Ohm's Law

Solving for Whichever of Voltage, Current, or Resistance You Don’t Know

Ohm’s Law describes the fundamental relationship between voltage, current, and resistance in any electrical circuit: voltage equals current multiplied by resistance. Enter any two of the three, and this calculator finds the third, along with the power the circuit is using.

This is the general-purpose electrical relationship, distinct from the

Voltage Drop Calculator, which answers a more specific question — how much voltage is lost over a run of wire, given its gauge, length, and material.

The Formula

• Voltage: $V = I \times R$
• Current: $I = V \div R$
• Resistance: $R = V \div I$
• Power (always computed): $P = V \times I$

Like the Circle Calculator, none of these three variables has a single fixed “input” role — whichever two you know solve for the third — so they’re left uncolored rather than assigning one a fixed color it wouldn’t consistently play across every direction.

Worked Example

A circuit with 2 amps of current flowing through a 6 ohm resistor:

1. Voltage: $2 \times 6 = 12$ V.
2. Power: $12 \times 2 = 24$ W.

All three solve-for modes agree: entering 12V and 6Ω finds 2A; entering 12V and 2A finds 6Ω — each direction gives the same consistent circuit.

Key Factors to Consider

• Ohm’s Law strictly applies to “ohmic” components — ones whose resistance stays constant regardless of voltage or current. Simple resistors are ohmic, but many real-world components (diodes, transistors, light bulb filaments as they heat up) have a resistance that changes with conditions — Ohm’s Law is still a foundational relationship, but it’s an approximation, not an exact description, for these nonlinear components.
• Power can be expressed three equivalent ways once any two of voltage, current, and resistance are known. P = V×I is the direct formula, but substituting Ohm’s Law gives two more forms: P = I²R and P = V²/R — all three describe the exact same power, just expressed using whichever two variables happen to be known in a given situation.
• This calculator models a single, simple resistive element — not a full circuit with multiple components. Real circuits often combine resistors in series or parallel, which changes the total resistance the circuit presents — see the Resistors in Series/Parallel Calculator for that specific combination math.
• Resistance itself depends on a conductor’s material, length, and cross-sectional area. A longer or thinner wire has more resistance than a shorter or thicker one of the same material — this is exactly the relationship the Voltage Drop Calculator models for a real wire run, building on the same underlying physics as Ohm’s Law.

Common Mistakes

• Mixing up which two values are actually known. This calculator solves for whichever quantity is missing — entering a guessed value for the third field instead of leaving it to be solved produces a nonsensical result, since all three are related by the same fixed equation.
• Assuming a component’s resistance stays constant when it may not. As the FAQ above notes, Ohm’s Law is exact for ohmic components like simple resistors but only approximate for nonlinear ones — a real reading can differ from a calculation based on a stated resistance value.
• Forgetting units when combining component specs from a datasheet. Resistance, current, and voltage all need to be in consistent base units (ohms, amps, volts) before applying the formula — a value given in milliamps or kilohms needs converting first.
• Ignoring power dissipation limits. Ohm’s Law alone doesn’t say whether a component can safely handle the power it calculates — always check a resistor’s or wire’s power/current rating separately before relying on a real circuit built to these numbers.

Useful to Know

• Combining multiple resistors in one circuit? Resistors in Series and Parallel Calculator extends this same math to series and parallel combinations.
• Need to identify a resistor’s rated value from its color bands? Resistor Color Code Calculator decodes that separately from the electrical relationship this calculator solves.
• Curious what a circuit’s power draw costs over time? Electricity Cost Calculator takes the wattage this calculator computes and turns it into a monthly bill estimate.