How we calculate

Formulas

Every calculator uses the same geometry contractors and suppliers use: area is length times width, volume is area times depth, and cubic yards are cubic feet divided by 27. We show the exact formula on each page so you can check the math or run it by hand.

Where a formula depends on a product specification (bag yield, box coverage, sheet size, bundles per square), we use the most common US retail spec and let you override it in the form.

Material specifications we assume

Concrete: 80 lb bag = 0.60 cu ft, 60 lb bag = 0.45 cu ft, 40 lb bag = 0.30 cu ft, density 150 lb per cu ft. Drywall: 4x8 sheet = 32 sq ft, 4x12 = 48 sq ft, about 1 screw per sq ft. Paint: 350 sq ft per gallon per coat; door = 20 sq ft, window = 15 sq ft. Roofing: 1 square = 100 sq ft, 3 bundles per square, 400 sq ft per roll of underlayment. Gravel and stone: 1.4 tons per cubic yard for crushed stone, 1.35 for pea gravel and river rock, 1.3 for sand, 1.1 for topsoil. Mulch: bags of 2 or 3 cu ft. Lumber: 1x6 = 5.5 in wide, 1x4 = 3.5 in wide, 5/4 deck board = 5.5 in wide.

Waste factors

We default to 10% waste, which is the standard recommendation for straight, rectangular layouts. Diagonal patterns, hip roofs, rooms with many angles and fragile materials (large-format tile, natural stone) warrant 15 to 20%. Every calculator lets you change the waste percentage, and results show the quantity with and without waste where it matters.

Units and rounding

Inputs are in US customary units: feet, inches, square feet, cubic yards, tons, gallons. Purchase quantities (bags, boxes, sheets, bundles, boards) are always rounded up to whole units because you cannot buy a fraction of a box. Intermediate values are shown to one or two decimals.

Sources

Formulas and specifications are cross-checked against manufacturer data sheets (Quikrete, Sakrete, USG, GAF, Owens Corning, Trex), retailer product coverage listings, and standard trade references such as the International Residential Code span tables for joist spacing. When manufacturers disagree, we use the most conservative common value.

Updates and corrections

Each page shows the date it was last reviewed. If you find an error or a product spec that has changed, email hello@calcbuilt.com and we will verify and correct it.