Plumbing Calculators
Plumbing is the least readable of the trades, and it is not because the arithmetic is hard.
The calculators
Every page on this site is built on a document a reader can open and check. That is straightforward for structural work, where the IRC span tables are reproduced by cities in free PDFs, and for insulation, where the Department of Energy publishes the code minimums across editions. It is unusually difficult for plumbing.
Two model codes, both paywalled, and jurisdictions split between them
American plumbing runs on the International Plumbing Code or the Uniform Plumbing Code depending where you are, and neither is free to read. Worse for our purposes, the cities that publish their adopted residential code as a PDF frequently omit the plumbing chapters: Seattle, which this site leans on throughout because it publishes chapter by chapter and states its edition on every page, adopts a separate plumbing code and its residential Chapters 25 to 33 simply are not there.
So the fixture unit tables, the drain sizing, the water service sizing and the vent rules that a plumbing calculator needs are all behind a paywall, and a page reproducing them from memory or from another calculator would be exactly the thing this site exists not to be.
What is readable, and it is more than you would think
Onsite wastewater is published. Septic systems are regulated by states rather than by the model codes, and state health and environment departments put their standards out as free documents. That is a whole design method, tank sizing through perc rates to setback distances, readable end to end.
Fuel gas is in the residential code. The IRC's own fuel gas chapter carries the pipe sizing tables, and cities that publish the residential code publish it.
And federal regulation reaches further into this trade than people expect. Fixture flow rates, appliance efficiency and product labelling are federal, and federal rules are free.
What that means for this section
It is smaller than the search volume suggests it should be, on purpose. Where the controlling document cannot be read, the page is not built, and the alternative would be a calculator that looks authoritative and cites nothing.