PencilMath is a set of free calculators for home and yard projects: concrete, gravel, soil, paint and more. Each one answers a single question — how much do I need to buy? — in the units stores and suppliers actually sell: bags, tons, cubic yards, gallons.

How the calculators are built

Every calculator runs on a small, published formula. Before a calculator goes live, its math is checked by automated tests against hand-worked examples and, where they exist, manufacturer figures (for example, QUIKRETE's 45 × 80 lb bags per cubic yard). The worked example on each page is one of those tests. See Methodology for every formula and constant we use.

What the results are — and are not

Results are estimates for planning and ordering. They are not engineering advice. Structural work, local building codes and permits are outside what a calculator can check: ask your supplier or a licensed contractor when it matters.

Corrections

Found a number that looks wrong? Tell us with the inputs you used. Confirmed errors are fixed and the page's "last reviewed" date is updated.