallon, so the figure never falls short: gallons = ⌈E ÷ R × 100⌉ ÷ 100. Fifth it derives the rest: litres = gallons × 3.78541, cost = gallons × price per gallon, 1-gallon cans = gallons rounded up to a whole number, and 5-gallon cans = gallons ÷ 5 rounded up. The spread rate hides a film thickness. A US gallon is 231 cubic inches, so spreading it over 350 sq ft (50,400 square inches) leaves a wet layer 231 ÷ 50,400 = 0.00458 inch thick, about 4.6 mils or 116 micrometres; at 400 sq ft per gallon it is about 4.0 mils. Stretching paint further than the label rate thins that film, which costs hide and durability rather than saving paint. Note that the cost line prices the exact gallons needed, not the whole cans you will carry out of the store.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Sherwin-Williams — How much paint do I need? Interior paint coverage technical bulletin.
2. [2]Behr Pro — Paint Coverage Calculator and Painting Resources.
3. [3]PPG Industries — Interior Latex Technical Data Sheets (coverage and wet film thickness specifications).
4. [4]Master Painters Institute — MPI #43, Interior Latex, Institutional Low Odour / VOC (coverage and application standards).
5. [5]US Environmental Protection Agency — National Volatile Organic Compound Emission Standards for Architectural Coatings (40 CFR Part 59 Subpart D).
6. [6]NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), unit conversions for gallons and square meters.