How We Calculate

Our approach

A calculator is only useful if you can see how it reached its answer. Each HomeBuildCalcs page explains its method in plain language, shows the formula where there is one and says which numbers are typical examples that you should replace. We use published formulas and standard practice where they exist, we label rules of thumb as rules of thumb, and we do not take payment to favor a product.

Area, volume and waste

Most quantity tools start with area (length times width) or volume (length times width times depth). Volumes in cubic feet divide by 27 to give cubic yards. Each tool then adds an allowance for waste, which you can change, because cuts, breakage and spills use up material. Rectangles and simple shapes are assumed unless the tool says otherwise.

Concrete and masonry

The concrete tools turn a slab, footing, column or step into cubic feet and cubic yards, then into bags using typical yields, for example about 0.6 cubic feet for an 80 pound bag, about 0.45 for a 60 pound bag and about 0.375 for a 50 pound bag. Block counts use the face area of the block, about 0.89 square feet for a standard 8 by 8 by 16 inch block, and mortar and rebar tools use typical consumption and spacing. Your product's label gives its exact yield.

Drywall, flooring and paint

A standard drywall sheet is 4 by 8 feet, or 32 square feet. Flooring and tile tools add a waste allowance that is higher for diagonal layouts. Paint tools subtract openings and apply the coverage on the can, typically about 350 to 400 square feet per gallon, times the number of coats.

Roofing, siding and insulation

Roofing is measured in squares of 100 square feet, and the roofing tool applies a pitch factor to the plan area and adds an allowance for waste. A square typically needs about three bundles of shingles. Siding and insulation tools use wall area less openings, plus waste, and R-value tools use the figures the manufacturer publishes.

Electrical

The wire gauge tool matches current and length to a conductor size using standard ampacity and voltage drop practice. Common guidance keeps voltage drop in a branch circuit to about 3 percent. The conduit fill tool applies the usual limit of 40 percent of the conduit area for three or more conductors. The load and generator tools add up running watts and allow for starting surges. These are estimates and are not code compliance.

Framing, stairs and decks

Board feet are thickness in inches times width in inches times length in feet, divided by 12. Roof pitch is the rise over a 12 inch run. The stair tool divides total rise by a riser height and the run by a tread depth. The deck and fence tools count boards, posts and hardware from the dimensions and spacing you give.

Converters and charts

The converters use exact definitions, such as 2.54 centimeters per inch, 0.3048 meters per foot and 43,560 square feet per acre. The cubic yards to tons converter depends on the material's density, and the tool says which density it uses.

Finders

Each finder asks a few questions, scores your answers against the options and shows the best match, often with a runner-up. Some answers can force a result when a single factor decides it. Each result explains why it fits, what to check and when it is not the right fit.

Limits

All results are estimates and general guidance. Real situations vary, defaults are examples and standards change. Please read our disclaimer and safety notes, and use the Report an Issue form if a result looks wrong.