TrueOhm – Ohm's Law & Power

509 Electric

About this app

Fast, clear Ohm's law and power calculations for working electricians, apprentices, estimators, and inspectors. TrueOhm solves for any unknown, shows the exact formula with your numbers plugged in, and flags anything that doesn't add up — so you trust the answer instead of guessing which formula to use.

WHAT IT DOES
Five focused calculators, each showing its work:
- Ohm's Law wheel — enter any two of volts, amps, ohms, or watts and TrueOhm solves the other two using the full 12-formula wheel (V=I*R, P=V*I, R=V2/P, I=sqrt(P/R), and the rest).
- AC Power (single- and three-phase) — relate volts, amps, kVA, kW, and power factor, then solve for the one you're missing. Three-phase correctly applies the square root of 3 to line-to-line voltage.
- Power Triangle — enter any two of kW (real), kVAR (reactive), kVA (apparent), power factor, or angle, and get the rest from S2 = P2 + Q2 and PF = cos(theta).
- Energy Cost — kW x hours x $/kWh gives kWh used and dollar cost for a run; edit the rate to match your utility.
- Efficiency — n = P_out / P_in, expressed as a percentage.

SHOWS ITS WORK
Every result opens a how-this-was-calculated panel with the chosen formula and your values substituted in — for example, I = P / V = 1440 / 120 = 12 A. No black-box answers. Great in the field and for teaching apprentices the why, not just the number.

CATCHES MISTAKES
Non-blocking sanity checks warn you without locking up the calculator: power factor outside 0 to 1, negative volts/amps/ohms/power, divide-by-zero inputs, an impossible power triangle (real power greater than apparent), and efficiency over 100%. You always get a best-effort answer and stay free to keep editing.

BUILT FOR THE FIELD
- Works fully offline — every calculation runs on your device.
- Light and dark mode.
- Clean, fast, single-purpose layout — no clutter, no ads.

WHO IT'S FOR
Journeymen and apprentices sizing up a load, estimators sanity-checking kVA and kW, and inspectors verifying a power-factor or three-phase figure on the spot. TrueOhm covers the electrical fundamentals that sit underneath every NEC sizing calculation — pair it with NEC-based 509 tools like TrueDrop (voltage drop) and TrueMotor (NEC 430 motor sizing) for code work.

ACCURACY
TrueOhm uses standard electrical formulas — Ohm's law, the power triangle, and single- and three-phase AC power (with the square root of 3). It is a field aid for qualified professionals, not a substitute for engineering judgment or the National Electrical Code. Inputs, assumptions, and warnings stay visible with every result.

PRICE
TrueOhm is completely free — no ads, no in-app purchases, no subscription, and no account.

PRIVACY
Every calculation runs on your device. TrueOhm collects no personal data, requires no account, and works fully offline. Nothing you enter leaves the phone.

Built by 509 Electric LLC, a licensed Washington electrical contractor (EL01) in Yakima — field tools made by people who wire buildings for a living. Part of the True suite of NEC field calculators for electricians.

What’s new

Initial closed testing release. Ohm's law, AC power, power triangle, energy cost, and efficiency calculator for electricians.