Construction calculators
Published:
Each calculator states its formula, walks a verified worked example, and answers the questions the search result didn't. All of them run in your browser, free, in imperial or metric — and each page links a plain-markdown mirror of itself from its , listed together in /llms.txt.
- Concrete Slab Calculator
Concrete slab volume in cubic yards or cubic meters, plus 80 lb, 60 lb, or 25 kg bag counts — free, imperial + metric, with waste factor.
- Rafter Length Calculator
Common rafter length from run and roof pitch (rise-in-12 or degrees) — line length, rise, and plumb-cut angle. Free, imperial + metric.
- Stair Layout Calculator
Riser count, exact riser height, treads, and total run from floor-to-floor rise — with IRC and DIN 18065 comfort checks. Free, imperial + metric.
How do you know the answers are right?
Each calculator is backed by a file of test vectors — an input set and the exact output it must produce — that runs on every single build. There are 125 of them today: 42 for the concrete slab, 42 for rafter length, 41 for the stair layout. A build whose engine disagrees with one of them fails, so a wrong answer cannot quietly reach this page.
That is a claim about THIS website, and it is worth being precise about the limit: these are the web calculators’ own vectors, checking the JavaScript that runs in your browser. The Tenon Calc mobile app is a separate implementation with its own vector suite, and the two are not the same engine. Where the app and this site have differed, that has been treated as a defect to resolve rather than a detail to leave in place.
Every page also shows its working. The formula is written out in plain terms, the worked example carries real numbers end to end, and the assumptions that drive a result — bag yield, waste factor, the code limit being applied — are on the page where you can see them and change them.
Do they work in metric?
Yes, and the switch is live rather than a separate page: change the unit and the same calculation re-reads in the other system. Rise in millimetres, slab thickness in centimetres, volume in cubic metres and litres of bag yield are all first-class inputs, not conversions bolted onto an imperial answer.
Where a national code sets the limits, the code is named. The stair calculator carries IRC R311.7.5 for US residential work and DIN 18065 for German work, because the comfortable riser in one is not the legal riser in the other.
What do these calculators deliberately not do?
They do not cover hip and valley rafters, trigonometric layout, or electrical sizing. They do not reproduce code tables from NEC or DIN — the physics is here, the tables belong to the bodies that publish them, and a copy of a table that goes stale is worse than no table.
And none of them is a substitute for the person signing off the work. Every result is an estimate to check against the drawing, the spec and the local code before anything gets cut or poured.
This page as plain text: markdown mirror.