Free Tool
Ohm's Law Calculator
Solve for voltage, current, resistance, or power — enter any two known values and get the answer instantly. All 12 Ohm's Law formulas included.
Ohm's Law Reference
V = I × RVoltage = Current × ResistanceI = V ÷ RCurrent = Voltage ÷ ResistanceR = V ÷ IResistance = Voltage ÷ CurrentP = V × IPower = Voltage × CurrentP = I² × RPower = Current² × ResistanceP = V² ÷ RPower = Voltage² ÷ ResistanceMore free tools coming — HVAC superheat calculator, pipe sizing tool
Pro Insights
- NEC branch circuit breaker size for this load:See in app
- Power factor correction for AC circuits:See in app
- Conduit fill % at calculated current:See in app
Electrician Prep
1,000+ NEC Practice Questions
Voltage drop, Ohm's Law, and full exam simulation — all offline
Ohm's Law and Watt's Law: The Electrician's Foundation
Ohm's Law states that voltage equals current multiplied by resistance: V = I × R. This single relationship produces 12 usable formulas when combined with Watt's Law (P = I × V), which defines power in watts as the product of current and voltage. Together they let electricians solve for any unknown given any two known values in a DC circuit.
On journeyman and master electrician license exams, Ohm's Law problems are typically embedded in circuit analysis questions: finding the current draw of a resistive load at a given voltage, determining the resistance of a heating element from its wattage and voltage rating, or calculating the power dissipated in a conductor with known resistance and current. These are 2–4 step calculations, not just formula recall.
Note: Ohm's Law in its basic form applies to DC circuits and purely resistive AC loads. In AC circuits with inductive or capacitive loads, impedance (Z) replaces resistance (R) and power factor must be accounted for — those calculations go beyond Ohm's Law alone.
Frequently Asked Questions
What are all 12 Ohm's Law formulas?
From V=IR and P=IV you can derive: V=IR, V=P/I, V=√(P×R) | I=V/R, I=P/V, I=√(P/R) | R=V/I, R=P/I², R=V²/P | P=IV, P=I²R, P=V²/R. The calculator handles all 12 — enter any two values to solve for the other two.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies to purely resistive AC loads (like heating elements and incandescent bulbs). For inductive loads (motors, transformers) and capacitive loads, the AC circuit uses impedance (Z = √(R² + X²)) instead of pure resistance, and the power formula must include power factor: P = V × I × cos(θ).
How is Ohm's Law used on the journeyman electrician exam?
License exam questions use Ohm's Law to find current draw (to size a breaker), resistance of a load (to check circuit balance), or power dissipation (to verify heat load). Know the power wheel (PIE/EIR diagram) for quick formula recall, and practice setting up multi-step calculations under time pressure.
What is the difference between resistance (R) and impedance (Z)?
Resistance (R) opposes current flow equally at all frequencies — it's a property of the conductor material and geometry. Impedance (Z) is the total opposition to AC current flow, combining resistance with reactance (inductive and capacitive). For DC circuits and purely resistive AC loads, Z = R.
Also try: Voltage Drop Calculator
NEC-compliant voltage drop with pass/fail status