How the numbers are made

Carpentry Calculators is 73 calculators, 33 printable charts and 59 FAQ pages for carpentry and woodworking. This page is about where their numbers come from, because a figure you are about to cut to is only as good as the method behind it.

Computed, never typed

Every calculated figure on the site is worked out by a small set of functions, and those are the same functions whether the number appears in a calculator, a chart or an FAQ page. A chart is not a table somebody retyped: it is generated when the page is built, by running those functions — or reading the same data tables they read — across a range of inputs. An FAQ page does the same for one case and prints its neighbours. So the three cannot disagree with each other — change the arithmetic and every page that uses it changes together.

Tested against published figures

Those functions are covered by automated tests, and the tests check them against numbers a tradesperson would recognise from outside this site rather than against whatever the code happens to produce. A few of them:

• —Crown molding with a 38° spring angle, cut flat in a square corner, is set at 31.6° miter and 33.9° bevel — the figures every compound-miter table prints.
• —A 6/12 roof takes 13.4164" of common rafter per foot of run — the figure the rafter table on a framing square gives, to the two decimal places it has room for.
• —A 2x4 is 1 1/2" × 3 1/2" and a 1x6 is 5 1/2" wide — the dressed sizes in the lumber standard below.

Where no single published number exists, the tests check a published relationship instead — that doubling a shelf's thickness cuts its sag to an eighth, for example — which catches the kind of mistake a single spot check can miss.

Sources

Where a figure is a property of wood or a published standard rather than geometry, it comes from one of these. The chart and FAQ pages that rely on one name it under their notes.

• Wood Handbook: Wood as an Engineering Material (FPL-GTR-282, 2021), USDA Forest Service, Forest Products Laboratory.

  Used for species densities, stiffness, hardness, seasonal movement coefficients and fastener withdrawal equations.

• 2021 International Residential Code, International Code Council.

  Used for stair, handrail and guard dimensions, roof slope limits for shingles, floor joist spans and the L/360 deflection limit.

• American Softwood Lumber Standard (DOC PS 20), National Institute of Standards and Technology, US Department of Commerce.

  Used for the dry, surfaced sizes of nominal softwood lumber.

• ASTM F1667, Standard Specification for Driven Fasteners: Nails, Spikes, and Staples, ASTM International.

  Used for nail shank diameters by penny size.

• National Design Specification for Wood Construction (2018 edition), American Wood Council.

  Used for bolt edge, end and spacing distances as multiples of bolt diameter.

Species figures are averages. Wood varies from tree to tree, so a species number is a good bet across a stack of boards and no promise about the one in front of you. Prices, where a calculator shows one, are rough national averages that go out of date, and every price field can be changed.

Building code figures are defaults, not law

Stair, handrail, guard, roof-slope and joist-span checks use the 2021 International Residential Code. That is a model code: your jurisdiction may have adopted a different edition, amended it, or require something stricter, and the building department is the one that decides. Treat the checks here as a starting point and confirm with your local building department before you build anything that has to pass an inspection. Anything structural beyond that belongs with an engineer.

Free, with no account and no server

The calculators are free to use, and they run entirely in your browser. There is no account to make and no server that receives what you type. A job you save stays on this device, in this browser — it does not follow you to another one, and clearing your browser data clears it, so a share link is the way to move a job and an export is the way to keep one. The privacy page has the details.

Found a wrong number?

Please say so. Email support@carpentrycalculators.xyz with the page, what you entered, what it said, and where the right figure comes from if you know. A correction to a published table is made against its source, so a reference is the most useful thing you can send.