Engineering calculator

Ohm's Law Calculator, Voltage, Current, Resistance & Power

Calculate any unknown in Ohm's Law, voltage (V), current (I), resistance (R), and power (W). Enter any two values to find the rest.

Published by · Reviewed against primary-source formulas

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Enter exactly 2 values. The third will be calculated automatically.

Voltage

–

V = I × R

Current

–

I = V / R

Resistance

–

R = V / I

Power

–

P = V × I

How CalcMesh applies Ohm’s law

Ohm’s law, first published by Georg Ohm in 1827, states that voltage equals current times resistance (V = I × R). From any two of the three quantities we solve for the third using this exact relationship.

We also derive power (P = V × I) so you can size components correctly; we assume a purely resistive, linear circuit, an assumption spelled out in our methodology.

Understanding Ohm's Law

The Basic Formula

Ohm's Law defines the relationship between three fundamental electrical quantities:

  • V = I × R - Voltage equals current times resistance
  • I = V / R - Current equals voltage divided by resistance
  • R = V / I - Resistance equals voltage divided by current

Power can also be derived: P = V × I = I²R = V²/R

Series vs Parallel Circuits

Series circuits: Components are connected end-to-end. The total resistance is the sum of all individual resistances (R_total = R1 + R2 + R3). The same current flows through each component, but voltage divides across them.

Parallel circuits: Components share the same two connection points. The total resistance is calculated as 1/R_total = 1/R1 + 1/R2 + 1/R3. Each branch gets the full voltage, but total current divides among the branches.

Practical Applications

  • LED circuits: Calculate the correct resistor value to limit current through an LED
  • Fuse sizing: Determine the expected current draw to select the right fuse
  • Wire sizing: Know the current to choose appropriate wire gauge
  • Battery life: Calculate how long a battery will last given the circuit's current draw

Note: This calculator assumes ideal resistive loads. For AC circuits with reactive components, use impedance calculations instead.

Methodology & Assumptions

This calculator implements standard formulas drawn from primary-source authorities. Values are point-in-time estimates; consult a licensed professional for high-stakes decisions. See the per-input definitions and source citations below.

How this works

Computations are deterministic and run client-side, no inputs leave your browser. Formulas are derived from standard published formulas for the calculator's domain (mortgage, taxes, energy, conversions, etc.). When the underlying agency publishes updated rates or thresholds we refresh defaults and update the page's lastmod timestamp.

Frequently Asked Questions

What is Ohm's Law?
Ohm's Law is a fundamental principle in electrical engineering that describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. It states that the voltage across a conductor is directly proportional to the current flowing through it, with resistance as the constant of proportionality. The formula is V = I × R.
How do I use this calculator?
Enter any two of the three values, voltage, current, or resistance, and the calculator will determine the missing value along with power. Leave the field you want to calculate empty or set to zero. For example, enter voltage and resistance to find the current, or enter current and resistance to find the voltage.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies directly to DC circuits and to AC circuits with purely resistive loads. For AC circuits with capacitors or inductors, impedance (Z) replaces resistance in the formula, and you must account for phase angles. The generalized form becomes V = I × Z, where Z is a complex number representing both resistance and reactance.
What is the difference between resistance and impedance?
Resistance (measured in ohms) opposes current flow in DC circuits and dissipates energy as heat. Impedance is the AC equivalent and includes both resistance and reactance (from capacitors and inductors). While resistance is a single real number, impedance is a complex number with both magnitude and phase angle, accounting for how components store and release energy.

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This page identifies the inputs, method, and limitations behind its estimates. Calculator outputs are not professional advice and should be checked against the relevant primary source for a consequential decision. This calculator's formula and defaults are drawn from standard published sources for its domain, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.

Inputs, defaults, and authoritative sources
Input Default Source / authority
All inputs Domain-typical defaults Editorial methodology, CalcMesh 2026