Free calculators for the trades
Three jobs you can finish in the browser: size a drain from fixture units, size a fire alarm standby battery, and check a scale ticket and slide your tandems. Each one runs the same math as our iPhone app for that trade, checked against the app’s own tests.
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To size a drain, add up the drainage fixture units (DFU) of every fixture it serves, then take the smallest pipe whose IPC rating covers that load. Building drains are also rated by slope, and a building drain that serves a water closet is never smaller than 3 inches. Enter your fixtures and the calculator does both steps.
Code basis: International Plumbing Code (IPC) Tables 709.1, 710.1(1) and 710.1(2), values as published in the 2015 to 2024 editions, as transcribed in DWV Pro. Not affiliated with or endorsed by the ICC. International Plumbing Code and IPC are trademarks of the International Code Council, Inc.
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Required battery capacity = (total standby current × standby hours + total alarm current × alarm minutes ÷ 60) × (1 + safety margin). The usual figures are 24 hours of standby, 5 minutes of alarm and a 20% margin. They come from the code edition your AHJ has adopted, so each one is editable here.
Code basis: the standby hours, alarm minutes and margin are inputs, not built-in code values. The defaults are the widely required figures (24 hours, 5 minutes, 20%). Not affiliated with or endorsed by NFPA.
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Federal limits are 12,000 lb on the steer axle, 34,000 lb on each tandem group and 80,000 lb gross (23 U.S.C. §127). If the drives are heavy, slide the trailer tandems forward; if the trailer tandems are heavy, slide them back. Each hole moves a set amount of weight, about 600 lb on a typical trailer, so divide the overage by that and round up.
Limits: the federal maximums in 23 U.S.C. §127, as applied in TandemIQ. States and permits can impose different ones.
The web tools cover each app’s free features. Saved jobs, PDFs and the rest of the toolkit are in the apps. Every tool is a planning aid: check the result against the code or rules your inspector or authority enforces.