Ohm's Law Calculator

What is Ohm's Law?

Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = I * R. This is the most fundamental equation in electrical engineering, relating voltage (volts), current (amperes), and resistance (ohms) in any resistive circuit.

Named after German physicist Georg Ohm who published the relationship in 1827, Ohm's Law applies to all linear (ohmic) conductors — materials where resistance remains constant regardless of voltage. Most metals and carbon-based resistors are ohmic. Semiconductors (diodes, transistors) and some other materials are non-ohmic — their resistance changes with voltage.

The Ohm's Law triangle

The three forms of Ohm's Law are: V = I * R (find voltage), I = V / R (find current), R = V / I (find resistance). A helpful memory aid is the VIR triangle — cover the quantity you want to find, and the remaining two show the formula. V on top, I and R on the bottom. Cover V to see I * R. Cover I to see V / R. Cover R to see V / I.

How to use this tool

Enter any two of the three values (voltage, current, resistance) and the calculator instantly computes the third, plus the power dissipated (P = V * I). All common unit prefixes are supported — milliamps, kilohms, millivolts, etc.

Practical examples

• An LED circuit: 5V supply, 220 ohm resistor, current = 5/220 = 22.7 mA.
• A heating element: 240V across 48 ohms draws 5A and consumes 1,200W.
• Finding wire resistance: 12V battery, 2A load, R = 12/2 = 6 ohms total circuit resistance.

Worked example

A 12-volt supply drives a 240-ohm resistor. Ohm's law, I = V ÷ R, gives 12 ÷ 240 = 0.05 A, or 50 mA. The power dissipated is P = V × I = 12 × 0.05 = 0.6 W, so a quarter-watt resistor would overheat and a half-watt part is the safe minimum. Rearranging V = I × R lets you size a resistor for a target current instead.

Common mistakes

A frequent mistake is mixing units — using milliamps with volts and ohms gives an answer 1,000 times off, so convert to base units first. Another is forgetting that Ohm's law applies to the resistance in question, not the whole circuit; in a series string you must use the voltage across that specific resistor. Finally, real resistors have a power rating: exceed it and the component overheats regardless of the correct current.

Frequently asked questions

Does Ohm's Law work for AC circuits?

The basic form V = IR works for purely resistive AC loads. For circuits with capacitors and inductors, the generalized form uses impedance (Z) instead of resistance: V = I * Z. Impedance is a complex number that includes both resistance and reactance (the opposition to current changes caused by capacitors and inductors).

Why does resistance cause heat?

When current flows through a resistor, electrons collide with the atoms in the material, transferring kinetic energy. This energy manifests as heat. The power dissipated as heat is P = I squared * R (Joule heating). This is why high-current wires must be thick (low R) and why resistors have wattage ratings — exceeding the rating causes overheating and failure.

Does Ohm's law apply to all components?

No. It holds for ohmic devices like resistors, where current is proportional to voltage. Diodes, transistors, and lamps are non-ohmic: their resistance changes with voltage or temperature, so a single fixed R does not describe them and you need their characteristic curve instead.