Ohm's Law Calculator

INPUT VALUES

Voltage (V)

Current (I)

Resistance (R)

Power (P)

What is an Ohm’s law calculator?

Ohm’s law is the fundamental relationship between voltage, current and resistance in electrical circuits. Combined with the basic power equation, it allows you to calculate any missing quantity when the others are known.

This calculator lets you enter any two known values (for example voltage and resistance, or current and power) and automatically computes the remaining quantities:

Voltage (V)
Current (I)
Resistance (R)
Power (P)

It is useful for everything from simple DC circuits and LED sizing to checking PLC I/O loads, power supplies and industrial wiring.

Ohm’s law and power relationships

The core equations are:

Ohm’s law: V = I × R
Power: P = V × I

From these, you can derive alternative forms:

I = V / R
R = V / I
P = I² × R
P = V² / R

The calculator uses these identities to solve for the unknown values based on whichever pair of inputs you provide.

Quick reference examples

The table below gives some typical combinations of voltage, current, resistance and power in low-voltage DC circuits:

Voltage (V)	Current (A)	Resistance (Ω)	Power (W)	Example
5 V	0.02 A	250 Ω	0.1 W	Small indicator LED with series resistor
12 V	0.5 A	24 Ω	6 W	Automotive or panel lamp
24 V	0.02 A	1.2 kΩ	0.48 W	PLC digital input or small relay coil
24 V	2 A	12 Ω	48 W	DC motor or power load

How to use this Ohm’s law calculator

Choose which values you know: For example, a 24 V supply and a 1 kΩ resistor, or a 12 V supply and a 2 A load.
Enter any two known quantities: Fill in two fields (V, I, R or P). The calculator will detect which ones are given.
Review the results: The remaining values are computed automatically using the appropriate formula set.
Apply to your design: Use the results to select resistor values, check current draw, or verify that power ratings are safely within component limits.

Practical design tips

Check resistor power ratings: After calculating current and resistance, always compute P = I² × R and choose a resistor with a sufficient wattage rating (typically at least 2× margin).
Watch cable and trace heating: For wiring and PCB traces, use the calculated current to verify ampacity and trace width, rather than only checking voltage.
Remember supply limits: If the calculated load current exceeds the power supply rating, you need a higher-rated supply or a different load design.
Estimate inrush and fault conditions: Ohm’s law also helps approximate short-circuit currents when you consider the total loop resistance, including source impedance.

Ohm’s law is the basic tool behind almost every electrical calculation. This calculator turns it into a quick, reliable design assistant for industrial control panels, embedded systems and general electronics.

FAQ about Ohm's Law Calculator

Can I use this Ohm’s law calculator for AC circuits?

Yes, for purely resistive AC loads (like heaters or incandescent lamps) you can still use Ohm’s law with RMS values of voltage and current.

However, when reactance is significant (inductors, capacitors, motors), you must use impedance instead of simple resistance and consider phase angle and power factor.

Why are there multiple power formulas (P = VI, P = I²R, P = V²/R)?

All three come from combining Ohm’s law with the basic power equation:

Start with <code>P = V × I</code>
Substitute <code>V = I × R</code> → <code>P = I² × R</code>
Or substitute <code>I = V / R</code> → <code>P = V² / R</code>

You can use whichever form matches the quantities you know.

What happens if my calculated current is higher than a component’s rating?

That indicates the component would be overloaded:

Resistors may overheat and fail open
Wires and traces may overheat or cause voltage drop
Devices (relays, LEDs, ICs) may exceed their maximum current ratings

You should increase resistance, reduce supply voltage, or choose components rated for higher current and power.