How do I calculate what resistor I need for an LED?

Use R = (Source Voltage - LED Forward Voltage) / Desired Current (Amps). For example, connecting a 2V red LED to a 9V battery at 20mA (0.02A) requires R = (9 - 2) / 0.02 = 350 Ohms.

Science & Engineering• Essential

Ohm's Law & Electrical Power Calculator

Enter any two known electrical values among Voltage, Current, Resistance, or Power to instantly solve for the remaining two. Complete with formula derivations, unit prefixes (mA, kΩ, MW), and circuit explanation.

Computed locally using client-side JavaScript math.

Ohm's Law Calculator

Physical science, electrical circuits, acoustics, and atomic data exploration.

Circuit Parameters

Edit any parameter to live-recalculate the rest using Ohm's Law (V = I × R) and Joule's Law (P = V × I).

Voltage (V) — Volts

Current (I) — Amperes

Resistance (R) — Ohms (Ω)

Power (P) — Watts (W)

Live Circuit Metrics

Voltage (V)

12 V

Current (I)

2 A

Resistance (R)

6 Ω

Power (P)

24 W

• V = I × R = 2 A × 6 Ω = 12 V

• P = V × I = 12 V × 2 A = 24 W

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How to Use Ohm's Law Calculator

Input Any Two Values

Provide any 2 known values: Voltage (V), Current (I), Resistance (R), or Power (P).

Select Unit Scale

Use micro (µ), milli (m), kilo (k), or mega (M) scale prefixes if needed.

View Solutions

Get the remaining 2 variables solved instantly along with step-by-step equations.

About Ohm's Law Calculator

Ohm's Law states that current through a conductor between two points is directly proportional to voltage across the two points and inversely proportional to resistance. Combined with Joule's first law (P = VI), any two known parameters fully resolve the four fundamental circuit characteristics.