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Ohm's Law Calculator

Calculate Voltage, Current, Resistance, and Power using Ohm's Law formulas.

Circuit Values
Results
Resistance —  
Current —  
Power —  

How It Works

1️⃣

1. Enter Values

Input any two known variables (Voltage, Current, Resistance, or Power).

2️⃣

2. Calculate

Instantly solve for the remaining two values using Ohm's Law formulas.

3️⃣

3. Visualize

See the relationship dynamically graphed to understand the circuit properties.

How to Calculate Ohm's Law

The Core Formulas

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. The three main formulas are:

  • Voltage (V) = I × R: Current multiplied by Resistance.
  • Current (I) = V / R: Voltage divided by Resistance.
  • Resistance (R) = V / I: Voltage divided by Current.

Understanding Power (Watt's Law)

Electrical power is the rate at which energy is transferred by an electric circuit. It is calculated using these common variations:

  • P = V × I: Voltage multiplied by Current.
  • P = I² × R: Current squared multiplied by Resistance.
  • P = V² / R: Voltage squared divided by Resistance.

How to Use This Calculator

1

Enter Known Values

Input any two of the four electrical quantities: voltage (V), current (I), resistance (R), or power (P).

2

Calculate Unknowns

The calculator solves for the remaining two values using Ohm's Law and the power equation.

3

Apply to Your Circuit

Use the results to select components, verify circuit behavior, or troubleshoot electrical problems.

Formula & Methodology

Ohm's Law

V = I × R

Voltage equals current multiplied by resistance. This fundamental relationship governs all resistive circuits.

Power Equation

P = V × I

Electrical power in watts equals voltage times current. Can also be expressed as P = I² × R or P = V² / R.

Resistance from Power

R = V² / P

Useful for calculating the resistance of heating elements or loads when voltage and power rating are known.

About this calculation · Accuracy & Methodology

Ohm's Law Quick Reference

To FindFormula 1Formula 2Formula 3
Voltage (V)V = I × RV = P / IV = √(P × R)
Current (I)I = V / RI = P / VI = √(P / R)
Resistance (R)R = V / IR = V² / PR = P / I²
Power (P)P = V × IP = I² × RP = V² / R

Key Terms

Voltage (V)
The electrical pressure or potential difference measured in volts. It is the force that pushes electrons through a conductor.
Current (I)
The flow rate of electric charge measured in amperes (amps). One amp equals one coulomb of charge per second.
Resistance (R)
The opposition to current flow measured in ohms. Higher resistance means less current for a given voltage.
Power (P)
The rate of energy consumption or conversion, measured in watts. A 60W light bulb consumes 60 joules of energy per second.
Conductance
The inverse of resistance, measured in siemens. A conductor with 0.01 ohms of resistance has a conductance of 100 siemens.

Real-World Examples

Example 1

Household Outlet Load

Voltage: 120V, Current: 15A

Result: Power = 1,800W, Resistance = 8 ohms. This matches a typical 15-amp household circuit at full load.

Example 2

12V Automotive Bulb

Power: 55W, Voltage: 12V

Result: Current = 4.58A, Resistance = 2.62 ohms. Knowing the current draw helps size the correct fuse (a 5A fuse would work here).

Ohm's Law in Everyday Electrical Work

The Foundation of Circuit Design

Georg Ohm published his law in 1827, and it remains the single most important relationship in electrical engineering. Whether you are wiring a house, designing a circuit board, or troubleshooting a car's electrical system, the relationship V = IR is the starting point for every calculation.

Practical Applications

Electricians use Ohm's Law to determine wire sizes, fuse ratings, and voltage drops over long cable runs. Electronics hobbyists use it to calculate resistor values for LED circuits. HVAC technicians use it to diagnose faulty heating elements by measuring resistance and comparing to specifications.

Limitations to Remember

Ohm's Law applies to purely resistive (ohmic) loads. Components like diodes, transistors, and LEDs have non-linear voltage-current relationships. For AC circuits, impedance (which includes inductance and capacitance) replaces simple resistance in the calculations.

How to Use This Calculator

  1. 1
    Choose the Unknown

    Select which quantity you want to solve for: Voltage, Current, Resistance, or Power.

  2. 2
    Enter Two Known Values

    Provide any two of the four quantities -- the calculator uses Ohm's Law to derive the remaining two.

  3. 3
    See the Result

    Results update instantly. Check the solved value in the highlighted output field.

  4. 4
    Note Power Dissipation

    Review the power (watts) to confirm resistors or wiring can handle the heat generated by the circuit.

  5. 5
    Check Safety Limits

    Compare against component ratings -- resistor wattage, wire ampacity, and fuse ratings -- to confirm the design is safe.

Key Terms

Voltage (V) — Electrical potential difference measured in volts; the "pressure" that drives current through a circuit.
Current (A) — The rate of charge flow measured in amperes; how much electricity is moving through the conductor.
Resistance (Ohm) — Opposition to current flow measured in ohms; materials with high resistance restrict current significantly.
Power (W) — Rate of energy conversion measured in watts; P = V x I or P = I^2 x R or P = V^2 / R.
Ohm's Law (V = IR) — The core relationship stating voltage equals current multiplied by resistance. Valid for all linear, resistive elements.
Kirchhoff's Laws — Two circuit laws: current into a node equals current out (KCL), and voltage around any closed loop sums to zero (KVL).