Ohm's Law Calculator

What the Ohm's law calculator does

Ohm's law is the single most important relationship in electronics, linking voltage, current, and resistance. This calculator applies it: enter the voltage and current, and it gives the resistance and the power. An advanced mode adds the material form of the law, finding resistivity from current density and electric field.

Below is what Ohm's law is, the equation behind it, how power follows from the same inputs, and a worked example.

How to use it

1. Enter the voltage across the component and the current through it, with their units.
2. For the material form, switch to advanced mode and enter the current density and electric field.
3. Press Calculate for the resistance and power, or Reset to clear them.

What Ohm's law is

Ohm's law describes how voltage, current, and resistance relate in an electrical circuit. Voltage is the electrical push that drives charge along; current is the rate at which charge flows; and resistance is how much the material opposes that flow. Ohm's law says these three are tied together in a simple, fixed way: for a given resistance, more voltage drives more current, and for a given voltage, more resistance allows less current. Named after Georg Ohm, who established it in the 1820s, it is the foundation on which nearly all circuit analysis is built.

The relationship is wonderfully intuitive once you picture it. Think of water in a pipe: voltage is like the water pressure, current is like the flow rate, and resistance is like the narrowness of the pipe. Raise the pressure and more water flows; narrow the pipe and less gets through. Ohm's law captures exactly this behaviour for electricity, and this calculator uses it to find whichever quantity you need from the others, starting with resistance from voltage and current.

The equation it uses

Ohm's law ties the three quantities together in one compact equation:

V = I × R

Here V is the voltage, I is the current, and R is the resistance. Rearranged to find resistance, it becomes R = V ÷ I, which is what the calculator computes from your inputs. The same equation can be solved for any of the three: voltage is current times resistance, current is voltage divided by resistance, and resistance is voltage divided by current. These three forms are the everyday workhorses of electronics, used constantly to size resistors, predict currents, and check circuit behaviour.

Getting power from the same inputs

Voltage and current give you more than just resistance; they also give you electrical power, the rate at which energy is being used or delivered. The calculator reports this alongside the resistance, because the same two inputs are all you need. Power is what determines how much heat a component must dissipate, how bright a bulb shines, or how quickly a battery drains, so it is just as important as resistance in practice.

Power is simply the voltage multiplied by the current. A component with a large voltage across it and a large current through it handles a lot of power, while one with either a small voltage or a small current handles little. This is why a component can be perfectly fine at low voltage but overheat at high voltage, even at the same resistance. By giving both resistance and power from one pair of inputs, the calculator lets you check not just how a component behaves electrically but whether it can handle the energy involved.

The material form of Ohm's law

Ohm's law has a deeper version that describes the material itself rather than a whole component. In advanced mode, the calculator works with current density, the current packed into each unit of cross-section, and electric field, the voltage drop spread over each unit of length. These local quantities are related by the material's resistivity, an intrinsic property that says how strongly the substance resists current regardless of its shape or size.

Resistivity is what makes copper a good conductor and rubber an insulator; it is a fixed property of each material. The familiar resistance of a component then depends on both the material's resistivity and its dimensions, a long thin wire resisting more than a short fat one of the same material. This microscopic view, relating electric field and current density through resistivity, is the form physicists use when studying conduction at the material level, and the advanced mode exposes it for those who want to go beyond the simple circuit picture.

Units and precision

The calculator takes voltage in units from millivolts to megavolts and current from microamperes to amperes, returning resistance in ohms and its multiples, and power in watts and its multiples. The simple mode applies Ohm's law and the power relation directly, which is the everyday use. The advanced mode works with current density and electric field for the material form. The relationships are exact; the results are as precise as your inputs.

A worked example

Suppose 12 volts is applied across a component, and a current of 2 amperes flows through it.

The resistance is R = V ÷ I = 12 ÷ 2 = 6 ohms. The power is the voltage times the current, P = 12 × 2 = 24 watts, so the component is dissipating 24 watts of heat, which tells you the kind of power rating it would need. From just the voltage and current, you have both how much the component resists and how much energy it is handling.

Questions people ask

What is Ohm's law?

The relationship V = I × R, linking voltage, current, and resistance. For a given resistance, more voltage drives more current; for a given voltage, more resistance allows less.

How do you find resistance from voltage and current?

Divide the voltage by the current: R = V ÷ I. A 12-volt drop with a 2-ampere current means a resistance of 6 ohms.

How do you get power from voltage and current?

Multiply them: P = V × I. This is the rate of energy use, which sets how much heat a component must handle, separate from its resistance.

What is resistivity?

An intrinsic property of a material saying how strongly it resists current, regardless of shape. It relates electric field to current density, the material form of Ohm's law.

References

A quick note on where the physics comes from. Ohm's law, electrical power, and resistivity are standard physics, set out in OpenStax's University Physics and in Georgia State University's HyperPhysics. The units follow NIST. The HyperPhysics link is worth a quick click to confirm it lands where you expect.

1. OpenStax, University Physics Volume 2, Section 9.4, Ohm's Law. https://openstax.org/books/university-physics-volume-2/pages/9-4-ohms-law
2. HyperPhysics, Ohm's Law. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html
3. National Institute of Standards and Technology (NIST), SP 811, Guide for the Use of the International System of Units. https://www.nist.gov/pml/special-publication-811