Ohm's law calculator
Enter any two of voltage, current, resistance and power — the calculator solves the other two and shows the formula it used. Computed fields are shaded; type into a field to claim it as an input and the least recently edited value is recomputed. The formula table is below the tool, printable for the bench wall; more bench math lives on the electronics hub.
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enter any two values to solve the rest
| Voltage | — |
| Current | — |
| Resistance | — |
| Power | — |
DC and purely resistive AC circuits only — heaters, resistors, incandescent lamps. Motors, transformers and switching supplies need impedance and power factor instead; see the FAQ below.
Ohm's law formula reference
The power wheel as a table: pick the quantity you are solving for, then the pair you know. All twelve rearrangements of V = I × R and P = V × I — print it and hang it next to the bench supply.
| Solve for | You know | Formula |
|---|---|---|
| Voltage V | I & R | V = I × R |
| P & I | V = P / I | |
| P & R | V = √(P × R) | |
| Current I | V & R | I = V / R |
| P & V | I = P / V | |
| P & R | I = √(P / R) | |
| Resistance R | V & I | R = V / I |
| V & P | R = V² / P | |
| P & I | R = P / I² | |
| Power P | V & I | P = V × I |
| I & R | P = I² × R | |
| V & R | P = V² / R |
Ohm's law at the bench
Most bench questions are two-knowns problems. A 24 W heater cartridge on a 12 V rail draws I = P / V = 2 A and sits at R = V² / P = 6 Ω hot — if the supply is rated 1.5 A, the arithmetic says no before you strip a wire. Run the same check backwards to size a drop or bleed resistor: burning 8 W from 12 V calls for R = V² / P = 18 Ω, pulling 0.67 A for as long as it is connected.
Dropping voltage with a series resistor is the other daily case. To lose 3 V at 350 mA you need R = V / I ≈ 8.6 Ω dissipating P = V × I ≈ 1.1 W, so fit a 2 W part and expect it to run warm. For LEDs the forward drop changes the arithmetic — the LED resistor calculator handles Vf and picks the next E24 value up. For a signal-level voltage rather than a power drop, use two resistors as a divider: the voltage divider calculator solves the ratio and checks the output under load.
Two limits are worth remembering. The law is exact only for DC and purely resistive AC loads — motors, transformers and switching supplies need impedance and power factor instead. And resistance drifts with temperature: a cartridge that measures 4 Ω cold still settles near 6 Ω at full power, with a matching inrush surge at switch-on. Identify unmarked parts with the resistor color code calculator before trusting what you measured.
Frequently asked questions
What is Ohm's law?
Ohm's law states that the current through a resistor is proportional to the voltage across it: V = I × R. Rearranged it gives I = V / R and R = V / I, so any one of voltage, current and resistance follows from the other two. Add the power identities and any two of V, I, R and P pin down all four — that is exactly what the calculator above solves.
How does the V-I-R triangle work?
Write V on top of a triangle with I and R side by side below it. Cover the quantity you want and the visible two give the formula: cover V and you see I × R, cover I and you see V / R, cover R and you see V / I. It is a memory aid for the three Ohm's law rearrangements, nothing more.
What are the three power formulas?
P = V × I is the definition of electrical power. Substituting Ohm's law into it produces the other two forms: P = I² × R for the power lost in a resistance at a known current, and P = V² / R for the power drawn by a resistance at a known voltage. All twelve rearrangements are listed in the formula table above.
What current and power does a 6 Ω load draw from 12 V?
Current is I = V / R = 12 / 6 = 2 A. Power is P = V × I = 12 × 2 = 24 W — the same answer P = V² / R = 144 / 6 gives directly. Enter 12 in the voltage field and 6 in the resistance field above and the calculator fills in 2 A and 24 W.
Does Ohm's law work for AC circuits?
Only for purely resistive loads — heaters, incandescent lamps, plain resistors — where voltage and current stay in phase. Motors, transformers, LED drivers and switching supplies are reactive or non-linear: there you need impedance, phase angle and power factor, and this calculator does not apply. For DC circuits it always holds.