How We Calculate

A calculator is only worth using if its numbers can be traced. Ours can: every figure behind these tools comes from a published source or from plain geometry, and every calculator page carries a Sources block linking the exact document its figures came from.

Where the numbers come from

Product yields and coverage rates are taken from the manufacturers' own documents. For example, QUIKRETE's Concrete Mix data sheet for bag yields, MAPEI's Keracolor S data sheet for grout coverage, and Belden Brick's dimensions guide for bricks per square foot. Industry standards come from the bodies that publish them: the Concrete Masonry & Hardscapes Association, the Tile Council of North America, the USDA Forest Products Laboratory, and building-code figures, such as stair riser limits, from the code text itself.

A few calculators rest on geometry alone: the volume of a bed of mulch or a load of gravel. Those pages say so in their method notes rather than dressing the maths up with a citation it doesn't have.

How each calculator is checked

Before a calculator goes live, its formula is traced back to the source document, then tested against worked examples calculated independently of the code. An automated test suite of more than 140 checks re-runs against every calculator before any change to this site ships. If an edit breaks a single expected result, the change doesn't go out. Results are always rounded up to whole purchasable units (bags, boxes, sheets and rolls), because a store won't sell you 0.6 of a bag.

Waste allowances

Real jobs waste material: cuts, breakage, spillage, uneven ground. Each calculator offers a waste allowance based on the trade's usual planning ranges. For concrete ordering, for instance, the ready-mix industry's guidance runs about 4–10%, and you can raise it, lower it, or set it to zero. The worked examples on every page state the allowance they used.

When sources disagree

Sometimes manufacturers publish different figures for the same job. Mortar is the honest example: makers' documents put an 80 lb bag at 35–40 standard bricks. Where that happens, the calculator uses the conservative end of the published range and the page says exactly that. You'd rather have one bag left over than be one short.

What we don't claim

Prices are planning estimates that you can edit. We don't track live store prices. Results are estimates for buying materials, not engineering: anything structural belongs under your local building code and, where the code requires it, a licensed professional.

Corrections

Spotted a figure that looks wrong? Tell us through the contact page. Corrections are re-checked against the source document before the calculator changes, and this page, not marketing copy, is where our method is on the record.