Ohm's Law Calculator

Calculate voltage, current, and resistance using Ohm's law. Enter any two values to find the third instantly.

Ohm's Law Calculator tool

Loading...

Example

Input: Known values: voltage 12 V and resistance 4 ohms

Result: Current (I) 3 A, power (P) 36 W

I = V / R = 12 / 4 = 3 A, and P = V x I = 12 x 3 = 36 W.

Ohm's Law Formulas for Every Pair

You knowFind the other two
Voltage and currentR = V / I, P = V x I
Voltage and resistanceI = V / R, P = V^2 / R
Voltage and powerI = P / V, R = V^2 / P
Current and resistanceV = I x R, P = I^2 x R
Current and powerV = P / I, R = P / I^2
Resistance and powerV = sqrt(P x R), I = sqrt(P / R)

These are the six cases the calculator solves.

  1. Select which two values you know from the Known Values menu - any pair of voltage, current, resistance, and power.
  2. Enter the two known values in volts, amps, ohms, or watts (convert first if needed, e.g. 20 mA = 0.02 A, 4.7 kiloohms = 4700 ohms).
  3. The calculator instantly computes the two missing quantities.
  4. Review all four results - voltage, current, resistance, and power - displayed below the inputs.
  5. Copy any result value to use in your circuit design or homework.

Ohm's Law: Voltage, Current and Resistance

Ohm's law says that voltage equals current times resistance: V = I x R. Know any two of voltage, current, resistance and power, and you can find the other two. For example, 12 volts across a 4 ohm resistor pushes 3 amps through it and turns 36 watts into heat.

Each quantity has a plain meaning. Voltage (V, in volts) is the push that drives charge around a circuit. Current (I, in amps) is how much charge flows each second. Resistance (R, in ohms) is how hard the part resists that flow. Power (P, in watts) is the rate at which energy is used, and it equals V x I.

Pick the pair you know from the Known Values menu, such as Voltage + Resistance, and type the two numbers. The other two appear at once, with the first unknown one highlighted. Use base units: type 20 mA as 0.02 and 4.7 kilohms as 4700. Current is shown to six decimal places so small values in milliamps stay readable.

Ohm's law holds well for resistors, wires and heaters at a steady temperature. It does not describe LEDs, diodes or transistors, whose current does not rise in step with voltage. For an LED, the LED Resistor Calculator does the right math. To read a resistor's value from its stripes, use the Resistor Color Code Calculator.

Formula / How It Works

V = I x R | I = V / R | R = V / I | P = V x I | P = I^2 x R | P = V^2 / R

Frequently Asked Questions

Divide the voltage by the resistance: I = V / R. A 9 V battery across a 450 ohm resistor gives 9 / 450 = 0.02 A, which is 20 mA. Be sure the resistance is in ohms, not kilohms, or the answer will be 1,000 times too large.
Use P = I^2 x R or P = V^2 / R. A 100 ohm resistor with 0.1 A through it uses 0.1 x 0.1 x 100 = 1 W. Choose a resistor rated for at least twice that power, here a 2 W part, so it runs cool and lasts.
The calculator works in base units: volts, amps, ohms and watts. If you type 20 for 20 mA, it reads it as 20 amps. Convert first: milliamps divide by 1,000, and kilohms multiply by 1,000. So 20 mA is 0.02 and 4.7 kilohms is 4700.
For pure resistors, yes, as long as you use RMS values for voltage and current. Coils and capacitors add a quantity called reactance. Then you need impedance (Z) in place of R, and power also depends on the phase angle. This calculator covers the resistive case.

Related Tools

Voltage Divider Calculator

Calculate output voltage, divider current, and resistor power dissipation for a two-resistor voltage divider.

Electrical and Engineering

LED Resistor Calculator

Find the correct resistor value and wattage to safely drive an LED from any supply voltage.

Electrical and Engineering

Resistor Color Code Calculator

Decode 4, 5, and 6-band resistor color codes to find the resistance, tolerance, and min/max value.

Electrical and Engineering

Power Consumption Calculator

Calculate electricity usage in watts, kilowatt-hours, and estimated cost for any appliance or device.

Electrical and Engineering

Wire Gauge Converter

Convert AWG (American Wire Gauge) sizes to metric. View diameter, cross-sectional area, and copper resistance.

Electrical and Engineering

Decibel Calculator

Calculate decibel values for power and voltage ratios. Convert dB to linear ratios and watts to dBm.

Electrical and Engineering