Ohm's Law Calculator (Voltage, Current, Resistance)

Use Ohm's law V=IR to calculate any one of voltage, current, or resistance from the other two values. Power (W) is calculated at the same time.

Usage Tips

  • Select the variable you want to find under "Value to solve for", and the other two input fields will appear. Enter the two known values from your circuit diagram or multimeter readings.
  • Power (P) is automatically calculated alongside using the formula P=VI, derived from Ohm's law. Check this field if you want to know the power consumption.
  • When resistance is close to 0Ω (a short circuit), current becomes extremely large and dangerous (overcurrent) in a real circuit, so this tool excludes input values of 0 or below from calculation.
  • AC circuits require impedance (which can include complex numbers) instead of simple resistance; this tool assumes a DC circuit.

Frequently Asked Questions

Ohm's law is a fundamental law describing the relationship between voltage (V), current (I), and resistance (R) in an electric circuit, expressed as V = I × R. It was published by the German physicist Georg Ohm in 1827.

Power can be calculated as P = V × I (voltage × current). Substituting Ohm's law V=IR gives P = I²R or P = V²/R, so power can also be found from just resistance and current (or voltage).

At a constant voltage, current decreases as resistance increases (I = V/R). Conversely, a smaller resistance allows a larger current to flow at the same voltage.

This tool handles simple Ohm's law calculations assuming a DC circuit. AC circuits require accounting for impedance (complex numbers) with phase shifts and frequency-dependent behavior, so accurate calculations can't be done with a simple resistance value alone.
ツールくん

Side Note — The Struggle Behind the Discovery of Ohm's Law

Georg Simon Ohm, who published Ohm's law, faced strong criticism from the German physics community when he published it in 1827. At the time, there was a prevailing view in academia that simply expressing electrical phenomena with a mathematical equation was "philosophically shallow," and Ohm is said to have faced such backlash that he was even pushed out of his university position.

It took more than a decade after publication for Ohm's law to become widely accepted. Eventually, in recognition of his achievement, his name was immortalized in the international unit of electrical resistance, the "ohm (Ω)." The Greek letter Ω (omega) is said to have been adopted as the unit symbol to avoid confusion with the letter O, the initial of "Ohm."

In modern electronics hobby projects and DIY work, such as determining the resistor value to connect in series with an LED, Ohm's law remains one of the most fundamental calculations used daily. The simplicity of instantly finding the third value once two of voltage, current, and resistance are known is one reason it hasn't lost its relevance even after nearly 200 years.

More tools in this category