Ohm's Law Calculator
Voltage, current, resistance, and power from any two known values
Last updated:
What this calculator computes
Ohm's Law is the foundational relationship in DC electrical engineering: the voltage across a conductor equals the current flowing through it multiplied by its resistance, written V = I × R. Named after the German physicist Georg Simon Ohm who published the relationship in 1827, it underpins almost every introductory circuit-analysis problem and remains the first equation any electrical engineer learns. This calculator solves for any one of the three primary variables — voltage in volts (V), current in amperes (A), or resistance in ohms (Ω) — given the other two, and it also outputs the power dissipated by the resistance in watts (W) using the related power equation P = V × I (equivalently I² × R or V² / R). Common applications include LED current-limiting resistor selection (where the desired LED forward voltage and current set the resistor value), battery-load analysis (where a known battery voltage and load resistance determine the drawn current and dissipated power), and bench troubleshooting (where measured voltage and current together imply the unknown resistance of a black-box component). The calculator assumes ideal DC behaviour and constant resistance, which is a reasonable approximation for everyday electronics work but breaks down in AC circuits with reactive elements, in components with strongly temperature-dependent resistance, and in non-linear devices like diodes and transistors.
Calculator
The formula
Formula
V = I × R P = V × I
Worked example
When to use this calculator
Use this calculator whenever you have any two of voltage, current, and resistance and need to find the third value plus the dissipated power. The most common use cases are sizing current-limiting resistors for LEDs, calculating the load current drawn by a known resistance from a known supply voltage, and reverse-engineering an unknown resistance from measured voltage-and-current readings during bench debugging. The calculator does not handle AC impedance, where reactive elements (capacitors, inductors) introduce frequency-dependent behaviour requiring complex-number arithmetic — for AC circuits with reactive components, use a dedicated impedance calculator. It also does not handle non-linear elements like diodes, transistors, and lamps where resistance changes with operating conditions; those require characteristic-curve lookup rather than Ohm's Law.
Common input mistakes
- Mixing unit prefixes without scaling. A "5 kΩ" resistance must be entered as 5000 Ω, and a "20 mA" current must be entered as 0.020 A. Failing to scale to base units introduces order-of-magnitude errors that produce nonsensical voltage or power figures.
- Selecting a resistor power rating below the dissipated power. The calculator returns dissipated power in watts; the chosen resistor must be rated for at least that wattage with derating headroom (typically 50% margin for reliability). A 1/4 W resistor in a 1.44 W circuit will overheat and fail within minutes.
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, with the constant of proportionality being the conductor's resistance: V = I × R. The relationship was published by Georg Simon Ohm in 1827 after experiments measuring the current driven by various voltages through wires of different lengths and materials. It is the foundational equation of DC circuit analysis and underlies nearly every introductory electrical engineering problem.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies to AC circuits only when the circuit elements are purely resistive — that is, no capacitors or inductors are present. When reactive elements appear, resistance is replaced by impedance (Z), a frequency-dependent complex quantity, and the relationship V = I × Z still holds but requires complex-number arithmetic to handle phase relationships. For AC circuits with significant reactance, use an impedance calculator rather than the simple Ohm's Law form.
How do I find power if I only know two of V, I, and R?
Power can be calculated directly from any two of the three primary variables using the equivalent forms P = V × I, P = I² × R, or P = V² / R. This calculator computes power automatically once two of the three are known (or once the third is solved). For a 12 V supply driving a 100 Ω load, all three forms give the same 1.44 W answer because the underlying values are consistent.
What units should I use for inputs?
Enter voltage in volts (V), current in amperes (A), and resistance in ohms (Ω). For milliamps (mA), divide the milliamp figure by 1000 to get amperes; for kilohms (kΩ), multiply by 1000 to get ohms; for milliwatts (mW), the same 1000-divisor applies. Sticking to SI base units (V, A, Ω, W) avoids the most common source of error in Ohm's Law calculations.
Why does my LED need a current-limiting resistor?
LEDs are non-linear devices whose resistance drops sharply once their forward-voltage threshold is reached, so a direct connection to a power supply at or above that threshold draws unbounded current and burns out the LED almost instantly. A series resistor sized to drop the supply voltage minus the LED forward voltage at the desired LED current limits the current to a safe value. Ohm's Law sets the resistor value: R = (V_supply − V_LED) / I_LED.