Ohms Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law. Enter any 2 values to find the remaining parameters instantly.

⚡ Ohm's Law Calculator

Enter any 2 values to calculate the remaining parameters

Key Formulas

V = I × R

P = V × I

I = V / R

P = I² × R

R = V / I

P = V² / R

What is Ohm's Law?

Ohm's Law is a fundamental principle in electrical engineering that describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Named after German physicist Georg Ohm, this law states that the current through a conductor between two points is directly proportional to the voltage across the two points.

The basic formula V = I × R forms the foundation of circuit analysis and design. When combined with power calculations (P = V × I), Ohm's Law becomes an essential tool for sizing components, ensuring safety, and troubleshooting electrical systems.

Our Ohm's Law Calculator simplifies these calculations by allowing you to input any two known values and instantly compute the remaining parameters. Whether you're a student, hobbyist, or professional engineer, this tool helps you quickly verify circuit designs and solve electrical problems.

How to Use the Ohm's Law Calculator

Select Your Known Values: Enter any two of the four electrical parameters: voltage (V), current (I), resistance (R), or power (P).
Choose Units: Select the appropriate units for each value from the dropdown menus (e.g., V, kV, mV for voltage; A, mA for current).
Click Calculate: Press the Calculate button to instantly compute all remaining values based on Ohm's Law and power formulas.
Review Results: The calculator displays all four parameters with automatic unit conversion for easy reading (e.g., large values shown in kΩ or MW).
Clear and Recalculate: Use the Clear button to reset all fields and perform new calculations as needed.

Latest Insights & Technical Guidelines

Understanding the scope and limitations of Ohm's Law is crucial for accurate circuit analysis:

Scope of Validity: Ohm's Law strictly applies to ohmic (linear) resistive components where current is proportional to voltage. It is designed for simple, primarily resistive DC circuits and resistive AC loads like heaters and incandescent lamps.
AC Circuits & Impedance: For AC circuits with significant inductance or capacitance, you must use impedance (Z) and phasor analysis. A simple Ohm's Law calculator provides only approximations unless it explicitly supports complex impedance calculations.
Power Dissipation & Safety: Modern design emphasizes checking resistor power ratings using P = V²/R or I²R to prevent component failure and fire hazards, especially in compact electronics and high-density circuit boards.
Regulatory Compliance: Use Ohm's Law calculations alongside safety standards for wiring, overcurrent protection, and touch-safe voltages. Calculators support but do not replace compliance with product datasheets and electrical codes.

Best Practices & Considerations

Use Correct Units: Always input values in the correct units (V, A, Ω, W) and convert between prefixes (mA↔A, kΩ↔Ω) before calculation to avoid order-of-magnitude errors.
Know Your Circuit Type: Only rely on simple Ohm's Law for DC or mainly resistive AC circuits. For reactive components, use impedance-based tools.
Include Power Checks: Always compute power (P) to ensure components, wires, and power supplies operate within safe limits and rated specifications.
Consider Tolerances: Real components have tolerances (±1%, ±5%) and temperature coefficients. Treat calculator outputs as nominal values and add appropriate safety margins.
Verify Measurements: When using measured values from a multimeter, ensure correct meter mode, range, and connection (series for current, parallel for voltage) before inputting into the calculator.
Document Your Work: Save or export calculated values into design notes or spreadsheets for later safety reviews, debugging, and design verification.

Understanding Ohm's Law in Detail

Core Formulas

V = I × R: Voltage equals current multiplied by resistance. This is the fundamental form of Ohm's Law.
I = V / R: Current equals voltage divided by resistance. Use this to find current when voltage and resistance are known.
R = V / I: Resistance equals voltage divided by current. Calculate resistance from voltage and current measurements.
P = V × I: Power equals voltage multiplied by current. This is the basic power formula for any electrical circuit.
P = I² × R: Power equals current squared multiplied by resistance. Useful for calculating power dissipation in resistors.
P = V² / R: Power equals voltage squared divided by resistance. Alternative power calculation when voltage and resistance are known.

Common Applications

Circuit Design: Select appropriate resistor values, component ratings, wire sizes, and power supply specifications for new circuit designs.
Safety Analysis: Estimate current and power to prevent overheating, overcurrent conditions, and fire risks. Choose proper fuses, circuit breakers, and safe operating conditions.
Troubleshooting: Compare measured values against calculated values to detect short circuits, open circuits, or out-of-tolerance components in DC or resistive AC circuits.
Education: Help students, hobbyists, and engineers learn and verify electrical calculations quickly without manual algebra.

Frequently Asked Questions

Why do I need to enter exactly 2 values?

Ohm's Law and power formulas create a system of equations that requires exactly 2 known values to solve for the remaining unknowns. With fewer than 2 values, the system is underdetermined. With more than 2, the values may be inconsistent.

Can I use this calculator for AC circuits?

This calculator works for DC circuits and resistive AC loads (like heaters and incandescent lamps). For AC circuits with significant inductance or capacitance, you need to use impedance (Z) instead of resistance (R) and consider phase angles.

What if my calculated power exceeds the component rating?

If the calculated power exceeds the component's rated power, the component will overheat and may fail or cause a fire. You must either reduce the voltage/current or use a component with a higher power rating.

Why are my results shown in different units?

The calculator automatically converts results to the most readable units. For example, 1000 Ω becomes 1 kΩ, and 0.001 A becomes 1 mA. This makes large and small values easier to read and understand.

Does Ohm's Law work for LEDs and semiconductors?

No. LEDs, diodes, transistors, and other semiconductors are non-ohmic devices with non-linear voltage-current relationships. Use manufacturer I-V curves and specialized LED calculators for these components.