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Plumbing Calculators

Plumbing is the least readable of the trades, and it is not because the arithmetic is hard.

The calculators

• Drain Slope CalculatorIPC Table 704.1 against UPC 708.1, and they disagree at 3 inches
• Gas Pipe Sizing CalculatorFuel Gas Code tables 402.4(2) and 402.4(28), checked against their own equation
• Pipe Pressure Drop CalculatorCDA Copper Tube Handbook, Tables 14.2, 14.6 and 14.7
• Pipe Size ChartCDA Copper Tube Handbook Tables 14.2a to 14.2c, and IPC Table E202.1 for plastic
• Pipe and Conduit Offset CalculatorThe triangle, against the bending table a manufacturer prints
• Plumbing Cost EstimatorHomeAdvisor's plumber and repipe guides, with the places they disagree
• Septic Tank and Drainfield CalculatorKansas KDHE Bulletin 4-2, a state regulator's own standard
• Sewer Line Size and Replacement CalculatorIPC Tables 709.1 and 710.1(1), and the size limits in Sections 716 and 717
• Water Heater Size CalculatorGeorgia Table 506, the federal first hour rating rule and A. O. Smith's sizing guide
• Water Pipe Size CalculatorIPC Appendix E sizing, and the tables in it that disagree with each other

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.