How we calculate

Every number on this site comes from the formulas below. Nothing is hidden, and every default can be changed in the planners.

Flooring

Area = Σ (length × width) − subtracted areas (never below zero per room)

Area to buy = Area × (1 + waste %). Defaults: 10% straight, 15% diagonal, 20% herringbone.

Boxes = round up (Area to buy ÷ coverage per box), rounded once for the whole job.

Leftover = Boxes × coverage − Area to buy. This is flooring paid for only because boxes come in fixed sizes, and the main reason two products with a similar price per square foot can cost different amounts.

Underlayment rolls = round up (Area × 1.05 ÷ coverage per roll); 5% covers seam overlap. Skipped for products with an attached pad.

Total = Boxes × price per box + Rolls × price per roll

Paint

Wall area = 2 × (length + width) × height − doors × door area − windows × window area

Defaults: 21 sq ft per door (3 × 7 ft) and 15 sq ft per window. Ceilings add length × width when selected.

Paint needed = paint area × coats ÷ coverage, where coverage is the figure on the can (often 350–400 sq ft per gallon, about 9–10 m² per liter).

Can mix: we test every combination of the can sizes you enter that covers the paint needed. With a price for every size we choose the lowest total cost (ties go to less leftover, then fewer cans). Without prices we choose the least leftover paint.

Tile, grout and thinset

Tiles = round up (Area × (1 + waste %) ÷ area of one tile)

Boxes = round up (Tiles ÷ tiles per box), or round up (Area to buy ÷ coverage per box) if you enter coverage instead.

Grout, the standard manufacturer formula: kg per m² = (A + B) ÷ (A × B) × C × D × 1.6, where A and B are the tile length and width, C the tile thickness and D the joint width, all in millimeters, and 1.6 is the typical density of grout in kg/dm³. We add 10% for mixing and cleanup. Imperial results are converted to pounds per square foot.

Thinset bags = round up (Area ÷ coverage per bag). Coverage depends on the trowel notch; use the figure on the bag.

Wallpaper

Strips per wall = round up ((wall width − openings) ÷ roll width)

Strip length = wall height rounded up to a whole number of pattern repeats + trim allowance

All strips are sorted from longest to shortest and placed into rolls one by one: each strip goes into the first roll with enough length left, otherwise a new roll is started (first-fit decreasing). This reuses offcuts from tall walls on shorter walls. If a single strip is longer than a roll, the planner shows a warning instead of a number.

Baseboard and trim

Each room has four walls. Door openings (doors × opening width) are taken off the longest walls first.

Each remaining wall gets the miter allowance. A wall longer than one piece uses as many full pieces as fit, and the remainders are packed into whole pieces with the same first-fit decreasing method used for wallpaper.

Pieces = round up ((full pieces + packed pieces) × (1 + extra %))

Units

1 m = 3.28084 ft · 1 m² = 10.7639 sq ft · 1 in = 2.54 cm = 25.4 mm · 1 gal (US) = 3.78541 L · 1 lb = 0.453592 kg. When you switch units, the values you already typed are converted. Standard sizes you have not changed (roll widths, can sizes, piece lengths) switch to the usual sizes of the other system.

Limits

These are planning estimates. Real quantities depend on the product, the room, the layout and the installer. Confirm with the product label, the manufacturer's instructions and your supplier before ordering.