OctaCalc / Ohm's Law Calculator

Ohm's Law Calculator

Solve for voltage, current, or resistance — leave one field blank and the Ohm's Law calculator will find the missing value from the other two.

Leave exactly one field blank to calculate that value.

Result
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For educational and reference use only. Always verify results before use in real-world designs or safety-critical applications. For more information, see Calculation Assumptions and Disclaimer.

How to Use This Ohm's Law Calculator

Fill in any two of the three fields and leave the third blank. The calculator will automatically solve for the missing quantity using Ohm's Law. All three modes (find V, find I, find R) are supported.

Ohm's Law Formula

Ohm's Law defines the relationship between voltage, current, and resistance in a linear circuit:

V = I × R
I = V / R
R = V / I

Example

Find current: 9V battery, 470 Ω resistor
I = V / R = 9 / 470
I ≈ 0.01915 A (19.15 mA)

Find resistance: 5V source, 100 mA load
R = V / I = 5 / 0.1
R = 50 Ω

Frequently Asked Questions

What is Ohm's Law?
Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, given a constant resistance. It is expressed as V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.

Can I use this calculator for AC circuits?
Ohm's Law applies directly to DC circuits and to the resistive parts of AC circuits. For AC circuits with inductors or capacitors, impedance (Z) replaces resistance (R), and the same formula applies: V = I × Z. This calculator assumes purely resistive loads.

What units does this calculator use?
Voltage is entered and displayed in volts (V), current in amperes (A), and resistance in ohms (Ω). If your values are in milliamps or kilohms, convert them first — for example, 47 mA = 0.047 A, and 4.7 kΩ = 4700 Ω.

Why do I get an error when all three fields are filled in?
The calculator needs exactly one unknown to solve for. If all three fields are filled, there is nothing to calculate. Leave the field you want to find blank and enter values in the other two.

How do I find resistance if I know voltage and current?
Use the formula R = V / I. For example, if your circuit runs at 12 V and draws 0.5 A, the resistance is R = 12 / 0.5 = 24 Ω. You can also just leave the Resistance field blank in the calculator and enter the voltage and current values.