Use this Ohm’s Law Resistance Calculator to find electrical resistance from voltage and current using the standard formula R = V / I. It also shows power and conductance, making it useful for circuit homework, electronics checks, and quick conversions when you want results in ohms, kilo-ohms, or mega-ohms.

Reviewed by: AjaxCalculators Editorial Team
Last updated: April 19, 2026
Method source: Standard Ohm’s law resistance calculation using voltage divided by current, plus conductance as the reciprocal of resistance
Editorial standards: AjaxCalculators Editorial Policy

What This Ohm’s Law Resistance Calculator Calculates

This calculator finds electrical resistance from:

• Voltage (V)
• Current (I)

It can also show:

• Power (P)
• Conductance (G = 1/R)

The tool supports common unit options for voltage, current, and resistance output, so you can work in volts, millivolts, kilovolts, amps, milliamps, microamps, ohms, kilo-ohms, or mega-ohms.

How the Ohm’s Law Resistance Calculator Works

The standard formula is:

R = V / I

• R = resistance
• V = voltage
• I = current

This means resistance increases when voltage increases for the same current, and it decreases when current increases for the same voltage.

The calculator can also show conductance:

G = 1 / R

Conductance is measured in siemens (S), which is the reciprocal of the ohm.

If power is shown, the standard electrical relationships are:

• P = VI
• P = I²R
• P = V² / R

Assumptions and Important Notes

• This calculator assumes a simple Ohmic relationship between voltage, current, and resistance.
• Current must be non-zero.
• Ohm’s law is very useful for ideal resistors and many circuit problems, but not for every electrical component.
• If signed voltage and current are allowed, a negative result can appear mathematically. That usually reflects a sign convention or active-circuit case, not a normal passive resistor value.
• Conductance is the reciprocal of resistance, so high conductance means low resistance.
• If power is shown as signed, interpret it according to circuit sign convention. Resistor heat dissipation is normally treated as nonnegative.

Worked Example

Suppose a circuit has:

• Voltage: 12 V
• Current: 3 A

Step 1: Apply Ohm’s law
R = V / I

Step 2: Substitute the values
R = 12 / 3 = 4 Ω

Step 3: Find conductance
G = 1 / 4 = 0.25 S

Step 4: Find power
P = VI = 12 × 3 = 36 W

So a circuit with 12 volts and 3 amps has a resistance of 4 ohms, a conductance of 0.25 siemens, and a power value of 36 watts.

How to Use This Ohm’s Law Resistance Calculator

1. Enter the voltage value.
2. Select the voltage unit.
3. Enter the current value.
4. Select the current unit.
5. Choose the resistance output unit you want.
6. Review the resistance, power, and conductance results.

How to Interpret the Result

The resistance result tells you how strongly the circuit element opposes current flow for the voltage and current you entered.

A larger resistance means less current would flow for the same voltage. A smaller resistance means current flows more easily.

The conductance result gives the inverse view of the same relationship: higher conductance means easier current flow.

If the calculator shows power, that tells you the rate of electrical energy transfer. In a passive resistor, that is usually interpreted as dissipated power.

Practical Uses of an Ohm’s Law Resistance Calculator

• Solving basic circuit homework
• Estimating resistor value from measured voltage and current
• Checking whether a circuit measurement is consistent with Ohm’s law
• Understanding the link between resistance and conductance
• Converting between Ω, kΩ, and MΩ quickly

References

1. OpenStax College Physics 2e – Ohm’s Law, Resistance, and Simple Circuits
2. OpenStax University Physics Volume 2 – Ohm’s Law
3. OpenStax University Physics Volume 2 – Electrical Energy and Power
4. NIST – Conductance in siemens as reciprocal of resistance in ohms

Related Calculators

• Ohm’s Law Current Calculator
• Electricity Cost Calculator
• Work and Power Calculator
• Ideal Gas Law Calculator

Note: This calculator is for educational and general calculation use. It applies Ohm’s law to simple resistor-style problems and does not replace full circuit analysis for AC impedance, reactive components, nonlinear devices, or active negative-resistance circuits.