Ohm's Law Calculator

How to use this Ohm’s law calculator

1. Choose the quantity you want to solve for: voltage (V), current (A), or resistance (Ω).
2. Enter the other two values with matching units so the Ohm's law equation stays consistent.
3. Leave the field for the unknown blank; that tells the calculator which variable to solve.
4. Press Solve to see the missing value and, if a graph is available, the corresponding V–I line.
5. Change the inputs to see how raising voltage or resistance changes current and power in a resistive circuit.

This Ohm's law calculator is most useful when you are working with one resistive path and need to switch quickly between volts, amps, and ohms. If the graph appears, it plots current along the horizontal axis and voltage along the vertical axis, so the slope still represents resistance and the highlighted point marks the operating point you entered.

Ohm’s law formula for voltage, current, and resistance

Ohm’s law ties voltage, current, and resistance together in a simple linear relationship for an ideal resistor. In this calculator, the equation is written as:

V = I × R

Where:

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

Rearranging the same Ohm’s law relationship lets you solve for the unknown quantity:

• I = V / R
• R = V / I

For browsers that support MathML, the same Ohm’s law relationships are shown below:

Because the current changes in direct proportion to voltage, a voltage-vs-current graph is a straight line. A larger resistance makes that line steeper, while a smaller resistance makes it flatter.

Interpreting the Ohm’s law results

When the calculator solves one missing value, the result tells you how that resistive circuit behaves at the chosen operating point. You can read each output like this:

• Calculated voltage (V): The supply or source voltage required to support the current through the stated resistance.
• Calculated current (A): The current that will flow when the stated voltage is applied across the resistance.
• Calculated resistance (Ω): The resistance implied by the voltage and current you entered.

Once voltage and current are known, power is easy to check as well:

P = V × I = I² × R = V² / R

That extra check is useful when you want to compare the result with resistor wattage, power-supply limits, or the heat a component may need to dissipate.

Worked example: finding current through a 200 Ω resistor

Suppose you are checking a 6 V battery or supply driving a single 200 Ω resistor and you want the current through it.

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 applies I = V / R:

I = 6 V / 200 Ω = 0.03 A

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

P = V × I = 6 V × 0.03 A = 0.18 W

Because the dissipation is below 0.25 W, a standard quarter-watt resistor would usually be comfortable in this example, assuming the part is otherwise appropriate for the circuit.

Typical Ohm’s law scenarios for resistive circuits

The table below shows a few resistive DC combinations and the current and power each one produces. It is a quick way to see how the same Ohm’s law equation behaves as voltage and resistance change.

These examples show the two most important patterns: if resistance stays the same, current rises with voltage; if voltage stays the same, current falls as resistance increases.

Ohm’s law assumptions and limitations

• Ideal resistive loads: This calculator treats the component as an ideal resistor, where Ohm’s law applies directly and the V–I line is perfectly straight.
• Constant resistance: Real parts can drift with temperature, aging, or applied power, so the result may differ from a measured value on a hot or heavily loaded component.
• DC or simple low-frequency AC: The tool is best for DC circuits or low-frequency cases where inductance and capacitance can be ignored. It does not model reactive or impedance effects.
• Non-ohmic devices: LEDs, diodes, lamps, and many semiconductors do not behave like fixed resistors. For those parts, the calculator can only give a rough first pass.
• Safety and ratings: Compare the calculated voltage and power with the component, wiring, and insulation ratings before building anything real. The calculator is not a substitute for electrical design or safety practice.
• Unit consistency: Enter values in volts (V), amperes (A), and ohms (Ω). For example, 10 mA must be written as 0.01 A, 1 kΩ as 1000 Ω, and 1 MΩ as 1,000,000 Ω.

Ohm’s law frequently asked questions

How many values do I need to enter?

Enter exactly two of the three quantities—voltage, current, and resistance—and leave the third blank. The calculator then applies Ohm’s law to solve the missing value.

Does this work for AC circuits?

It works as a good shortcut for simple resistive AC loads when voltage and current are in phase and entered as RMS values. It does not handle reactance, impedance, or power factor in more complex AC circuits.

How accurate are the results?

The calculation is exact for the ideal Ohm’s law model. In real hardware, component tolerance, temperature, and non-ideal behavior can shift the measured result.

Can I use this for LEDs or other semiconductors?

You can use it to estimate the current through a series resistor once you know the device’s approximate forward voltage. The device itself is not ohmic, so treat the result as an estimate and verify it against a datasheet.

What does the graph represent?

If the graph appears, it shows the straight-line voltage-current relationship for the resistance you entered. The line’s slope equals the resistance, and the highlighted point marks your operating voltage and current.