Ohm's Law Calculator | Solve for V, I & R

Quick Start Guide

  1. Choose what to calculate: Select Voltage, Current, Resistance, or Power — the one you want to solve for.
  2. Enter known values: Input any two known parameters — including power, e.g. 1,200 W and 120 V. All SI units (V, A, Ω, W).
  3. View results: See all four electrical values: voltage, current, resistance, and power, plus the interactive Ohm's Law wheel.
  4. Try a preset example (optional): Load a real-world scenario like an LED series resistor or car headlight to see the formulas applied.
  5. Switch calculation target: Try solving for different variables to explore circuit relationships — expand the Formula Reference if you want to see the underlying equations.

Understanding Ohm's Law

The core relationship

Ohm's Law says the current through a conductor is directly proportional to the voltage across it, and inversely proportional to its resistance: V = I × R. Knowing any two of voltage, current, and resistance always lets you solve for the third.

Where power fits in

Power (P, in watts) is the rate energy is used, and it combines with Ohm's Law in three equivalent forms: P = V × I, P = I² × R, and P = V² ÷ R. Which form is most useful depends on which two values you already know — the calculator always shows all three.

Reading the Ohm's Law wheel

The wheel in the results panel groups all twelve V/I/R/P formulas by which value they solve for. Click a quadrant to jump straight to that calculation — it is a fast way to find the right formula without memorizing all twelve.

Short circuits and open circuits

Zero resistance (a short circuit) implies infinite current for any nonzero voltage — a real short is limited only by wiring and source resistance, which is exactly why it is dangerous. Infinite resistance (an open circuit) implies zero current no matter the voltage.

What this calculator does not cover

This calculator models simple resistive DC circuits. For AC circuits with reactive components (capacitors, inductors), the equivalent relationship uses impedance (Z) instead of plain resistance — see the LC Circuit calculator for resonance and impedance in reactive AC circuits.

Features

Flexible Solver: Solve for any of the four variables — voltage, current, resistance, or power — from any two of the others, so watts and volts give you amps directly.

Instant Results: All four electrical parameters update instantly as you change inputs.

SI Units & Precision: Full support for SI units with configurable precision for engineering work.

Formula Reference: An expandable reference box shows the underlying V=IR and power formulas whenever you want them.

Export: Export your calculation to PDF, JSON, or CSV.

Common Use Cases

Circuit Design & Validation: verify voltage drop across circuit components, calculate required current for a given load, and determine wire gauge and safety margins.

Power & Energy Calculations: calculate power consumption (watts) for electrical devices, estimate energy usage and operating costs, and size power supplies and check rating adequacy.

Troubleshooting & Analysis: diagnose circuit faults from known measurements, find resistor values for specific current limits, and verify component ratings under operating conditions.

Educational & Learning: understand relationships between V, I, R, and P, explore how changing one parameter affects others, and verify textbook examples and homework problems.

Frequently Asked Questions

Ohm's Law states that V = I × R (Voltage = Current × Resistance). It describes how current flows through a conductor proportional to voltage and inversely proportional to resistance.
Power (measured in Watts) can be calculated as P = V × I, P = I² × R, or P = V² / R. Our calculator provides all four power formulas automatically based on your known values.
Select Power (P) as the value to solve for, then enter the voltage and current — P = V × I, so a 10 A load on a 120 V circuit is 1,200 W. For watts to amps, select Current (I) and enter the power and voltage instead: I = P ÷ V, so that same 1,200 W appliance at 120 V draws 10 A.
Standard units: Voltage in Volts (V), Current in Amperes (A), Resistance in Ohms (Ω), Power in Watts (W). This calculator also accepts millivolts (mV), microamps (µA), kilohms (kΩ), and milliwatts (mW) — just select the unit prefix from the dropdown.
Ohm's Law (V=IR) applies directly to DC circuits. For AC circuits, resistance becomes impedance (Z), which includes capacitive and inductive reactance. This calculator covers DC and resistive AC. Use the Power Calculator for AC phase angle and power factor analysis.
Preset examples (e.g., LED series resistor, car headlights, smartphone charger) show common real-world uses. Click one to fill the inputs and see how Ohm's Law is applied — then edit the values for your own circuit.

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