Free Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law (V = I ร R). Solve for any value in electrical circuits.
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Voltage (V)
12.00 V
Power (P = V ร I)
24.00 W
Formula
V = I ร R โ where V = voltage (volts), I = current (amps), R = resistance (ohms). Power: P = V ร IHow to Use Ohm's Law for Circuit Calculations
Ohm's Law is the foundation of electrical engineering. The relationship V = I ร R lets you calculate any one of three values โ voltage, current, or resistance โ when you know the other two. This calculator also shows power (P = V ร I), which tells you the energy consumption of a circuit.
Select which value you want to solve for, enter the two known values, and the calculator instantly gives you the result along with power consumption in watts.
Understanding the Electrical Units
- Voltage (V) โ Volts: The electrical "pressure" that pushes electrons through a circuit. US outlets provide 120V; dryer outlets provide 240V.
- Current (I) โ Amps: The flow rate of electrons. A typical household circuit handles 15-20 amps before the breaker trips.
- Resistance (R) โ Ohms (ฮฉ): Opposition to current flow. Thinner wires, longer wires, and certain materials have higher resistance.
- Power (P) โ Watts: The rate of energy use. Multiply volts by amps to get watts. Your electric bill is based on kilowatt-hours (kWh).
Common Electrical Calculations
Here are practical scenarios where Ohm's Law helps:
- Home wiring: A 20-amp, 120V circuit can handle up to 2,400 watts (120 ร 20). Factor in 80% safety margin = 1,920 watts max continuous load.
- LED projects: Calculate the right resistor value to protect LEDs from burning out
- Car electronics: 12V automotive systems โ figure out fuse sizes and wire gauges
- Solar panels: Determine how much current flows at a given voltage and resistance
The Water Analogy
The easiest way to understand electricity is the water pipe analogy. Voltage is like water pressure โ higher pressure pushes more water through. Current is the flow rate of water. Resistance is like a narrow section of pipe that restricts flow. Just as higher pressure with a wider pipe moves more water, higher voltage with lower resistance produces more current.
Frequently Asked Questions
What is Ohm's Law in simple terms?
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. Voltage (V) is the electrical pressure, current (I) is the flow of electrons, and resistance (R) opposes that flow. Higher voltage means more current; higher resistance means less current. V = I ร R.
How do I calculate amps from watts and volts?
Use the power formula: Amps = Watts รท Volts. For example, a 1,500-watt space heater on a 120V circuit draws 12.5 amps (1500 รท 120 = 12.5A). This is important for sizing circuit breakers and wiring.
What is the difference between AC and DC in Ohm's Law?
Ohm's Law applies directly to DC circuits. For AC circuits, it still works but you use impedance (Z) instead of simple resistance. Impedance accounts for capacitance and inductance in addition to resistance. For basic resistive loads (heaters, incandescent bulbs), Ohm's Law works the same for both.
Why is power important in electrical calculations?
Power (watts) tells you how much energy a circuit uses per second. It determines wire sizing, breaker capacity, heat generation, and electricity costs. A circuit drawing too much power overheats wires and trips breakers. Power = Voltage ร Current (P = V ร I).
How do I size a resistor for an LED?
Use R = (Supply Voltage - LED Voltage) รท LED Current. For a typical LED on a 5V supply with a 2V forward voltage and 20mA current: R = (5 - 2) รท 0.020 = 150 ohms. This calculator helps you verify each step.