liminfoOhm's Law Calculator
Free web tool: Ohm's Law Calculator
Ohm’s Law Wheel
V
I × R
√(P × R)
P / I
I
V / R
P / V
√(P / R)
R
V / I
V² / P
P / I²
P
V × I
V² / R
I² × R
About Ohm's Law Calculator
The Ohm's Law Calculator solves the fundamental relationships between voltage (V), current (I), resistance (R), and power (P) in electrical circuits. Enter any two of the four quantities and the calculator instantly derives the remaining two values using the core equations: V = I × R, P = V × I, and their algebraic variants. This covers all six possible two-known-value combinations, making it suitable for DC circuit analysis, component selection, power dissipation calculations, and basic electronics troubleshooting.
Electronics engineers, electricians, hobbyists building Arduino or Raspberry Pi projects, students studying for exams, and technicians sizing resistors and fuses are the primary users of this tool. The multi-unit selectors allow inputs in practical real-world scales — millivolts to kilovolts, microamps to amps, ohms to megaohms, milliwatts to kilowatts — so values do not need to be manually converted before entry. Results are displayed in an automatically scaled format (e.g., 0.015 A is shown as 15 mA) to aid readability.
The tool also includes an Ohm's Law Wheel reference panel showing all formula variants for V, I, R, and P side by side. This serves as a quick visual reminder for users who need to derive formulas rather than just plug in numbers. All calculations are performed in the browser with no data sent to any server, making it fast, private, and available offline after the page has loaded.
Key Features
- Solves for all four quantities (V, I, R, P) from any two known inputs using all six formula combinations
- Multi-unit input selectors: voltage in mV/V/kV, current in µA/mA/A, resistance in Ω/kΩ/MΩ, power in mW/W/kW
- Auto-scaled result display — large values shown in k/M prefix, small values in m/µ prefix
- Built-in Ohm's Law Wheel showing all formula variants for V, I, R, and P at a glance
- Real-time calculation — results update instantly as inputs change, no submit button needed
- Handles edge cases: requires at least two known values and prevents division-by-zero errors
- 100% client-side processing — no data leaves the browser, works offline after first load
- Responsive layout with dark mode support for use on any device or in any lighting condition
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, with the constant of proportionality being the resistance (R): V = I × R. Combined with the power equation P = V × I, these relationships allow any one of the four quantities to be derived from any two others.
How many values do I need to enter?
You need to enter exactly two values. The calculator supports all six combinations: V+I, V+R, V+P, I+R, I+P, and R+P. If fewer than two values are entered, no result is shown. If more than two are entered, the first two non-empty values are used for the calculation.
What units does the calculator support?
Voltage: mV, V, kV. Current: µA, mA, A. Resistance: Ω, kΩ, MΩ. Power: mW, W, kW. Select the appropriate unit from the dropdown next to each input field before entering the numeric value.
How are the results formatted?
Results are automatically scaled for readability. Values of 1,000,000 or more are shown with an M prefix, values of 1,000 or more with a k prefix, values less than 0.001 with a µ prefix, and values less than 1 with an m prefix. This mirrors how values are typically expressed in electronics datasheets.
Can I use this calculator for AC circuits?
Ohm's Law in the V = IR form applies directly to pure resistive DC circuits. For AC circuits with reactive components (inductors, capacitors), impedance (Z) replaces resistance (R) in the formula. This calculator does not model AC reactance or phase angles, so it is best suited for DC analysis or the resistive component of AC circuits.
What is the Ohm's Law Wheel shown in the calculator?
The Ohm's Law Wheel is a reference diagram showing all the formula rearrangements for each of the four quantities. For example, voltage can be computed as I × R, √(P × R), or P / I. The wheel serves as a quick visual aid for users who want to understand the underlying relationships rather than just use the calculator as a black box.
Is this calculator accurate enough for professional use?
The calculator uses standard double-precision floating-point arithmetic (IEEE 754), which provides approximately 15–16 significant digits of precision. This is more than sufficient for typical electronics design and educational purposes. For safety-critical or high-precision engineering applications, always verify results using calibrated instruments.
Can I calculate the power dissipated by a resistor?
Yes. Enter the resistance value (R) and either the voltage across it (V) or the current through it (I). The calculator will compute the power dissipation using P = V²/R or P = I²×R respectively. This is useful for selecting resistors with adequate power ratings to prevent overheating.