Enter any two of the four values below — voltage, current, resistance, or power — and the rest solve automatically.
Ohm's Law describes how voltage, current, and resistance relate in an electrical circuit: voltage equals current multiplied by resistance. Push more voltage through a fixed resistance and current rises with it; increase the resistance while holding voltage steady and the current drops. It's the single most-used equation in electronics, which is why it's usually the first thing taught in any electrical course.
Add power (P) to the mix and Ohm's Law expands into a full set of power formulas, since power is simply voltage multiplied by current:
Say a 5V supply drives an LED strip drawing 0.5A. Resistance works out to 5V ÷ 0.5A = 10Ω, and the power dissipated is 5V × 0.5A = 2.5W. That wattage figure matters just as much as the ohms — a resistor or wire rated below 2.5W will overheat and fail even though the resistance math checks out. Always size components for at least double your calculated power.
Ohm's Law only holds for linear, "ohmic" components — plain resistors and wires at a roughly constant temperature. Diodes, LEDs, and transistors are non-linear: their effective resistance changes with the current passing through them, so V = I × R can't predict their behavior directly. For those parts you need the manufacturer's datasheet curves instead of this formula.
Yes, but substitute impedance (Z) for resistance (R). Impedance accounts for the extra phase effects that capacitors and inductors add on top of plain resistance.
Resistors are only manufactured in standard series (E12, E24, etc.), not every possible value. Round your result up to the nearest standard size, then check it against your tolerance needs using the Resistor Color Code Calculator.
Voltage and resistance are usually easiest to measure directly with a multimeter — enter those two above and this calculator solves current and power automatically.
Ohm's Law is the single most fundamental relationship in electrical circuits, describing how voltage, current, and resistance interact. Formulated by Georg Ohm in 1827, it states that the current through a conductor is directly proportional to the voltage across it, and inversely proportional to its resistance.
V = I × R — where V is voltage in volts, I is current in amps, and R is resistance in ohms. This single equation rearranges into three useful forms depending on what you're solving for:
Voltage: V = I × R
Current: I = V ÷ R
Resistance: R = V ÷ I
Power (P, in watts) relates to Ohm's Law through P = V × I. Combining this with V=IR gives two more useful forms: P = I²R (useful when you know current and resistance) and P = V²/R (useful when you know voltage and resistance). This calculator solves for all four quantities — V, I, R, and P — from any two known values.
Say you have a 9V battery and want to limit current through a component to 20mA (0.02A). Using R = V/I: R = 9 ÷ 0.02 = 450Ω. The power dissipated would be P = V × I = 9 × 0.02 = 0.18W, so a standard 1/4W (0.25W) resistor would handle it safely.
A common point of confusion: if V=IR, why does voltage seem to decrease when current increases (like lights dimming when a heater turns on)? The answer is that real circuits include the resistance of wiring and the power source itself, not just the load. As current rises, more voltage gets "used up" by that wiring resistance, leaving less for everything downstream — Ohm's Law still holds true at every point, it's just being applied to a different resistance than you might expect.
In a series circuit, the same current flows through every component, and voltages add up. In a parallel circuit, voltage is the same across every branch, and currents add up. Ohm's Law applies to each individual component or to the circuit as a whole, depending on what you're analyzing — see our Series/Parallel Resistance Calculator for combining multiple resistors.
Does Ohm's Law apply to AC circuits? Yes, with a modification — for AC circuits with capacitors or inductors, resistance is replaced by impedance (Z), which accounts for phase differences between voltage and current. For purely resistive AC loads, the basic V=IR relationship still applies directly.
What if my calculated resistance isn't a standard value? Round up to the nearest standard resistor value (see our Resistor Color Code Calculator) — this typically gives you slightly less current than calculated, which is usually the safer direction.
Why does my multimeter reading not match my calculation? Real components have tolerances (typically ±5% or ±10% for resistors), and wiring/contact resistance adds small additional resistance not accounted for in ideal calculations.
Is Ohm's Law always true? It holds for "ohmic" materials and components (most resistors, wires) across normal operating conditions. Some components (diodes, transistors, LEDs) are "non-ohmic" — their resistance changes with voltage/current, so V=IR doesn't directly apply to them in the same simple way.
What's the difference between this and the Voltage Divider calculator? This tool solves the basic V/I/R/P relationship for a single component or circuit. The Voltage Divider Calculator specifically solves the two-resistor voltage divider configuration, a common sub-circuit built from this same underlying law.
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