Ohm's Law & Power
Calculates voltage, current, resistance or power from any two known quantities.
V
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A
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Ω
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W
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Ohm's Law Wheel
What is Ohm's Law
Formulated by German physicist Georg Ohm in 1827, Ohm's Law describes the relationship between voltage (V, in volts), current (I, in amperes) and resistance (R, in ohms) in a conductor: V = I × R. From it, electrical Power (P, in watts) — the rate of energy consumed or dissipated, defined as P = V × I — combines with Ohm's Law itself to produce four more equivalent formulas (P = I²R, P = V²/R, and the two ways of isolating V or I from P and the third quantity). In total, there are four quantities and six possible pairs of "two known, two calculated" — the "Ohm's Law wheel", above, gathers all twelve formulas (three per quantity) into a single diagram.
Limitations
This calculator assumes an ideal ohmic component (a resistor) under direct current (DC) — resistance is treated as a fixed value, the same at any applied voltage/current. Non-linear components (LEDs, diodes, transistors) don't follow V = I × R directly: their effective resistance changes with the applied voltage, which is why the LED example uses the voltage already left over after the LED's own forward drop (the voltage entered is only what remains for the resistor). Under alternating current (AC), capacitors and inductors introduce reactance — the vector sum of resistance and reactance is called impedance, and it isn't covered by this tool (see "Frequently asked questions", below).
Frequently asked questions
Voltage, current, resistance and power are tied together by two independent equations (V = I × R and P = V × I) — mathematically, two equations with four unknowns become fully determined as soon as two of the unknowns become known values. It doesn't matter which pair: all six possible combinations (V+I, V+R, V+P, I+R, I+P, R+P) solve the other two uniquely, always through the most direct route (see the "Ohm's Law wheel", above).
Resistance (R, in ohms) is a component's opposition to direct current (DC) flow — a single, fixed value, independent of frequency. Impedance (Z, also in ohms) is the equivalent concept under alternating current (AC): it adds resistance to reactance (the extra opposition capacitors and inductors introduce, which varies with the signal's frequency). Every resistor has resistance; only circuits with a capacitor/inductor under AC have meaningful impedance. This calculator only works with resistance/DC — out of scope for this version.
20 mA (milliamps) is the typical operating current of a common indicator LED (5 mm, red/green/yellow) — a widely used reference value when the specific LED's datasheet isn't at hand. High-brightness or RGB LEDs may call for different currents (always check the datasheet when available); a current well above the LED's limit burns it out, and one well below leaves it dim.
A field stays blank when fewer than two quantities currently have a valid typed value — with zero or one known quantity, the system of equations isn't determined yet, so there's nothing to calculate. Filling in the second quantity makes the other two fields calculate automatically.
The last two edited fields are always the current "inputs" — typing into a third field automatically releases the older of the previous two, which goes back to being calculated from the two most recent. There's no "all four locked" state: the calculator always recalculates on top with the most recent pair, without asking you to clear anything first.
Yes — type the number followed by an International System prefix letter (m = milli, k = kilo, M = mega, among others) and the value is interpreted automatically in the correct unit: "4.7k" in Resistance becomes 4700 Ω, "20m" in Current becomes 0.02 A. Calculated fields already display the result formatted the same way (e.g. "150 Ω" instead of "150").
Nexinon Principles
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