Ohm's Law Calculator — Free Online Electrical Calculator

How to Use the Ohm's Law Calculator

1. Choose what to solve for: Select which electrical quantity you want to find using the radio buttons. You can solve for resistance (R), voltage (V), current (I), or power (P). The calculator adjusts the input fields to show only the values you need to provide.
2. Enter two known values: Fill in the known values in their fields. Enter voltage in volts (V), current in amps (A), and resistance in ohms. The calculator requires exactly two inputs to solve for the unknowns. Use decimal values for milliamps (0.020 for 20 mA) or kilohms (enter 1000 for 1 k ohm).
3. Read the result: The computed value appears in large text at the top of the results panel. The calculator automatically applies appropriate unit prefixes: milliamps (mA) for small currents, kilohms (k ohm) for large resistances, and kilowatts (kW) for high power values.
4. Review the complete circuit analysis: Below the main result, the "Formula Used" section shows which equation was applied. The "All Values" section displays voltage, current, resistance, and power simultaneously, giving you a complete picture of the circuit's electrical characteristics from just two known values.

The calculator handles all valid combinations of two inputs and automatically computes all four values (V, I, R, P). This is useful for both analyzing existing circuits and designing new ones where you need to determine component values.

Ohm's Law Formulas

Variables Explained

• V — Voltage (Volts): The electrical potential difference between two points in a circuit. Voltage is the "pressure" that pushes electrons through the conductor. Common voltages include 1.5V (battery), 5V (USB), 12V (car battery), 120V (US household), and 240V (European household).
• I — Current (Amperes): The rate of electron flow through the circuit. One ampere means approximately 6.24 × 10^18 electrons passing a point per second. Current flows from higher potential to lower potential (conventional current direction). Typical currents range from microamps (sensor circuits) to hundreds of amps (electric vehicle motors).
• R — Resistance (Ohms): The opposition to current flow in a material. Higher resistance means less current for a given voltage. Resistance depends on the material (copper vs. steel), dimensions (longer/thinner = more resistance), and temperature. Resistors provide precise, controlled resistance in circuits.
• P — Power (Watts): The rate at which electrical energy is converted to other forms (heat, light, motion). Power determines component ratings and energy consumption. A 60W light bulb converts 60 joules of energy per second. Power dissipation generates heat, which is the primary concern in circuit design.

Step-by-Step Example

A 12V power supply drives current through a 240-ohm resistor. Find the current and power:

1. Apply Ohm's law for current: I = V / R
2. Substitute: I = 12 / 240 = 0.05 A = 50 mA
3. Calculate power: P = V × I = 12 × 0.05 = 0.6 W
4. Verify with alternative formula: P = V² / R = 144 / 240 = 0.6 W (confirmed)

The resistor passes 50 milliamps and dissipates 0.6 watts of heat. A standard 1-watt resistor would be sufficient for this application, providing a comfortable safety margin above the 0.6W dissipation.

Practical Examples

Example 1: Sizing an LED Current-Limiting Resistor

Kevin wants to power a blue LED (3.2V forward voltage, 20 mA rated current) from a 9V battery. He needs to calculate the resistor value and power rating:

• Voltage across resistor: 9V - 3.2V = 5.8V
• Required resistance: R = V / I = 5.8 / 0.020 = 290 ohms
• Nearest standard value: 300 ohms
• Actual current: I = 5.8 / 300 = 19.3 mA (close enough to target)
• Power dissipation: P = 0.0193 × 5.8 = 0.112W

Kevin selects a 300-ohm, 1/4-watt resistor. The 0.112W dissipation is well within the 0.25W rating, ensuring the resistor stays cool. Without this resistor, the LED would draw excessive current and burn out within seconds.

Example 2: Diagnosing a Car Electrical Problem

Sara, an automotive technician, measures 11.5V at the headlight connector of a 12.6V system. She uses Ohm's law to diagnose the 1.1V voltage drop with the headlamp drawing 4.5A:

• Resistance causing the drop: R = V / I = 1.1 / 4.5 = 0.244 ohms
• Power wasted as heat: P = V × I = 1.1 × 4.5 = 4.95W

The 0.244-ohm parasitic resistance indicates corroded connectors or damaged wiring. Healthy automotive wiring should have near-zero resistance. The 4.95W wasted as heat explains why the connector feels warm. Sara cleans the corroded connector, reducing resistance to 0.02 ohms and the voltage drop to 0.09V. For power unit conversions, try our horsepower calculator.

Example 3: Home Electrical Circuit Analysis

Marco wants to know if his 15-amp kitchen circuit can handle a 1,200W microwave and a 800W toaster running simultaneously on a 120V circuit:

• Microwave current: I = P / V = 1,200 / 120 = 10.0A
• Toaster current: I = 800 / 120 = 6.67A
• Total current: 10.0 + 6.67 = 16.67A
• Circuit capacity: 15A (with 80% continuous rating = 12A)

At 16.67A, the combined load exceeds the 15A circuit breaker and far exceeds the 12A continuous rating. Running both appliances will trip the breaker. Marco should use separate circuits or avoid running them simultaneously. This is exactly the kind of analysis electricians perform using Ohm's law every day.

Example 4: Speaker Impedance Matching

Lisa has a stereo amplifier rated at 100W into 8 ohms and wants to know the maximum voltage and current it delivers:

• Peak voltage: V = sqrt(P × R) = sqrt(100 × 8) = sqrt(800) = 28.3V
• Peak current: I = P / V = 100 / 28.3 = 3.54A
• Verify: P = V × I = 28.3 × 3.54 = 100.2W (confirmed)

If Lisa connects a 4-ohm speaker instead, the current doubles to about 7A while voltage drops. Some amplifiers cannot handle this increased current demand and may overheat or clip. Speaker impedance matching is critical for audio quality and equipment longevity.

Ohm's Law Quick Reference Table

Tips and Complete Guide

The Ohm's Law Wheel

The Ohm's law wheel (or pie chart) is a visual memory aid showing all twelve formulas for V, I, R, and P. The wheel is divided into four quadrants, each showing the variable in the center and three formulas around it. For voltage: V = IR, V = P/I, V = sqrt(PR). For current: I = V/R, I = P/V, I = sqrt(P/R). For resistance: R = V/I, R = P/I^2, R = V^2/P. For power: P = VI, P = I^2R, P = V^2/R. Memorizing even the basic V = IR and P = VI allows you to derive all others through algebra.

Safety Considerations

Understanding Ohm's law is critical for electrical safety. Current kills, not voltage alone. As little as 10 mA through the heart can be lethal. By Ohm's law, with dry skin resistance of about 100,000 ohms, 120V household voltage could push about 1.2 mA through the body. But wet skin drops resistance to 1,000 ohms, allowing 120 mA, which is potentially fatal. This is why GFCI outlets (which trip at 5 mA of leakage current) are required in bathrooms and kitchens. Always de-energize circuits before working on them.

Ohm's Law in AC Circuits

In alternating current (AC) circuits, Ohm's law extends to V = I × Z, where Z is impedance (measured in ohms). Impedance includes resistance (R) from resistive components, reactance (X) from capacitors and inductors, and is calculated as Z = sqrt(R^2 + X^2). At household frequency (60 Hz in the US, 50 Hz elsewhere), capacitive and inductive reactance must be considered when analyzing circuits with motors, transformers, or power factor correction equipment. For basic DC calculations, standard Ohm's law applies directly. For more physics calculations, try our engine horsepower calculator.

Common Mistakes to Avoid

• Mixing up units: Entering milliamps instead of amps is the most common error. 20 mA = 0.020 A. If you enter 20 instead of 0.020, your resistance calculation will be off by a factor of 1,000. Always convert to base units (V, A, ohms) before calculating.
• Forgetting that power is not linear with voltage: Doubling the voltage across a fixed resistance quadruples the power (P = V^2/R). This is why voltage regulators are critical in electronics: small voltage variations cause larger power variations.
• Applying Ohm's law to non-ohmic devices: LEDs, diodes, and transistors do not follow V = IR. An LED has a roughly fixed forward voltage regardless of current. Use Ohm's law only for the resistive portions of circuits containing these components.
• Ignoring temperature effects: Resistance changes with temperature. Incandescent bulb filaments have about 10x lower resistance when cold than when hot. This means the initial inrush current is much higher than the steady-state current, which is why bulbs often burn out at turn-on.
• Confusing voltage across with voltage source: The voltage in V = IR is the voltage across the component, not necessarily the source voltage. In a series circuit with multiple resistors, each resistor has a fraction of the source voltage across it.

Frequently Asked Questions