Ohm's Law Calculator
Solve for voltage, current, resistance, or power — V = IR, P = VI
Frequently Asked Questions
Ohm's Law — voltage equals current times resistance (V = IR) — and the related power formula (P = VI) together relate four electrical quantities, and this calculator solves for whichever one is unknown given the other two or three, following the same multi-directional solve pattern as other physics ratio formulas on this site.
Why one relationship solves for four different variables
V = IR is the core relationship between voltage, current, and resistance — rearranged algebraically, current = voltage ÷ resistance, and resistance = voltage ÷ current, three expressions of the same underlying law. Power (P = VI) brings a fourth variable into the same system, and combined with Ohm's Law, power can also be expressed in terms of current and resistance, or voltage and resistance, giving several equivalent ways to calculate power depending on which two of the four variables (voltage, current, resistance, power) are actually known in a given circuit situation. This is standard practical circuit analysis — real electrical problems rarely hand you all four values directly, so being able to solve for any missing one from whichever two or three are actually measured or specified is the practically useful skill.
A worked example
A circuit with a known voltage source and a known resistor value can have its current calculated directly via Ohm's Law without ever needing to measure current directly — and once current is known, power dissipated by that resistor can be calculated from either the voltage-current or current-resistance form of the power formula, both giving the identical correct answer since they're mathematically equivalent expressions of the same relationship.
How this connects to your other technical calculations
For component-level circuit work specifically, the Resistor Color Code Calculator decodes a resistor's actual resistance value from its physical color bands — that resistance value is exactly the kind of input this Ohm's Law calculator uses to solve for current, voltage, or power in a circuit containing that resistor.
Common mistakes
Mixing up which of the several equivalent power formula forms to use for a given known-variable combination — using a formula that requires a variable that isn't actually known in the specific problem, rather than selecting the power formula form that matches the two variables actually available — is a common point of confusion; the correct approach is choosing whichever equivalent formula uses only the values already known, not defaulting to memorizing just one form.