Free electrical calculators
Code-ready electrical math. Free.
Voltage drop, conduit fill, box fill and Ohm’s law, built on the NEC tables and fast enough to use on a ladder. No app to install, no account, no paywall.
The toolkit
Four calculators you’ll use every week.
Most trade apps charge a subscription to multiply three numbers. We think the math should be free, and the explanation should come with it.
Built like a pro tool. Priced like a pencil.
Code tables built in
Resistance, conductor area and box volumes come straight from NEC Chapter 9 and Article 314. Each page lists the exact table it uses.
Made for your phone
Big inputs, instant results and your last numbers remembered on your device. It loads fast on a weak jobsite signal.
Shows its work
Formulas, worked examples and the limits of each method, so you can check the answer and explain it to an inspector.
Electrical calculators that explain themselves
Circuit Math puts the calculations electricians do most into one fast, free website. The voltage drop calculator sizes conductors for long runs against the 3% and 5% recommendations. The conduit fill calculator checks THHN in EMT or PVC against the 40% rule. The box fill calculator applies every counting rule in NEC 314.16. The Ohm’s law calculator solves volts, amps, ohms and watts from any two values.
Every tool works in any browser, remembers your last inputs on your own device and never asks you to sign up. Under each calculator is a plain-English guide covering the formula, a worked example, the code section it relates to and the mistakes we see most often.