Ohm's law
V = IR
Voltage equals current times resistance for an ideal linear resistor.
Ohm’s Law Calculator
Solve for voltage, current, or resistance in an ideal resistive element and calculate power and conductance.
Access: Free to use, no installation, and No account required.
Inputs
This is an ideal DC resistance calculation. Temperature coefficients, source impedance, reactive effects, component tolerances, and nonlinear loads are not included.
Results
Ohm’s law
For an ideal resistor, voltage, current, and resistance are related by V = IR. Electrical power follows P = VI, and conductance is the reciprocal of resistance.
Engineering reference
Ohm’s Law Calculator: background and worked detail
Use the Ohm's Law Calculator to solve an ideal resistive relationship for voltage, current, or resistance and to calculate electrical power and conductance from the resulting state.
V, I, R, and P
Electrical power
P = VI = I²R = V²/R
Equivalent power forms follow after substituting Ohm's law.
Conductance
G = 1/R
Conductance is the reciprocal of resistance and is measured in siemens.
12 V across 120 Ω
Set the calculator to solve current using 12 V and 120 Ω.
- I = 12/120 = 0.100 A.
- P = VI = 1.20 W.
- G = 1/120 ≈ 0.00833 S, or 8.33 mS.
Result: The same state satisfies all three standard Ohm's-law power forms.
Where the ideal-resistor model breaks down
Assumptions
- The element is linear and ohmic.
- Voltage and current represent compatible instantaneous DC or RMS resistive quantities.
- Resistance is treated as constant.
Limitations
- Does not model reactive impedance, semiconductor junctions, lamp temperature coefficients, or frequency dependence.
- A power result does not by itself determine a safe resistor wattage because thermal design and derating still matter.
Ohm’s Law Calculator FAQ
What values do I need for Ohm's law?
Any two of voltage, current, and resistance determine the third for an ideal resistor.
Can Ohm's law be used for AC?
For a purely resistive AC load using RMS voltage and current, yes. General AC circuits require complex impedance rather than resistance alone.
Why is resistor power important?
Electrical power becomes heat in a resistor. A real design needs a resistor power rating with appropriate temperature and reliability margin.
Engineering theory and interpretation
Ohm's law describes a linear resistive element with V = IR. Once any two of voltage, current, and resistance are known, the third follows algebraically; electrical power can then be written as VI, I²R, or V²/R.
Real components can depart from an ideal constant resistance because of temperature, frequency, self-heating, tolerance, and nonlinear voltage-current behavior.
Validation checks
Unit resistance
Case: Set R = 1 Ω and I = 1 A while solving voltage.
Expected: The result should be 1 V and 1 W.
Power identity
Case: Calculate a nonzero resistive state.
Expected: VI, I²R, and V²/R should agree within numerical rounding.
Where this calculation comes from
The same resistive-network solver backs the Introductory Circuits Workbench, which adds multi-branch circuits and nodal analysis.
Open the source workbench →Read calculation and source methodology →