Step 1 — the area you measured
Paint: the walls (length and width of the room, times the wall height, all four walls) minus the door and window area you measured. Wallpaper: one wall’s width and height. Flooring: one rectangle of floor. Example: a 12 ft × 10 ft room with 8 ft walls has 352 sq ft of wall; with 20 sq ft of openings, 332 sq ft to paint.
Step 2 — the product’s own number
Paint: coverage per gallon printed on the can (a range that depends on the surface; the calculator uses the value you type). Wallpaper: the physical roll’s width and length, the pattern repeat and the match type. Flooring: square feet per carton printed on the box.
Step 3 — coats, strips or allowance
Paint multiplies the area by the number of coats and adds the extra percentage you chose. Wallpaper divides the wall width by the roll width and rounds up to whole strips, then adds trim to the height (and rounds the cut up to a whole pattern repeat for straight match). Flooring adds the allowance percentage you chose.
Step 4 — round up to what you can buy
Paint: 332 sq ft × 2 coats ÷ 350 sq ft per gallon × 1.10 = 2.087 gallons, rounded up to the next quarter gallon: 2 one-gallon cans + 1 quart (or 3 gallons if only gallon cans are sold). Wallpaper: strips per physical roll = usable roll length ÷ cut length, rounded down; rolls = strips ÷ strips per roll, rounded up; packs = rolls ÷ rolls per pack, rounded up. Flooring: 120 sq ft × 1.10 = 132 sq ft ÷ 23.64 sq ft per box = 5.58, rounded up to 6 boxes (141.84 sq ft).
What the shopping list separates
The amount the math says you need, the amount you will buy, and the difference from rounding are shown as separate lines. For flooring, the part of the surplus that comes from your allowance is separated from the part that comes from whole-box rounding.
What the calculators leave out
Ceilings, trim and primer unless you measure them separately; half-drop and offset wallpaper patterns, murals and reusing offcuts; cutting layouts, subfloor and installation decisions; prices. When an input is outside what the calculator supports, it says so instead of inventing a number. Every number on this site is arithmetic on what you typed; nothing is estimated by an AI model behind the scenes.
Checked against the engine
The three examples above were calculated by hand and compared with the site’s calculation code on 2026-09-07; they matched. The code is fixed and covered by automated tests, so the same inputs always give the same answer.