Ohm’s Law Calculator — Voltage, Current & Resistance Online

How to use the Ohm’s Law Calculator

Fill in exactly two of the three fields — Current (I) in amps, Voltage (V) in volts, Resistance (R) in ohms — and leave the third one blank. Click “Calculate” and the missing value appears, rounded to two decimal places. If you fill in fewer than two fields, or all three, the tool tells you plainly: “Please enter two values to calculate the third.” That message is not an error — it is the tool reminding you it needs exactly two inputs to solve for the third.

Worked example: three directions, one formula

Take a 12 volt battery connected to a 4 ohm resistor. Enter Voltage 12 and Resistance 4, leave Current blank, and the tool gives Current (I) = 3.00 Amps, because I = V ÷ R = 12 ÷ 4 = 3.

Now flip it. A 9 volt battery drives 0.50 amps through an unknown resistor. Enter Voltage 9 and Current 0.50, leave Resistance blank, and you get Resistance (R) = 18.00 Ohms, because R = V ÷ I = 9 ÷ 0.50 = 18.

One more direction: 2 amps flowing through a 10 ohm resistor. Enter Current 2 and Resistance 10, leave Voltage blank, and the answer is Voltage (V) = 20.00 Volts, because V = I × R = 2 × 10 = 20. Same triangle, three uses — and every result shows two decimal places, so 3 amps reads as 3.00.

Where it helps

Physics and electronics students use it to check homework before handing it in — a wrong rearrangement of the formula shows up at once when the tool disagrees. Hobbyists building LED circuits use it to pick a resistor: knowing the supply voltage and the current the LED needs gives the resistor value directly. Technicians fault-finding a circuit use it to sanity-check a reading: if the measured voltage and resistance imply a current far from what is measured, something else in the circuit is wrong. Teachers use it for quick classroom demos, because changing one number and recalculating shows the relationship between the three quantities live.

Honest limitations

This calculator treats resistors as ideal and circuits as direct current (DC). It does not account for resistance changing as a component heats up, for alternating current, or for reactance from capacitors and inductors — those need their own calculators, such as the capacitive reactance and inductive reactance tools. It does not calculate electrical power either: if you need watts, use P = V × I on the values you found here. Results are rounded to two decimal places, which is fine for homework and hobby circuits but not for precision engineering. And it solves one resistor at a time — for combinations of resistors, use the series and parallel resistance calculator.

Ohm’s Law Calculator — frequently asked questions

Which two values do I enter?

Any two of voltage, current and resistance. Leave the value you want to find blank, and the calculator works out that one.

What happens if I enter all three values?

The tool asks you to enter two values to calculate the third. It solves for a missing value; it does not check three values against each other, so leave one field empty.

Does it calculate power in watts?

No. This tool only solves V = I × R. For power, multiply the voltage and current yourself with P = V × I.

Can I use it for AC circuits?

Only as a rough guide. Ohm’s law in this simple form applies to DC and to resistive AC loads; circuits with capacitors or inductors need reactance calculations instead.