Overview
Ohm's law is one of the most fundamental relationships in electronics: voltage (V), current (I), and resistance (R) in many circuits are linked by the equation V = IR. If you know any two of the three quantities, you can solve for the third — which is exactly what makes Ohm's law so useful for both circuit design and troubleshooting.
This guide covers how to use the relationship, why units matter, and where the "ideal" model this formula describes starts to diverge from real-world components.
Know two, find the third
For a resistor that behaves in an ohmic (linear) way, the three quantities are always related by V = IR. Rearranged, this also gives I = V/R (solve for current) and R = V/I (solve for resistance). Which form you use just depends on which two quantities you already know and which one you are solving for.
Watch units
Ohm's law requires consistent units — volts for voltage, amperes for current, and ohms for resistance — before you divide or multiply. A very common source of error is forgetting to convert milliamps (mA) to amps (dividing by 1000) or kilohms (kΩ) to ohms (multiplying by 1000) before applying the formula. Getting the units wrong by a factor of 1,000 is one of the most common mistakes in basic circuit calculations.
Ideal model
Ohm's law describes an idealized linear relationship that holds well for many common resistors under normal conditions, but real components can deviate from it — resistance can change with temperature as a component heats up, and some components (diodes, transistors, and other non-ohmic devices) do not follow a linear V-I relationship at all. Treat the calculator's output as the ideal relationship, and expect real measured values to be close but not always identical, especially at higher power levels.
A quick practical example
Suppose you have a 12-volt power source connected to a 470-ohm resistor and want to know the current flowing through the circuit. Using I = V/R, that is 12 ÷ 470 ≈ 0.0255 amps, or about 25.5 milliamps. This kind of quick check is common when selecting a resistor to limit current to a safe level for an LED or other sensitive component.
Frequently asked questions
Does Ohm's law apply to all electrical components?
No — it applies specifically to ohmic components, where resistance stays roughly constant regardless of voltage or current. Non-ohmic components like diodes, transistors, and light bulbs (whose resistance changes as they heat up) do not follow a simple linear V = IR relationship.
What happens if I use the wrong units in Ohm's law?
The formula will still calculate a number, but it will be wrong by whatever factor the unit mismatch introduces — commonly a factor of 1,000 when milliamps or kilohms are mixed with amps and ohms without converting first. Always convert to base units (volts, amps, ohms) before calculating.
How is power related to Ohm's law?
Power (in watts) can be calculated from Ohm's law variables using P = VI, and by substitution, also as P = I²R or P = V²/R. These related formulas are useful when you need to check whether a resistor or component can safely handle the power involved in a circuit.
Try the calculator
Use the free Ohm's Law Calculator for live numbers, then come back to this guide anytime you need a quick refresher.