Ohm's Law Calculator
Use this interactive Ohm's Law calculator to solve voltage, current, and resistance with the V = IR formula and clear circuit examples.
Ohm's Law formula
V = I × R
The Ohm's Law formula says voltage equals current multiplied by resistance. Voltage is measured in volts (V), current is measured in amps (A), and resistance is measured in ohms (Ω). This V = IR calculator uses that relationship in each direction, so any two known values can solve the third.
What is Ohm's Law?
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. Voltage is the electrical push, current is the flow of charge, and resistance is what limits that flow.
This Ohm's Law Calculator is useful for quick answers, but it is also meant to make the relationship visible. Raise the resistance and current falls. Raise the voltage and current rises. Those changes are the formula playing out in the circuit.
How to calculate voltage
To calculate voltage, multiply current by resistance: V = I × R. For example, if a circuit has 0.5 A of current through a 20 Ω resistor, the voltage is 0.5 × 20 = 10 V.
Use this when you know how much current is flowing and the resistance of the load, and you want to find the voltage across it.
How to calculate current
To calculate current, divide voltage by resistance: I = V / R. A 9 V battery connected to a 300 Ω resistor gives 9 / 300 = 0.03 A, or 30 mA.
This is the most common use for a voltage current resistance calculator because it helps check whether a circuit is drawing a safe amount of current.
How to calculate resistance
To calculate resistance, divide voltage by current: R = V / I. If a 5 V supply should produce 0.02 A, the needed resistance is 5 / 0.02 = 250 Ω.
This is useful when choosing a resistor value for a simple load or checking whether a measured voltage and current make sense together.
Common mistakes
The easiest mistake is mixing amps and milliamps. The formula uses amps, so 20 mA should be entered as 0.02 A. Entering 20 instead would mean 20 A, which is a thousand times larger.
Another mistake is applying Ohm's Law as if every part behaves like a fixed resistor. LEDs, motors, and sensors often need extra context, but the formula is still a good starting point for the resistor or load you are actually calculating.
Practical examples
Small battery circuit
A 9 V battery and a 1,000 Ω resistor give I = 9 / 1000 = 0.009 A, or 9 mA.
Finding resistor voltage
If 0.25 A flows through a 40 Ω resistor, V = 0.25 × 40 = 10 V.
Choosing a resistance
If a circuit should draw 50 mA from 12 V, convert 50 mA to 0.05 A, then calculate R = 12 / 0.05 = 240 Ω.
FAQ
What does an Ohm's Law Calculator solve?
It solves voltage, current, or resistance when you know the other two values. The calculator uses the Ohm's Law formula V = I × R and rearranges it as needed.
What units should I use?
Use volts for voltage, amps for current, and ohms for resistance. Convert milliamps to amps before calculating: 100 mA is 0.1 A.
Can I use this as a V = IR calculator?
Yes. The tool works as a V = IR calculator for voltage, and it also rearranges the same formula to calculate current or calculate resistance.