Resistance Calculator

1. Enter the voltage across the component in volts.
2. Enter the current through it in amperes.
3. Read the resistance, the power dissipated and the conductance, all updating as you type.

Each output describes the circuit:

Electrical resistance is a measure of how much a component opposes the flow of electric current. It is measured in ohms, and a higher resistance means less current flows for a given voltage. Every wire, resistor and device has some resistance, which is why circuits are designed around it.

Resistance links the two other core quantities: voltage, the push that drives current, and current, the flow of charge. Ohm's law ties them together, so knowing any two lets you find the third, which is the foundation of nearly all circuit analysis.

It applies Ohm's law, dividing the voltage by the current, and also works out the power from voltage times current.

Ohm's law states that R = V / I, where V is the voltage in volts and I the current in amperes. Power follows as P = V * I, and conductance is simply the reciprocal of resistance, G = 1 / R.

1. Divide the voltage by the current for the resistance.
2. Multiply voltage by current for the power.
3. Take the reciprocal of resistance for the conductance.

The current cannot be zero, since dividing by zero would imply an infinite resistance.

A 12 volt supply drives 2 amps through a component. The resistance is 12 / 2 = 6 ohms, and the power is 12 * 2 = 24 watts, so it turns 24 joules into heat each second.

A small signal part might see 5 volts and only 0.02 amps, giving 5 / 0.02 = 250 ohms and just 0.1 watts. The higher resistance and tiny current keep the power low, which is typical of electronics rather than power circuits.

A mains appliance at 230 volts drawing 10 amps has a resistance of 23 ohms and dissipates 2,300 watts, showing how the same law spans tiny and large scales.

These three quantities are the core of any circuit, and Ohm's law connects them.

A useful analogy is water in a pipe: voltage is the pressure, current is the flow, and resistance is how narrow the pipe is. Rearranging Ohm's law also gives voltage as current times resistance.

The value you calculate reflects the component, but several physical factors set a conductor's resistance.

Material

Copper and silver conduct well and have low resistance, while nichrome and carbon resist more, which is why they are used in heaters and resistors.

Length and Thickness

A longer wire has more resistance, and a thicker one has less, in the same way a longer or narrower pipe restricts flow.

Temperature

For most metals, resistance rises as temperature increases, so a hot wire resists more than a cold one.

The Component Itself

Resistors are made to a fixed value, while other parts change their resistance with light, heat or voltage.

Learning Circuit Basics

See Ohm's law in action and build intuition for how voltage, current and resistance relate.

Designing Simple Circuits

Pick a resistor to set a target current, or check the power a component must handle.

Troubleshooting

Measure voltage and current and back out the resistance to spot a faulty or unexpected component.

1. Mixing Up the Units

Use volts and amperes. Milliamps must be converted to amps first, or the resistance is off by a factor of a thousand.

2. Dividing by Zero Current

Zero current implies infinite resistance, which is not a real value to compute.

3. Forgetting Power Ratings

A resistor must handle the power it dissipates; ignoring watts can burn out a component.

4. Assuming Resistance Is Constant

Many components change resistance with temperature or voltage, so Ohm's law is an idealisation.

5. Confusing Series and Parallel

This finds a single resistance. Combining resistors needs the series or parallel rules.

The math is exact for Ohm's law, but that law is an idealisation of real components.

Divide the voltage across a component by the current through it: R = V / I. For 12 volts and 2 amps, the resistance is 6 ohms. This calculator does it for you.

Ohm's law states that voltage equals current times resistance, or V = I x R. Rearranged, R = V / I, which is how resistance is found from voltage and current.

The ohm, written with the Greek letter omega. One ohm is the resistance that lets one ampere flow under one volt.

Multiply them: power in watts equals voltage times current. A 12 volt, 2 amp component dissipates 24 watts.

Because resistance is voltage divided by current, and dividing by zero is undefined. Zero current would imply infinite resistance.

Conductance is the reciprocal of resistance, measured in siemens. It shows how easily current flows, so a high conductance means a low resistance.

It applies to steady direct current. For alternating current you use impedance, which combines resistance with reactance from capacitors and inductors.

This calculator finds a single resistance. Resistors in series add up, while resistors in parallel combine through the reciprocal rule.

No. The calculation runs in your browser, and nothing you enter is sent anywhere unless you Save a result, which stays on this device only.

• Ohm's law (Wikipedia).
• Electrical resistance and conductance (Wikipedia).
• Basal metabolic rate (Wikipedia, for the units convention).