For study estimates only, not engineering design
This Ohm’s Law & Power calculator solves the two universal electrical relationships V = I × R and P = V × I for you. Enter any two of voltage, current, resistance, or power, and the tool computes the other two instantly – no algebra required.
These formulas hold in any electrical system, AC or DC, any country. This tool does not make any wire-size, ampacity, or jurisdiction-specific claim.
Ohm’s Law & Power Calculator
Pick which two values you know, enter them below, and press Calculate.
Result
- Voltage: V
- Current: A
- Resistance: Ω
- Power: W
On the Texas Journeyman exam
Where this calculation shows up
On the PSI Texas Journeyman exam this is part of the Calculations part: Electrical Theory. That part has 26 questions in 110 minutes, and you need 70% on it on its own.
Try an exam-style calculation (our own numbers): A 1,500 W heater is connected to 120 V. How much current does it draw?
- A. 0.08 A
- B. 1.25 A
- C. 12.5 A
- D. 180 A
Show the answer
C. 12.5 A. I = P / V = 1,500 / 120 = 12.5 A. Its resistance is V / I = 120 / 12.5 = 9.6 ohms.
Try 70 free Journeyman questions How the exam tests calculations
Exam format from the PSI candidate bulletin, read October 3, 2026.
How the formulas work
Every DC circuit (and, approximated resistively, most simple AC circuits) follows two relationships:
- Ohm’s Law: V = I × R (voltage = current × resistance)
- Power: P = V × I = I² × R = V² ÷ R
Given any two of the four quantities (V, I, R, P), the other two are fully determined – that’s exactly what this calculator solves.
Worked examples
- Example 1 – a simple LED circuit: a 12V supply pushes 20mA (0.02A) through a resistor. Enter Voltage=12V, Current=20mA → Resistance = 600Ω, Power = 0.24W.
- Example 2 – a heating element: a 1000W (1kW) heater is rated for 120V. Enter Voltage=120V, Power=1000W → Current ≈ 8.33A, Resistance ≈ 14.4Ω.
- Example 3 – reverse-engineering a rating plate: a motor draws 5A through 8Ω of winding resistance. Enter Current=5A, Resistance=8Ω → Voltage=40V, Power=200W.
What this tool does not do
This calculator is universal physics (V=IR, P=VI) and carries no jurisdiction dependency. It does not select a wire size, does not state an ampacity rating, and does not convert between AWG and mm² – those are separate, jurisdiction-specific questions (see our Voltage Drop Calculator for conductor sizing against a specific code basis, cited with its source and revision date).
Next step
If you’re working through Ohm’s Law problems as part of studying toward an electrician career, a judged, personal read on your specific background can save months of guessing.
FAQ
Q1. Does it matter which two values I pick?
A. No – any two of voltage, current, resistance, or power fully determine the other two. Pick whichever two you actually know from your circuit or datasheet.
Q2. Does this work for AC circuits?
A. It works exactly for DC and for purely resistive AC loads. For AC circuits with significant inductance or capacitance (motors, transformers, most real household loads), true impedance replaces plain resistance – see our Impedance guide for that distinction.
Q3. Why does entering resistance = 0 sometimes give an error?
A. A short circuit (R=0) with a nonzero current would require infinite power, which isn’t a real, finite answer – the calculator fails closed on that combination instead of showing a misleading number.
Further reading
Back to the Electrical Calculator Toolbox
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