Ohm's Law Calculator: Voltage, Current and Resistance

What does this quantity describe?

Ohm's law describes a linear relationship between the voltage U across a component and the current I through it: U = R·I. The proportionality factor R, the electrical resistance, is a material constant for ohmic components at constant temperature and does not itself depend on U or I.

A simplified but useful picture is a water circuit: voltage corresponds to a pressure difference, current to the flow rate, and resistance to how narrow a pipe is. A narrower pipe (higher resistance) allows less flow at the same pressure — just as a higher ohmic resistance reduces current at the same voltage. The analogy explains the proportionality but does not replace circuit analysis involving capacitors or inductors.

Formula and variables

U = R · I

• U = R · I
• I = U / R
• R = U / I
• P = U · I
• P = R · I²
• P = U² / R

Choose the inputs correctly

Enter two of the three electrical quantities U, I or R; the calculator finds the third and also derives power P. Make sure all values belong to the same operating point — for nonlinear components R itself changes with U or I.

How to use the calculator

Select the target quantity. Enter the two known values with matching units (V, A or Ω). The calculator automatically also computes power P. For AC loads, check whether the load is truly ohmic (purely resistive), since otherwise reactive power and phase shift must be considered separately.

Worked example

Given U = 12 V and I = 2 A, resistance follows as R = U/I = 12 V / 2 A = 6 Ω. Power follows from P = U·I = 12 V · 2 A = 24 W. As a check, P = I²R = 2² · 6 = 24 W and P = U²/R = 12²/6 = 24 W agree.

Understand the result and units

A resistance of 6 Ω means that 12 V drives exactly 2 A through the load; doubling the voltage to 24 V doubles the current to 4 A for the same R. The 24 W power figure shows the energy converted to heat each second and determines the required cooling or rated power of the component.

Voltage in volts (V), current in amperes (A) and resistance in ohms (Ω) are coherent SI units, so U = R·I needs no conversion factor. In practice mV, kΩ and MΩ also appear; a 4.7 kΩ resistor is 4,700 Ω.

Which components do not follow Ohm's law?

Semiconductor components such as diodes, transistors and thermistors, as well as vacuum tubes, show a nonlinear relationship between voltage and current; their differential resistance changes with the operating point. A metal conductor heating up significantly also increases its resistance, so R no longer stays constant during operation. In these cases U = R·I only gives a snapshot at one operating point, not a general-purpose equation.

Typical applications

Ohm's law is used to size LED series resistors, calculate heating elements, dimension fuses and cable cross-sections, and troubleshoot simple DC circuits.

Assumptions, limits and common mistakes

The linear relationship holds only for ohmic (linear) components at constant temperature, such as many metal resistors under normal operation. Semiconductor components like diodes, transistors or thermistors, as well as conductors at strongly varying temperature, show a nonlinear U-I behaviour where R itself depends on U or I. Superconductors below their critical temperature do not follow the law either, since their resistance becomes zero.

Common mistake: Do not apply Ohm's law uncritically to semiconductor components, since R is not constant there. For AC, do not equate R with impedance Z once coils or capacitors are involved. Prefixes like kΩ or mA must be converted to SI units before substitution.