Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's law. Solve for any two known values with step-by-step solutions for electrical circuits.

Enter any 2 known values (Voltage, Current, Resistance, or Power) to calculate the remaining unknowns using Ohm's law.

⚡ Voltage (V) — Volts

🔌 Current (I) — Amps

🛡️ Resistance (R) — Ohms

💡 Power (P) — Watts

Real-World Examples

💡 Light Bulb Circuit

A light bulb has a resistance of 240 Ω and is connected to a 120 V household outlet.

Current = V ÷ R = 120 ÷ 240 = 0.5 A

Power = V × I = 120 × 0.5 = 60 W

A standard 60W incandescent bulb draws 0.5 amps from a 120V supply.

🔋 Battery-Powered Device

A device draws 2 A from a 12 V car battery.

Resistance = V ÷ I = 12 ÷ 2 = 6 Ω

Power = V × I = 12 × 2 = 24 W

The device has an effective resistance of 6 ohms and consumes 24 watts.

🔌 Resistor in a Circuit

A 100 Ω resistor carries a current of 0.1 A.

Voltage = I × R = 0.1 × 100 = 10 V

Power = I² × R = 0.01 × 100 = 1 W

A 1-watt resistor is appropriate since the power is exactly 1 watt.

📡 Power Amplifier Design

An amplifier delivers 100 W of power into an 8 Ω speaker.

Voltage = √(P × R) = √(100 × 8) = √800 = 28.28 V

Current = √(P ÷ R) = √(100 ÷ 8) = √12.5 = 3.54 A

The amplifier must output approximately 28.3V RMS into the 8-ohm speaker.

Understanding Ohm's Law

V = I × R

Voltage = Current × Resistance

P = V × I

Power = Voltage × Current

P = I² × R

Power = Current² × Resistance

P = V² ÷ R

Power = Voltage² ÷ Resistance

How to Calculate Step by Step

Identify known values: Determine which two quantities (V, I, R, P) you already know from your circuit.

Apply Ohm's Law: Use V = IR to find the missing voltage, current, or resistance.

Calculate power: Use P = VI, P = I²R, or P = V²/R depending on which values are available.

Verify units: Check that your results use the correct units — volts (V), amps (A), ohms (Ω), and watts (W).

Quick Tips

⚡ Ohm's Law Triangle

Cover the value you need: V = I×R, I = V÷R, R = V÷I. The triangle makes it easy to remember.

🔄 Power Wheel

P = VI, P = I²R, P = V²÷R. Use the right form based on which two values you know.

🎯 Enter Any Two

You only need any 2 of the 4 values. The calculator will determine the other 2 automatically.

📏 Consistent Units

Always use volts, amps, ohms, and watts. For millivolts or kilo-ohms, convert to base units first.

⚡

Four Variables

Calculate Voltage (V), Current (I), Resistance (R), or Power (P) from any two known values in your circuit.

🔄

Any Two Inputs

Enter any 2 known values and instantly get all missing values calculated using Ohm's law and power formulas.

🎯

Instant Results

All missing values are calculated and displayed with their units, color-coded for easy reading.

📚

Circuit Analysis

Step-by-step derivation shows exactly how each value was computed, helping you learn circuit analysis.

What is Ohm's Law?

Ohm's Law is a fundamental principle in electrical engineering that describes the relationship between voltage, current, and resistance in an electrical circuit. Named after the German physicist Georg Ohm, it states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The law is expressed mathematically as V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.

Ohm's Law is essential for understanding how electrical circuits behave. It allows engineers and technicians to predict how much current will flow given a certain voltage and resistance, or what voltage is needed to drive a specific current through a given resistance. Combined with the power formulas (P = VI, P = I²R, P = V²/R), it provides a complete picture of the electrical behavior of any circuit element.

Why Is Ohm's Law Important?

Ohm's Law is the foundation of all electrical and electronic engineering. It's used in designing everything from simple flashlight circuits to complex computer processors. Understanding this relationship helps you choose the right components, ensure circuits operate safely within their limits, and troubleshoot problems when things go wrong. Whether you're a student, hobbyist, or professional engineer, mastering Ohm's Law is an essential skill for working with electricity and electronics.

When to Use an Ohm's Law Calculator

Our Ohm's Law calculator is useful in countless real-world scenarios. Here are some of the most common applications:

💡 Circuit Design

Determine the correct resistor values, voltage requirements, and current limits when designing electronic circuits.

🔋 Power Supply Selection

Calculate the voltage and current requirements for power supplies based on the load resistance and power needs.

🔧 Troubleshooting

Verify if measured values in a circuit match expected values to identify faulty components or wiring issues.

📡 Audio Systems

Match amplifier output to speaker impedance and calculate power handling for audio system design.

🏠 Home Wiring

Calculate current draw of appliances, determine proper wire gauge, and ensure circuit breakers are appropriately rated.

🎓 Education

Learn and verify Ohm's Law calculations for physics and electrical engineering coursework and lab experiments.

Frequently Asked Questions

What is Ohm's Law exactly?

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance. It is expressed as V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. This fundamental relationship is the basis for all DC circuit analysis.

Can I calculate power using Ohm's Law?

Yes! Power (P) in watts can be calculated using combinations of voltage, current, and resistance. The most common formulas are P = V × I (voltage times current), P = I² × R (current squared times resistance), and P = V² ÷ R (voltage squared divided by resistance). Our calculator automatically applies the appropriate formula based on which two values you enter.

What if I enter more than 2 values?

Our calculator is designed to accept exactly 2 known values. If you enter 3 or 4 values, the calculator will use the first two it finds to compute the others — but we recommend entering exactly 2 values for the most reliable results. The values you enter should be consistent with Ohm's Law (e.g., if you enter V=12, I=2, and R=6, the calculator will calculate P=24W).

Does Ohm's Law work for AC circuits?

Ohm's Law applies to AC circuits as well, but with one important caveat: in AC circuits with reactive components (inductors and capacitors), you must use impedance (Z) instead of pure resistance (R). Impedance is a complex quantity that includes both resistance and reactance. For purely resistive AC circuits (like heating elements and incandescent bulbs), Ohm's Law works exactly the same as for DC circuits.

What are the SI units for Ohm's Law?

The standard SI units used in Ohm's Law are: Volt (V) for voltage/potential difference, Ampere (A) for electric current, Ohm (Ω) for electrical resistance, and Watt (W) for power. For very small or very large values, prefixes like milli- (m, 10⁻³), kilo- (k, 10³), and mega- (M, 10⁶) are common. Always convert to base units before calculating.

How do I remember the Ohm's Law formulas?

The easiest way is the Ohm's Law triangle: draw a triangle and write V at the top, I at the bottom left, and R at the bottom right. Cover the quantity you want to find — if V is covered, you see I×R (V = IR). If I is covered, you see V÷R (I = V/R). If R is covered, you see V÷I (R = V/I). For power, use P = VI, and substitute using Ohm's Law to get P = I²R or P = V²/R.

⚠️ Important Note: While our Ohm's Law Calculator provides accurate mathematical results, always consider real-world factors such as wire resistance, temperature effects, and component tolerances. Use this tool as a guide for ideal calculations and verify critical circuit designs with professional engineering tools and measurements.