How We Calculate

Our approach

A calculator is only useful if you can see how it reached its answer. Each LumberCalcs 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.

Sizes, board feet and plywood

Lumber is named by its nominal size and sold in smaller actual sizes: a 2x4 is 1.5 by 3.5 inches, a 2x6 is 1.5 by 5.5 inches, and a 1x6 is three-quarters of an inch by 5.5 inches. One board foot is a piece one foot long, one foot wide and one inch thick, so board feet equal thickness in inches times width in inches times length in feet, divided by 12. A standard sheet of plywood is 4 by 8 feet, which is 32 square feet. The size charts and the board foot and plywood tools use these standard definitions.

Framing, rafters and stairs

The stud count tool uses the spacing you choose, usually 16 or 24 inches on center, plus extra for corners, openings and plates. Rafter length comes from the Pythagorean relationship between the run and the rise of the roof pitch, plus the overhang. Stair stringer tools divide the total rise by a riser height and the run by a tread depth, and the results are checked against typical residential limits that you must confirm locally. The joist span, header and beam tools are simplified estimates based on commonly published span tables and loads, and they are not engineering calculations.

Fencing

Post and picket counts come from the fence length, the spacing and the picket width and gap. Post depth follows the common practice of burying about a third of the post, plus extra for frost and soil, and concrete volume is the hole volume minus the post, converted to bags using typical bag yields. Local frost depth and soil can change these figures.

Decking and finishing

Deck board quantities divide the deck area by the board's coverage width and add an allowance for waste and cuts. Baluster spacing divides the opening so that no gap is larger than the maximum you enter. The miter and crown molding tools use angle geometry. The stain tool uses the coverage per gallon on the product label, the wood movement tool uses approximate shrink and swell rates, and the Janka and weight tools use published figures for each species.

Converters

An inch is exactly 2.54 centimeters and exactly 25.4 millimeters, which is how the converters work.

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.