Methodology
This page explains how every number on this site is produced, what has been checked, and where the boundary of our competence sits. If a claim on a calculator page is not supported here, treat it as unverified.
One arithmetic layer, run in your browser
Every calculator is a plain JavaScript function of the values you enter. There is no server round trip, no model, no stored profile. The same function produces the result you see and the worked example printed on the page, so you can reproduce any figure with a pencil and check it against the formula shown above it.
Because the arithmetic is deterministic, a result is either right or wrong for a given set of inputs — there is no confidence interval to report. What varies is whether the inputs match your site, which is why each page lists its assumptions explicitly.
What each page must contain
- The formula, written out, not just a description of the result.
- A worked example with real numbers, computed by the same function.
- An assumptions note — what the tool takes as given.
- A "what this does not do" section, which is the part most calculators omit.
- Code references, where a dimension is governed by building code.
What we check, and what we do not
Checked: unit consistency (inches vs feet is the single largest error source in this domain), rounding direction (material counts round up, spans round down), bag yields against manufacturer data, and the worked examples against hand calculation.
Not checked, because we cannot: your local code amendments, your soil, your frost depth, your lumber's actual moisture content, or whether the board you bought is the width the label claims. Where these matter, the tool says so rather than assuming a default.
Code citations
Where we quote a dimension — guard height, the 4-inch sphere rule, riser and tread limits, minimum footing diameter — it comes from the International Residential Code as published, and the clause is cited on the page. We do not paraphrase code into our own words when a number is involved, because paraphrase is where errors enter.
We do not claim that compliance with the quoted values means your project complies. Jurisdictions adopt different editions and amend them. The inspector's measurement governs.
What we deliberately refuse to model
Three things, consistently:
- Structural sizing. Joist, beam and footing sizing depend on species, grade, spacing, live load, deflection limit and soil bearing. A form that accepts four inputs cannot produce a safe answer to a six-variable question. Our joist tool counts and lays out; it does not approve a span.
- Soil and frost. Both are site-specific and both are published locally. Guessing an average would be worse than saying nothing.
- Cost as a quote. Our cost tool uses national planning rates. Regional variation is 30–50%, and lumber is a commodity. It is a budgeting range, labelled as one.
Corrections
If a formula is wrong, we want to know, and we will say so publicly. A corrected page gets a new review date and, where the change is material, a note describing what changed. We do not silently edit numbers — a calculator that changes without saying so is worse than one that is openly wrong, because you cannot tell which state you are looking at.
Pages are reviewed when a formula changes, when a cited code value changes, or when a reader reports a discrepancy that we can reproduce.
Funding
The site is supported by advertising. Advertisers have no input into formulas, results, rankings or recommendations, and we do not accept payment to alter a calculated output. See the disclosure page for the detail.
Known limits
- The calculators assume rectangular, level, conventional wood-framed decks. Curved, multi-level and steel-framed structures are out of scope.
- Imperial units only. Metric input is not supported, which is a deliberate scope decision rather than an oversight.
- No account, no saved projects, no history. Refreshing the page clears your inputs.
- Bag yields are manufacturer averages; actual yield varies with mix and water.
Questions people actually ask
How do you verify the formulas?
By computing each worked example independently by hand and comparing it to the function output, and by checking unit consistency — the most common error in deck arithmetic is mixing inches and feet.
Do you use AI to generate content?
The site uses tooling to help draft and check pages. Every formula is arithmetic that can be verified by hand, and every worked example is computed by the same function the tool runs, so a reader can check any result without trusting us.
Why do you not publish joist span tables?
Because a span table is only valid for one combination of species, grade, size, spacing, load and deflection limit. Publishing a general one invites people to use it outside its range. We point you to the table instead.