About the Ohm's Law Calculator
Ohm's Law Calculator solves a simple circuit from any two known values. Type two of voltage, current, resistance and power, in the units you have, and the other two appear straight away, highlighted so you can tell them from your own entries. The formulas used are shown beside the answer, and a table lists all twelve equations. Units include millivolts, kilovolts, milliamps, kilohms, megohms, milliwatts and kilowatts, and typing a value never changes when you switch a unit: 5 with mA stays 5 milliamps. The rules are for ideal resistive parts at a fixed temperature. They do not describe AC loads with reactance, diodes and LEDs, or the power rating and tolerance of a real resistor.
Updated September 2026
How to use the Ohm's Law Calculator
- 1Choose which two values you know.
- 2Type them into their boxes and pick the units.
- 3Leave the other two boxes empty.
- 4Read the two calculated values and the formulas used.
- 5Press Reset, or Clear if three boxes are filled, to start again.
The Ohm's law triangle in words
Voltage equals current times resistance. To find current, divide voltage by resistance. To find resistance, divide voltage by current. Many people remember this as a triangle with V on top and I and R below, covering the quantity they want.
Power adds a second triangle: watts equal volts times amps. Substituting one triangle into the other gives the twelve forms listed on the page, so any pair of values leads to the remaining two.
Calculating power in a resistor
A resistor turns electrical power into heat. A 100 Ω resistor across 10 V carries 0.1 A and dissipates 1 W. Ordinary through-hole resistors are rated at about a quarter or half a watt, so this one would overheat, and you would need a 2 W part or a different design.
A common rule of thumb is to choose a rating at least double the calculated power. Enter the voltage and resistance here and read the watts from the results.
Picking a resistor for an LED
- Subtract the LED's forward voltage from the supply voltage to find the voltage across the resistor.
- Choose the current: 10 to 20 mA is typical for a small LED.
- Enter those two numbers as voltage and current to get the resistance.
- Round up to the nearest standard value; the Resistor Color Code Calculator shows the bands.
- Check the resistor's power using the same page.
When Ohm's law does not apply
Ohm's law describes parts whose resistance stays the same as voltage changes. Diodes, LEDs, transistors and lamp filaments do not behave that way, and resistance also drifts with temperature. Motors and transformers involve inductance, which brings in impedance and power factor.
For mains circuits with power factor, see the Watts, Amps and Volts Calculator. For the cost of running a device, use the Electricity Cost Calculator.
Features
- Any two of four values solve the other two
- Millivolts to kilovolts, milliamps, ohms to megohms, milliwatts to kilowatts
- Formulas shown next to the answer
- Table of all 12 Ohm's law equations
- Clear messages for impossible inputs
- Copy the result in one tap
- Runs in your browser
Frequently asked questions
What is Ohm's law?
Ohm's law says the current through a resistor is the voltage across it divided by its resistance: I = V / R. Together with the power formula P = V × I, it lets you find any of voltage, current, resistance and power when you know two of them.
How do I find resistance from voltage and current?
Divide the voltage by the current. A 12 V supply pushing 2 A through a load means 6 Ω. Enter 12 in the voltage box and 2 in the current box, and the resistance appears with the power, 24 W.
How do I calculate the resistor for an LED?
Take the supply voltage, subtract the LED's forward voltage, and divide by the current you want. With a 5 V supply, a 2 V LED and 20 mA, that is 3 V over 0.02 A, or 150 Ω. Type 3 V and 20 mA here to check it, and pick a standard value at or above the result.
What is the difference between resistance and impedance?
Resistance opposes current in the same way at every frequency. Impedance also includes the effect of coils and capacitors in AC circuits, which changes with frequency. This calculator handles resistance only.
How do I calculate power in a resistor?
Multiply voltage by current, or use current squared times resistance, or voltage squared divided by resistance. All three give the same watts. Pick a resistor rated for at least twice that power so it stays cool.
Why does the calculator need exactly two values?
Four quantities are linked by two equations, so any two determine the other two. With only one value there is not enough information, and with three or four the values might contradict each other, so the tool asks you to keep two.