Short answer: the coverage number is printed on the can, usually as a range such as 350–400 sq ft per gallon. Type the number from your can into the calculator. Do not use a website default, including ours.
Where to find it on the can
Look on the back or side label for “coverage”, “covers up to” or “spread rate”. It is given per gallon (or per quart on small cans) and often comes with a condition such as “on smooth, sealed surfaces”. If the label gives a range, the lower number is the safer input for a porous or textured wall.
Why it is a range, not a constant
Two examples from one manufacturer’s technical data sheets: Regal Select Interior (550) lists “Coverage per Gallon at Recommended Film Thickness 400 – 450 Sq. Ft. … Depending on surface texture and porosity” (data sheet, US, 2018), and Ultra Spec 500 Interior Eggshell (N538) lists 350 – 400 sq. ft. with the same caveat (data sheet, US, 2019). Different products, finishes and walls give different numbers. These are dated examples of how a data sheet reads, not current product claims; the can you buy is the source of truth.
Same room, two coverage numbers
A 12 ft × 10 ft room with 8 ft walls and 20 sq ft of openings has 332 sq ft to paint. With two coats and a 10% allowance:
| Coverage on the label | Paint the math needs | What you buy (gallons + quarts) | Left over from rounding |
|---|---|---|---|
| 350 sq ft/gal | 2.087 gal | 2 one-gallon cans + 1 quart (2.25 gal) | 0.163 gal |
| 400 sq ft/gal | 1.826 gal | 2 one-gallon cans (2 gal) | 0.174 gal |
Fifty square feet per gallon on the label changed the shopping list by one quart. That is why the calculator asks for your label instead of assuming one.
Keep area and coats apart
Two coats on 332 sq ft is 664 “coat-square-feet” of paint work, but the wall did not get bigger. The calculator shows the wall area once and multiplies by coats in the formula. Same room at 350 sq ft/gal: one coat needs 1.043 gal (1 one-gallon can + 1 quart); two coats need 2.087 gal (2 one-gallon cans + 1 quart). Follow the coat count on the can for your surface; new drywall usually needs two.
Primer and finish are separate products
Primer has its own coverage and its own coats. Run the calculator once per product you will buy; do not add a primer coat to the finish-paint calculation.
When 2.01 gallons becomes “2 gallons + 1 quart”
Cans come in fixed sizes, so the calculator rounds up to the next can it can buy. The rounding depends on which can sizes you tell it are available. Examples at 1 coat, 100 sq ft/gal, no allowance (so the gallons equal the square feet ÷ 100):
| Area to paint | Paint the math needs | Gallons + quarts available | Gallons only |
|---|---|---|---|
| 200 sq ft | 2.00 gal | 2 gal | 2 gal |
| 201 sq ft | 2.01 gal | 2 gal + 1 qt (2.25 gal) | 3 gal |
| 225 sq ft | 2.25 gal | 2 gal + 1 qt (2.25 gal) | 3 gal |
| 226 sq ft | 2.26 gal | 2 gal + 2 qt (2.5 gal) | 3 gal |
| 300 sq ft | 3.00 gal | 3 gal | 3 gal |
| 301 sq ft | 3.01 gal | 3 gal + 1 qt (3.25 gal) | 4 gal |
One extra square foot past a whole gallon adds a quart; if only gallon cans are sold, it adds a whole gallon. The calculator rounds by volume, not by price: two quarts may cost more than a gallon at your store, and a slightly larger allowance may push you over a boundary. Check the boundary before you choose the allowance.
Questions people ask
The label says “up to 400 sq ft”. Which number do I type?
“Up to” is the best case. Use a lower number if your wall is unpainted drywall, patched, textured or a darker colour change; the label itself usually says the coverage is lower on porous surfaces.
Do I subtract windows and doors?
Only the openings you actually measured (width × height of each). The calculator has a field for that; leaving it at 0 simply buys a little more paint.
Does the quart price matter?
Not to this calculator. It gives the smallest purchase volume that covers the math. Compare prices at the store.
Numbers in the tables were calculated by hand and checked against the site’s calculation code and its automated tests on 2026-09-08. They describe the examples shown, not any specific product.
Sources: Benjamin Moore Regal Select 550 TDS; Benjamin Moore Ultra Spec 500 N538 TDS. Arithmetic: this site’s calculator, see how Wall Math calculates.