Ohm's Law Calculator
This Ohm's Law calculator solves for Voltage (V), Current (I), Resistance (R), and Power (P) from any two values you know. Enter any two of the four quantities and it instantly computes the remaining two, so you never have to rearrange a formula by hand.
It is built around the Power Wheel — Ohm's Law (V = I × R) combined with the power equation (P = V × I) — which produces 12 formulas covering every combination of V, I, R, and P. Whether you are a student, technician, electronics hobbyist, or electrician, the tool handles the unit conversions and the algebra for you.
How to Use the Ohm's Law Calculator
Enter any two known values
The calculator has four fields: Voltage (V), Current (I), Resistance (R), and Power (P). Type values into any two of them.
Pick the right unit
Each field has a unit dropdown — voltage in mV, V, or kV; current in µA, mA, or A; resistance in mΩ, Ω, kΩ, or MΩ; power in mW, W, or kW. Conversions are handled for you.
Read the results
The remaining two values are calculated automatically as you type. Fields you fill in show an Input badge, while computed fields show a Result badge.
Try the quick examples
Click Examples to load real circuits like an LED, USB charging, or household power. Use the Decimals selector to set the precision and Reset to clear everything.
Features
Any 2 of 4 Values
Enter any two of Voltage (V), Current (I), Resistance (R), and Power (P) to compute the other two.
SI Unit Prefixes
Work in real-world values — mV/V/kV, µA/mA/A, mΩ/Ω/kΩ/MΩ, mW/W/kW — with conversions handled automatically.
Real-Time Calculation
Results update the moment you type, with clear Input and Result badges marking each field.
Interactive Power Wheel
A visual wheel of all 12 formulas highlights the active inputs and results so you can see the relationships.
Formula Display
See exactly which equation was used and its SI-formatted result, so you can verify or learn the math.
6 Quick Examples
Load ready-made circuits — LED, USB charging, car battery, household, heater, and resistor — in one click.
Adjustable Precision
Choose 2, 3, 4, or 6 decimal places to control how many digits the results display.
Full Formula Reference
A built-in table lists every equation for V, I, R, and P as a handy Ohm's Law reference.
Smart Input Pairs
Only two inputs are used at once — enter a third and the oldest is cleared, keeping the workflow fast.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the voltage across a conductor is directly proportional to the current through it, with resistance as the constant of proportionality: V = I × R. It applies to most everyday electrical circuits and components.
How do I calculate voltage, current, or resistance?
Enter any two of the four values and the calculator solves for the rest using V = I × R: voltage is V = I × R, current is I = V / R, and resistance is R = V / I. You only need two knowns to find the other two.
How do I find power with Ohm's Law?
Power combines Ohm's Law with the power equation P = V × I. Depending on which values you know, it can also be calculated as P = V² / R or P = I² × R. The calculator picks the right formula automatically and shows it under the inputs.
What is the Power Wheel?
The Power Wheel (also called the Ohm's Law Wheel) is a circular diagram that combines Ohm's Law (V = IR) with the power equation (P = VI) to show all 12 formulas for V, I, R, and P. Each quadrant holds three formulas for one quantity.
Why can I only enter two values?
In a simple circuit, any two of the four quantities (V, I, R, P) uniquely determine the other two. Entering a third value would be redundant or contradictory, so the calculator keeps only the two most recent inputs and clears the oldest.
What units does the calculator support?
It supports standard SI prefixes: millivolts to kilovolts for voltage, microamps to amps for current, milliohms to megaohms for resistance, and milliwatts to kilowatts for power. All conversions are handled internally, so you can enter 4.7kΩ instead of 4700Ω.
Why does entering 0 not produce a result?
A value of zero leads to division by zero in most formulas — for example, 0Ω of resistance would imply a short circuit with infinite current. To avoid undefined results, the calculator requires positive values.
Is my data stored anywhere?
No. All calculations happen entirely in your browser. Nothing you enter is sent to any server, and nothing is saved after you close the page.
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