Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law and the power formula.

Ohm's Law Calculator

V = I × R·P = V × I·P = I² × R

Solve for:

Voltage (V)

Current (A)

Power (W)

Resistance (Ω)

6 Ω

Voltage

12 V

Current

2 A

Resistance

6 Ω

Power

24 W

Example Scenarios

How to Use

Identify your two known values

Determine which two of Voltage (V), Current (I), Resistance (R), or Power (P) you already know.

Enter the known values

Input your two known quantities in their respective fields. Leave unknown fields blank.

Calculate remaining quantities

The tool applies the correct formula combination to solve for the two unknown electrical values.

Use results in circuit design

Apply the calculated values for component selection, safety checks, or circuit troubleshooting.

Ohm's Law

Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R). Combined with the power formula P=VI, knowing any two of the four electrical quantities lets you calculate the other two. This is fundamental to electronics design and circuit analysis.

Real-World Examples & Use Cases

Selecting the Correct Resistor for an LED Circuit

LEDs require a current-limiting resistor to prevent burnout. An LED rated for 20mA with a forward voltage of 2.1V connected to a 5V supply needs a resistor calculated as (5V - 2.1V) / 0.020A = 145 ohms. Ohm's Law determines the required resistance, while the power formula confirms the resistor's power rating: P = I² × R = 0.020² × 145 = 0.058W — meaning a standard 1/8W (0.125W) resistor is adequate. Without this calculation, incorrect resistors destroy LEDs or overheat components.

Troubleshooting Electrical Circuit Problems

When a circuit component fails or behaves unexpectedly, measuring two electrical values and calculating the others helps diagnose the problem. A motor drawing 3A instead of its rated 1.5A on a 12V supply is dissipating 36W (P = VI = 12V × 3A) instead of the expected 18W — indicating a shorted winding or mechanical jam. Comparing measured values against Ohm's Law predictions isolates whether the issue is the power supply, wiring, or the component itself.

Home Electrical Wiring and Appliance Safety

Electricians and homeowners use Ohm's Law to verify circuit safety. A 20A circuit breaker on a 120V circuit limits power to 2,400W (P = VI). Adding appliances: refrigerator (800W) + microwave (1,200W) = 2,000W draws 16.7A — within the 20A limit. Adding an electric kettle (1,500W) pushes the total to 3,500W drawing 29.2A — exceeding the breaker rating and triggering a trip. Ohm's Law calculations prevent overloaded circuits and electrical fires.

DC Power Supply and Battery System Design

Engineers designing battery-powered systems calculate current draw to estimate runtime. A device drawing 500mA from a 12V battery pack with 10,000mAh capacity runs for approximately 20 hours (10,000mAh ÷ 500mA). Power dissipation P = VI = 12V × 0.5A = 6W. If the device includes a 50-ohm load, verification: I = V/R = 12/50 = 0.24A, P = 12 × 0.24 = 2.88W. Ohm's Law ties together all design parameters for battery, power supply, and load component selection.

How It Works

Ohm's Law and the power formula provide six calculation combinations: Fundamental relationships: - Ohm's Law: V = I × R - Power formula: P = V × I Derived formulas: - Voltage: V = I × R = P / I = √(P × R) - Current: I = V / R = P / V = √(P / R) - Resistance: R = V / I = V² / P = P / I² - Power: P = V × I = I² × R = V² / R Given any two values, solve for the other two: - Know V and I → R = V/I, P = V×I - Know V and R → I = V/R, P = V²/R - Know V and P → I = P/V, R = V²/P - Know I and R → V = I×R, P = I²×R - Know I and P → V = P/I, R = P/I² - Know R and P → V = √(P×R), I = √(P/R) Units: V in volts, I in amperes (amps), R in ohms (Ω), P in watts (W)

Frequently Asked Questions

What is Ohm's Law and who discovered it?▼

Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, with the proportionality constant being resistance (R): V = IR. It was published by German physicist Georg Simon Ohm in 1827. The law applies to linear (ohmic) conductors at constant temperature. Non-ohmic components like diodes, transistors, and thermistors do not follow Ohm's Law because their resistance changes with voltage or temperature.

What is the difference between watts, volts, amps, and ohms?▼

These are the four fundamental electrical quantities: Voltage (volts, V) is electrical pressure — the force pushing current through a circuit. Current (amperes/amps, I) is the flow rate of electrons. Resistance (ohms, Ω) is the opposition to current flow. Power (watts, W) is the rate of energy conversion. Analogy: voltage is water pressure, current is flow rate, resistance is pipe friction, power is the rate of work done by the water flow.

Does Ohm's Law work for AC circuits?▼

Ohm's Law in its basic form (V = IR) applies to DC circuits with purely resistive loads. In AC circuits, the opposition to current flow is called impedance (Z) rather than resistance, and includes inductive and capacitive reactance that shift the phase between voltage and current. For AC circuits: V = IZ (where Z = √(R² + X²), X being reactance). For purely resistive AC loads, R = Z and Ohm's Law applies directly.

How do I choose the right resistor wattage rating?▼

Calculate the power dissipated in the resistor using P = I²R or P = V²/R. Then select a resistor rated for at least double that calculated power to ensure safe operation with headroom. Standard ratings: 1/8W (0.125W), 1/4W (0.25W), 1/2W (0.5W), 1W, 2W, 5W, 10W. Undersized resistors overheat and fail. Example: a 100Ω resistor carrying 50mA dissipates P = (0.05)² × 100 = 0.25W — use a 0.5W or 1W rated component.

What happens if resistance in a circuit approaches zero?▼

When resistance approaches zero, current approaches infinity (I = V/R; as R→0, I→∞). This condition is called a short circuit. In practice, a short circuit causes massive current flow limited only by internal resistance of the power supply and wiring. This generates extreme heat, may damage components, and can cause fires. Circuit breakers and fuses are designed specifically to interrupt current when short circuit conditions are detected, protecting equipment and preventing electrical hazards.

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