Tip: The two values you type are used as inputs; the other two are calculated from them.
Given any two of the four quantities, the remaining two follow directly:
| Known | Voltage V | Current I | Resistance R | Power P |
|---|---|---|---|---|
| V & I | — | — | V / I | V × I |
| V & R | — | V / R | — | V² / R |
| V & P | — | P / V | V² / P | — |
| I & R | I × R | — | — | I² × R |
| I & P | P / I | — | P / I² | — |
| R & P | √(P × R) | √(P / R) | — | — |
Ohm's law is the single most important relationship in electronics. Discovered by the German physicist Georg Simon Ohm in 1827, it states that the current flowing through a conductor between two points is directly proportional to the voltage across those points, and inversely proportional to the resistance between them. In its everyday form it is written V = I × R — voltage equals current multiplied by resistance. From this one equation, together with the power relationship P = V × I, you can solve for any electrical quantity in a simple resistive circuit as long as you know two of them. That is exactly what the calculator above does: enter any two of voltage, current, resistance, or power, and it instantly derives the other two.
Example 1 — sizing an LED resistor. A 5 V supply drives an LED that needs about 2 V across it at 20 mA. The resistor must drop the remaining 3 V. Using R = V / I = 3 V ÷ 0.02 A = 150 Ω. Its power dissipation is P = V × I = 3 V × 0.02 A = 0.06 W, so a standard ¼ W resistor is more than adequate.
Example 2 — finding current draw. A 12 V car bulb has a hot resistance of 6 Ω. The current it draws is I = V / R = 12 ÷ 6 = 2 A, and it consumes P = V × I = 12 × 2 = 24 W. This tells you the fuse and wiring must comfortably handle 2 A.
Example 3 — working backwards from power. A heating element is rated 1100 W on a 220 V mains supply. Its operating current is I = P / V = 1100 ÷ 220 = 5 A, and its resistance is R = V² / P = 220² ÷ 1100 = 44 Ω.
Because power and Ohm's law can be combined, there are twelve rearrangements in total (shown in the reference table above). For example, if you only know current and resistance you can still get power directly with P = I² × R, and if you know power and resistance you can find voltage with V = √(P × R). The calculator automatically picks the correct formula based on whichever two fields you fill in most recently.
Every practical circuit relies on it: choosing current-limiting resistors, sizing fuses and wire gauges, designing voltage dividers and biasing networks, calculating battery run-time, estimating heat-sink requirements, and diagnosing faults (an unexpected current usually points to a wrong resistance somewhere). It applies exactly to purely resistive, DC or instantaneous circuits; for AC circuits with capacitors and inductors you replace resistance with impedance, but the same V = I × Z form still holds.
Can I use this for AC circuits? For simple resistive loads, yes. For reactive AC circuits (with inductance or capacitance) use impedance in place of resistance.
What if I enter three or four values? The tool always solves from the two you entered most recently, so you can freely change your mind and it recomputes the rest.
Does Ohm's law apply to every component? It applies to "ohmic" components whose resistance is constant (resistors, wires). Diodes and transistors are non-ohmic — their current–voltage relationship is not a straight line — so Ohm's law only approximates a single operating point.