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.