Ohm's Law Calculator: Voltage, Current, Resistance, and Power
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. It's the most fundamental equation in electronics and is used constantly in electrical work, electronics design, and troubleshooting.
The Equations
Ohm's Law: V = I × R
Where:
- V = Voltage (volts)
- I = Current (amperes, amps)
- R = Resistance (ohms, Ω)
Power equation: P = V × I
And derived:
- P = I² × R
- P = V² ÷ R
Given any two values, the other two can be calculated.
Practical Examples
What resistor limits an LED to safe current?
LED supply: 5V. LED forward voltage: 2V. Desired current: 20mA.
Voltage across resistor: 5V - 2V = 3V
R = V ÷ I = 3V ÷ 0.02A = 150Ω
How much current does a 60W bulb draw at 120V?
I = P ÷ V = 60W ÷ 120V = 0.5 amps
What's the power dissipated by a 100Ω resistor carrying 50mA?
P = I² × R = (0.05)² × 100 = 0.25 watts
Why This Matters for Practical Work
Fuse sizing: Know the current draw of a circuit, use Ohm's law to verify the fuse is appropriate.
Wire gauge: Current-carrying capacity of wire gauges is based on the resistance of copper at different cross-sections.
LED resistors: Every LED needs a current-limiting resistor; Ohm's law calculates the correct value.
Battery life estimation: Current draw × battery capacity (amp-hours) = operating time.
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Frequently Asked Questions
How do I calculate the resistor needed for an LED?
Subtract the LED's forward voltage from your supply voltage to find the voltage across the resistor, then use R = V ÷ I with your desired current. For example, with a 5V supply, 2V LED forward voltage, and 20mA target current: R = 3V ÷ 0.02A = 150Ω. Use the nearest standard resistor value available.
What's the difference between using P = V × I versus P = I² × R?
Both formulas calculate power, but you use whichever one matches the values you have. Use P = V × I when you know voltage and current, like calculating a 60W bulb's current at 120V. Use P = I² × R when you know current and resistance, like finding power dissipated by a 100Ω resistor carrying 50mA (0.25 watts).
Can I use Ohm's Law to check if my wire gauge is safe for the current?
Ohm's Law doesn't directly verify wire safety, but it helps you calculate the current your circuit draws. Once you know the current, you compare it against wire gauge ampacity charts—for example, knowing a 60W load at 120V draws 0.5 amps tells you the minimum wire gauge needed to safely carry that current without overheating.
How do I estimate how long a battery will last?
Multiply the battery's capacity in amp-hours by the battery voltage, then divide by the power draw of your device. Alternatively, if you know the current draw in amps, divide the battery's amp-hour rating by the current—for example, a 2000mAh battery powering a 100mA device would last roughly 20 hours.
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