Sizing calculators for the trades

Duct, wire, conduit, gas pipe and service sizing, built from the published code tables and checked against their worked examples. Free, no sign-up, and they run in your browser.

HVAC

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Duct size calculator

Round and rectangular duct sizes from airflow and friction rate, with a CFM chart for every size.

Plumbing

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Gas pipe sizing calculator

Schedule 40 steel pipe size for a natural gas load by the longest-length method.

Electrical

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Voltage drop calculator

Drop in volts and percent for copper or aluminum, single-phase, three-phase or DC.

Conduit fill calculator

Fill percentage and the smallest EMT, rigid or PVC conduit for a mix of THHN conductors.

Electrical load calculator

Dwelling service size by the NEC 220.82 optional method, with every demand factor shown.

How these calculators are built

Every number comes from a published source, and each calculator cites it on the page: NEC Chapter 9 tables for conductors and raceways, Table 310.16 for ampacity, the IFGC sizing equation for gas pipe, and the ASHRAE friction relationships for duct. Before a calculator goes live, its results are checked against the code's own worked tables, such as Annex C for conduit fill and the IFGC capacity tables for gas pipe.

They are built for estimating, checking a design, and answering the question on site. They do not replace the code adopted where you work, a stamped design where one is required, or the inspector's call.