Ohm's Law Calculator
Calculate the voltage across a component by entering the current flowing through it and its resistance. Essential for electronics hobbyists, electrical engineers, and students working with circuits.
About this calculator
Ohm's Law describes the fundamental relationship between voltage, current, and resistance in an electrical conductor: V = I × R. Voltage (V) is measured in volts, current (I) in amperes (A), and resistance (R) in ohms (Ω). The law states that the voltage across a resistor is directly proportional to the current passing through it, with resistance as the proportionality constant. It can be rearranged to solve for any variable: I = V / R or R = V / I. Ohm's Law applies to linear (ohmic) components such as resistors at constant temperature; non-linear devices like diodes and transistors do not strictly obey it. It is the starting point for analysing virtually every electrical and electronic circuit.
How to use
Suppose a resistor carries a current of 2 A and has a resistance of 15 Ω. Using V = I × R: V = 2 × 15 = 30 V. Now reverse the problem: if you have a 12 V battery connected to a 4 Ω resistor, the current is I = V / R = 12 / 4 = 3 A. For a third scenario, measuring 5 V across a component with 0.5 A flowing gives R = V / I = 5 / 0.5 = 10 Ω. All three rearrangements follow directly from the same law.
Frequently asked questions
What is Ohm's Law and how do voltage, current, and resistance relate to each other?
Ohm's Law states that V = I × R — voltage equals current multiplied by resistance. This means that for a fixed resistance, increasing the current increases the voltage proportionally, and for a fixed voltage, increasing resistance decreases current. The relationship is linear for ohmic materials, meaning a graph of voltage versus current produces a straight line whose slope equals the resistance. It is the most fundamental tool for analysing and designing electrical circuits.
When does Ohm's Law not apply to a circuit component?
Ohm's Law does not apply to non-ohmic components whose resistance changes with voltage or current. Diodes, transistors, light bulbs (at varying temperatures), and thermistors all exhibit non-linear V–I characteristics. For example, a diode allows significant current only when forward voltage exceeds a threshold (~0.7 V for silicon), and its resistance is not constant. In these cases, more complex device models or characteristic curves must be used instead of a simple V = IR calculation.
How do I use Ohm's Law to choose the correct resistor for an LED circuit?
An LED requires a specific forward voltage (typically 2–3.5 V) and a safe operating current (commonly 20 mA). If you are powering it from a 5 V supply, the resistor must drop the excess voltage. Using Ohm's Law: R = V / I = (5 − 2.0) / 0.020 = 150 Ω for a red LED with a 2 V forward voltage. You would then select the nearest standard resistor value (e.g., 150 Ω or 180 Ω) to keep the current within safe limits and protect the LED from burning out.