Ohm's Law Calculator

Use this Ohm’s law calculator to find voltage, current, or resistance in a DC circuit. Enter any two of the three values (V, A, Ω) and leave the one you want to solve for blank. This is useful for electronics students, hobbyists, and technicians working with simple resistive circuits.

How to use the Ohm’s law calculator

1. Decide which quantity you want to calculate: voltage (V), current (A), or resistance (Ω).
2. Enter known values in the other two fields with consistent units (volts, amperes, ohms).
3. Leave the target field blank so the calculator can solve for it.
4. Run the calculation to see the missing value and, if available, a line showing the V–I relationship on the graph.
5. Adjust inputs to explore how changing voltage or resistance affects current and power in a simple circuit.

If a visual graph is shown with the calculator, it typically plots current on the horizontal axis and voltage on the vertical axis. The line’s slope represents the resistance, and the highlighted point shows your specific operating condition.

Ohm’s law formula

Ohm’s law describes the linear relationship between voltage, current, and resistance in an ideal resistor. The basic form is:

V = I × R

Where:

• V is voltage in volts (V)
• I is current in amperes (A)
• R is resistance in ohms (Ω)

You can rearrange this equation to solve for any variable:

• I = V / R
• R = V / I

The same relationships can be written in MathML for clarity:

Because the relationship is linear, plotting voltage against current gives a straight line. Higher resistance corresponds to a steeper slope, while lower resistance gives a shallower line.

Interpreting the calculator results

After you enter any two values and solve, the calculator returns the missing quantity. You can interpret the result as follows:

• Calculated voltage (V): The potential difference your supply must provide to push the chosen current through the given resistance.
• Calculated current (A): The flow of charge that will result from applying the given voltage across the given resistance.
• Calculated resistance (Ω): The effective resistance implied by the measured voltage and current.

Once you know voltage and current, you can also estimate power:

P = V × I = I² × R = V² / R

This helps you check whether components such as resistors or power supplies are operating within their ratings.

Worked example

Imagine a simple circuit where a 6 V battery is connected to a 200 Ω resistor. You want to know the current.

1. Leave the current field blank.
2. Enter 6 in the voltage field (V).
3. Enter 200 in the resistance field (Ω).
4. Run the calculation.

The calculator uses I = V / R:

I = 6 V / 200 Ω = 0.03 A

So the current is 0.03 A, or 30 mA. The power in the resistor is:

P = V × I = 6 V × 0.03 A = 0.18 W

A standard 0.25 W (1/4 W) resistor would be suitable here because the power is below its rating.

Comparison of typical scenarios

The table below shows how changing voltage and resistance affects current and power in purely resistive DC circuits.

You can reproduce these values by entering the voltage and resistance from any row and solving for current. Then, estimate power using the formulas above.

Assumptions and limitations

• Ideal resistive loads: The calculator assumes Ohm’s law applies exactly, which is true for ideal resistors and many metal-film or carbon resistors operated within their ratings.
• Constant resistance: Real components can change resistance with temperature, aging, or bias. High power or high temperature operation may make real behavior differ from these ideal calculations.
• DC or low-frequency AC: The tool is intended for DC circuits or low-frequency cases where inductance and capacitance are negligible. Reactive components like capacitors and inductors are not modeled.
• Non-ohmic devices: LEDs, diodes, filament lamps, and many semiconductor devices do not follow a simple linear V–I relationship. For them, this calculator only provides a rough approximation at best.
• Safety and ratings: Always compare computed power and voltage with component and insulation ratings. This calculator does not replace proper electrical design or safety practices, especially at mains or high-voltage levels.
• Unit consistency: Enter values in base units: volts (V), amperes (A), and ohms (Ω). For example, 10 mA = 0.01 A, 1 kΩ = 1000 Ω, and 1 MΩ = 1,000,000 Ω.

Frequently asked questions

How many values do I need to enter?

You must enter exactly two of the three values (voltage, current, resistance). Leave the third one blank so the calculator can solve for it using Ohm’s law.

Does this work for AC circuits?

It can approximate AC behavior for simple resistive loads where voltage and current are in phase and specified as RMS values. It does not account for reactance, impedance, or power factor in complex AC circuits.

How accurate are the results?

The math is exact for the ideal Ohm’s law model. Differences between calculated and real-world measurements usually come from component tolerances, temperature changes, and non-ideal behavior.

Can I use this for LEDs or other semiconductors?

You can use it to estimate current through a series resistor once you know the approximate forward voltage of the LED or device. However, the device itself is not ohmic, so results are only an approximation and you should check a datasheet.

What does the graph represent?

The graph, when present, typically shows a straight line of voltage versus current for the calculated resistance. The slope of the line equals the resistance, and a marked point shows your specific operating voltage and current.