MasterTool

LED Resistor Calculator

Find the series resistor for an LED from supply voltage, LED colour and target current, rounded to a real E12 value with a safe wattage rating.

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What this tool does

Find the series resistor for an LED from supply voltage, LED colour and target current, rounded to a real E12 value with a safe wattage rating. Forward voltages are published typical values by colour; a specific LED's datasheet value at your operating current takes precedence. Current above ~20 mA needs a driver, not a bigger resistor calculation.

How to use the LED Resistor Calculator

  1. Enter or select led colour (typical forward voltage).
  2. Enter or select custom forward voltage vf (v).
  3. Enter or select supply voltage (v).
  4. Enter or select target led current (ma).
  5. Read the calculated result; change any measurement to compare alternatives.

Formula

R = (Vs − Vf) ÷ If; standard value = nearest E12; resistor power = (Vs − Vf) × If; recommended rating = first standard rating ≥ 2 × power
colour
LED colour (typical forward voltage)
vfCustom
Custom forward voltage Vf (V)
voltage
Supply voltage (V)
current
Target LED current (mA)

Forward voltages are published typical values by colour; a specific LED's datasheet value at your operating current takes precedence. Current above ~20 mA needs a driver, not a bigger resistor calculation.

Worked example

For led resistor calculator, the following measurements illustrate the exact method: LED colour (typical forward voltage): red; Custom forward voltage Vf (V): 2; Supply voltage (V): 12; Target LED current (mA): 20.

Inputs

  • LED colour (typical forward voltage)Red — Vf ≈ 2.0 V
  • Custom forward voltage Vf (V)2
  • Supply voltage (V)12
  • Target LED current (mA)20

Result

  • Standard E12 resistor (ohms)470
  • Exact required resistance (ohms)500
  • Resistor power dissipation (W)0.2
  • Recommended minimum wattage rating (W)0.5
  • Actual LED current with standard resistor (mA)21.28

Results explained

Standard E12 resistor (ohms)
Standard E12 resistor (ohms) from the formula above. Forward voltages are published typical values by colour; a specific LED's datasheet value at your operating current takes precedence. Current above ~20 mA needs a driver, not a bigger resistor calculation.
Exact required resistance (ohms)
Exact required resistance (ohms) from the formula above. Forward voltages are published typical values by colour; a specific LED's datasheet value at your operating current takes precedence. Current above ~20 mA needs a driver, not a bigger resistor calculation.
Resistor power dissipation (W)
Resistor power dissipation (W) from the formula above. Forward voltages are published typical values by colour; a specific LED's datasheet value at your operating current takes precedence. Current above ~20 mA needs a driver, not a bigger resistor calculation.
Recommended minimum wattage rating (W)
Recommended minimum wattage rating (W) from the formula above. Forward voltages are published typical values by colour; a specific LED's datasheet value at your operating current takes precedence. Current above ~20 mA needs a driver, not a bigger resistor calculation.
Actual LED current with standard resistor (mA)
Actual LED current with standard resistor (mA) from the formula above. Forward voltages are published typical values by colour; a specific LED's datasheet value at your operating current takes precedence. Current above ~20 mA needs a driver, not a bigger resistor calculation.

Frequently asked questions

Divide the voltage left after the LED's forward drop by the target current: R = (supply − Vf) ÷ If. A 12 V supply, red LED (Vf ≈ 2 V) at 20 mA needs (12 − 2) ÷ 0.02 = 500 Ω, and the nearest E12 value is 470 Ω.

Published typical values used here: infrared ≈1.2 V, red ≈2.0 V, orange ≈2.1 V, yellow and standard green ≈2.2 V, blue and white ≈3.2 V, ultraviolet ≈3.6 V. High-brightness green is usually ≈3.2 V. Always check your LED's datasheet — Vf varies by part and current.

Resistors are sold in the E12 series (10% steps: …470 Ω, 560 Ω…). The tool picks the E12 value nearest to the exact resistance on a logarithmic scale, the way the series is spaced, and then shows the actual LED current that value will give.

The resistor dissipates (supply − Vf) × current as heat. Standard practice is to derate by at least 2×, so this tool recommends the first common rating (⅛, ¼, ½, 1, 2 W…) that is at least twice the calculated dissipation.

An LED connected directly across a supply draws current limited only by the supply and its own tiny dynamic resistance, which usually destroys it quickly. A series current-limiting resistor (or a constant-current driver) is essential.