How We Calculate

Formulas

Each calculator uses the standard geometry for its job: area for flat surfaces, volume for concrete and bulk materials, and unit counts based on the actual or nominal size of each product.

Assumptions

Coverage and yields come from manufacturer data and trade associations, for example 350–400 sq ft per gallon for interior paint and about 0.60 cu ft per 80 lb bag of concrete. Each page lists its assumptions and sources.

Waste and rounding

A waste percentage is added before rounding up to whole boxes, bags, sheets or pieces, because stores sell whole units.

Units

Metric inputs are converted with NIST factors (1 ft = 0.3048 m, 1 in = 2.54 cm, 1 gal = 3.785 L).

Testing

Every formula has automated unit tests, and every page is checked in a browser at desktop and phone widths before release.