Ohm's Law Calculator
Enter any two of voltage, current, or resistance to calculate all four values.
Enter exactly two values. Leave the unknown field blank.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Ohm's Law relates voltage (V), current (I), and resistance (R) through the equation V = I × R. This calculator lets you solve for any unknown when you know two of the three values. It also calculates power (P = V × I) automatically.
- Enter two values. Fill in exactly two of the three fields: voltage in volts, current in amps, or resistance in ohms. Leave the unknown field blank.
- Read the results. The calculator shows power in watts along with all four values: V, I, R, and P.
- Use SI prefixes. Results are displayed with appropriate prefixes (mA, kΩ, mW, etc.) for easy reading at any scale.
Results update instantly as you type. Use the Share button to send your inputs to a colleague or save the URL for reference.
About Ohm's Law
Ohm's Law, formulated by Georg Simon Ohm in 1827, is the foundational equation of electronics. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The four key relationships are: V = IR, I = V/R, R = V/I, and P = VI = I²R = V²/R.
These relationships apply to resistive (ohmic) components at constant temperature. Non-linear components like diodes and transistors do not follow Ohm's Law directly, but the law still applies to resistive elements in those circuits. All calculations run entirely in your browser with no data stored or transmitted.
Frequently Asked Questions
What is Ohm's Law and why does it matter?
Ohm's Law (V = IR) describes the relationship between voltage, current, and resistance in an electrical circuit. It matters because it lets you predict circuit behavior, size components correctly, and troubleshoot problems. Every resistor calculation, LED circuit, power supply design, and motor drive relies on Ohm's Law as its foundation.
How do I calculate power from voltage and resistance?
Power equals voltage squared divided by resistance: P = V²/R. For example, 12 V across a 100 Ω resistor dissipates 144/100 = 1.44 W. This is equivalent to P = I²R and P = VI once you calculate the current. This calculator computes power automatically when you enter any two of voltage, current, or resistance.
What units should I use for this calculator?
Enter voltage in volts (V), current in amps (A), and resistance in ohms (Ω). For milliamp circuits, convert first: 20 mA = 0.02 A. For kilohm resistors: 10 kΩ = 10000 Ω. The results are displayed with SI prefixes automatically, so a 0.02 A result will show as 20.00 mA.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies directly to purely resistive AC circuits using RMS voltage and current values. For circuits with capacitors or inductors (reactive components), the impedance (Z) replaces resistance (R), and the formula becomes V = IZ. This calculator handles the resistive case, which covers most practical applications including lighting, heating elements, and resistive loads.