Circuit Math is a set of free electrical calculators built for the way people actually work: on a phone, on the job, with gloves half off. It covers the everyday math (voltage drop, conduit fill, box fill and Ohm’s law) that trade apps often lock behind a subscription.
Each calculator uses the tables in Chapter 9 and Article 314 of the National Electrical Code (NEC), and every page explains the formula, shows a worked example and lists its limits. We'd rather you understand the answer than just trust it.
What's here
- Voltage Drop Calculator: Volts lost, % drop and the smallest wire that stays within 3% or 5%.
- Conduit Fill Calculator: THHN in EMT or PVC: fill %, the 40% rule, and the smallest size that passes.
- Box Fill Calculator: Applies every NEC 314.16 counting rule and checks your box size.
- Ohm’s Law Calculator: Enter any two of V, I, R or P and get the other two instantly.
How we keep the numbers right
Every tool on Circuit Math shows the formula it uses, and each guide explains where the numbers come from. We test each calculator against worked examples from the sources we cite before it goes live, and recheck it when those sources change. Each page shows the date it was last reviewed.
If you spot a result that looks wrong, please tell us. Corrections are the most useful message we get, and we fix confirmed errors quickly.
Why it's free
Circuit Math is paid for by advertising. We don't sell accounts, subscriptions or "pro" upgrades. Every feature is free and works without signing up. Ads are kept away from the tool's buttons and inputs so you don't click one by accident.
Who runs it
Circuit Math is an independent site operated by Bemonix. It isn't affiliated with, or endorsed by, any app, manufacturer or marketplace mentioned in our guides. Product names are used only to describe compatibility or comparisons.
Questions, corrections or ideas for a new tool: hello@circuitmath.example.