Ohm's Law & Power Calculator: what this tool is for
Ohm's Law is the foundational cornerstone of physics, electronics, and electrical engineering. Formulated in 1827 by German physicist Georg Simon Ohm, it establishes the linear relationship between electric voltage (V in Volts), current intensity (I in Amperes), and electrical resistance (R in Ohms).
Coupled with Joule's Law for electrical power (P in Watts), Ohm's law empowers engineers, technicians, and hobbyists to design, size, and troubleshoot both direct current (DC) circuits and purely resistive alternating current (AC) setups.
This free online Ohm's Law and Power Calculator enables you to instantly compute any 2 missing electrical variables from any known 2-variable pair among Voltage (V), Current (I), Resistance (R), and Power (P).
With full multi-unit prefix support (micro, milli, kilo, mega) and detailed step-by-step arithmetic solutions, dimension your circuits, LED limiting resistors, battery packs, and power supplies with complete precision and confidence.
No file stored
Nothing is saved on our servers, during or after processing.
Local processing
The computation runs in your browser whenever the technology allows it.
Instant result
No waiting and no export queue: the result appears live.
Free, no account
No sign-up, no daily limit and no intrusive ads.
What this tool gives you
Universal 2-from-4 variable solver
Input any combination of 2 known electrical parameters (V-I, V-R, V-P, I-R, I-P, or R-P) to calculate the remaining two values instantly.
Full multi-unit prefix conversions
Seamlessly convert between microamps (µA), milliamps (mA), millivolts (mV), kilovolts (kV), kilo-ohms (kΩ), megawatts (MW), and horsepower (hp).
Step-by-step formula breakdown
Inspect exact formulas and intermediate calculation steps derived from the comprehensive 12-formula Ohm's Law wheel.
Joule thermal power rating guide
Evaluate heat dissipation in Watts and receive direct component wattage recommendations (1/4W, 1/2W, 1W, 5W) to avoid thermal breakdown.
Real-world examples
Review the master formula relationships and benchmark parameters for popular electrical circuits:
How to use Ohm's Law & Power Calculator
Select your known 2-variable pair
Choose which two values you currently have (e.g., Voltage V and Resistance R, or Voltage V and Power P).
Input the numeric values and units
Enter your figures and pick appropriate engineering units (e.g., 5 V and 20 mA).
Inspect real-time synchronized results
Review the calculated values for V, I, R, and P displayed in both base SI units and convenient scaled prefixes.
Check component power rating guidance
Verify thermal heat dissipation to ensure your physical resistor rating complies with recommended safety margins.
Tips and mistakes to avoid
Maintain a 50% power safety margin on physical resistors
If a circuit dissipates 0.18 W, avoid pushing a standard 1/4 W (0.25 W) resistor close to its thermal limit. Select a 1/2 W (0.5 W) rated component to ensure long-term reliability and cool operation.
Account for voltage drops along extended wiring
In low-voltage, high-current circuits (such as 12V automotive or solar off-grid setups), wiring internal resistance causes noticeable voltage drops (ΔV = R_wire × I). Use heavier gauge wire to minimize line loss.
Avoid : Applying DC Ohm's law to inductive AC loads without power factor
In alternating current circuits with electric motors or transformers, active power requires the power factor: P = V × I × cos(φ). Direct Ohm's law applies strictly to DC or purely resistive AC circuits.
Avoid : Misinterpreting metric prefix multipliers
Entering 10 Ω instead of 10 kΩ (10,000 Ω) results in calculating 1,000 times higher current and 1,000 times greater power dissipation, risking immediate component destruction.
Frequently asked questions
Answers to the questions people ask about Ohm's Law & Power Calculator.
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Updated on August 2026. See all tools