Methodology: where our numbers come from

We take numbers from the people who publish them as their own: federal standards such as NIST’s lumber and plywood standards and the USDA Forest Products Laboratory’s reports, state transportation departments’ drawings, the driveway, sidewalk and fence rules cities publish as their own law, and the data sheets and install guides of the companies that make the product. A trade association’s printed value is used one at a time, with its document named. Wage and climate files from the Bureau of Labor Statistics and NOAA are collected for features still to come.

The newest document was checked on Oct 13, 2026. 512 documents are saved as files with a hash of their bytes, so we can tell later whether a document changed. 50 we have seen only as a search excerpt; their facts carry a mark and are checked against the page itself before a planner uses them. 121 would not open, and we did not get around a block to read them.

Some sources are left out on purpose: retailers’ sites that forbid collection, paid standards we can’t quote, and sites whose terms forbid automated reading or use by AI. Where a free government document repeats a number from a paid standard, we cite the government document.

A fact is one value as a document prints it, never a whole table. Each one records the value and its unit; the words as printed; whether it is exact, a maximum, a minimum, a range or approximate; the product and conditions it applies to; the document, the place in it and a quote to find it again; the kind of evidence; the day it was checked; and a status. A value printed for one size is not stretched to cover another.

A planner never holds a number of its own. Its constants sit in one place in the code, and each names the fact it comes from by its id. A test reads our data files and stops the build when a fact is missing, comes from another document, or holds a different number than the constant. A conversion, such as feet to inches, is written beside the constant with its reason and checked the same way.

Today the planners’ constants cite 163 facts from 68 documents. 56 of those facts are marked disputed; the next section shows what a plan does with them.

The arithmetic is code with tests of its own. No number on a plan is typed in by hand or written by a language model.

When documents disagree, even two editions of the same maker’s sheet, we keep every value with the document that prints it. A plan that relies on one of those numbers names the value it uses and gives its reason in one line, so you can check the choice against the document.

A choice is made where two documents print different values for the same quantity and a plan needs one of them. The rule is written in the planner’s code, beside the value it takes, and the reason it gives is printed on the plan. A value taken from a state or city code is the text that place published; a federal standard’s value is taken as printed.

The numbers where a plan chooses between values are in the next section, each with every value it weighed. Values our data marks disputed are counted by topic after them. The same cases were also run through other calculators, further down.

265 values in our data are marked disputed, across 15 topics and 111 documents. Each is shown as its document prints it, or as our arithmetic on a printed value.

14 numbers are chosen between on the planners’ pages, in the cases below. Each card gives every value, its document, and the one the plan uses.

Each topic’s count is in the table, and its first values open under its name. The rest are in our data and are not printed here. Values a plan uses are not repeated in these lists; they are the cards above.

We ran the same cases by hand through free online calculators whose terms allow it, with the same inputs, and wrote down what each one printed. Where they differ, the cause is method: whether a gate is counted, whether waste is added, which density turns a volume into tons. Each case below opens to its table.

Some widely used calculators count bagged concrete by its weight at an assumed density instead of the yield printed on the bag, and some leave the post out when they work out the concrete in a post hole. Their terms forbid commercial use of their results, so their numbers are not shown here.

A plan is a starting point for buying and digging. These are things it can’t know:

• Local codes and permits. We record rules some cities and states publish, but your city, county and homeowners’ association decide. Ask your permit office before you build.
• Your ground. Soil, drainage, slope and frost depth at your address change how deep to dig and how much base to lay.
• Engineering. Spans, loads, retaining walls and anything a person stands on or behind need a design checked by a qualified person. The deck planner waits for span values from a state’s or city’s published code, the only kind of code value we take.
• Prices. Most stores don’t publish prices openly, and the ones we found change by region and week. The planners show no prices for now.
• Buried lines. Only your state’s call-before-you-dig service can mark them.

If a number here or on a plan doesn’t match its document, we want to know. A contact address is coming; until it is published, this site has no way to send us a note, and we won’t pretend otherwise. Fixes are listed on the Corrections page with the date and what changed.