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DCC Decoder Function Resistor Calculator

Use this tool to calculate the value (ohms) and wattage (watts) of a dropping resistor for the small bulbs and LEDs commonly used for locomotive function lighting.  The DCC decoder output voltage can be modified; however, the default value of 12.5-volts is the standard decoder common (blue wire on decoder) output voltage and shouldn't need to be changed.

Note: If you select any bulb type other than "Generic incandescent bulb", then the recommended voltage and current rating is selected for you automatically.  All you need to do is hit the "Calculate" button for your result. 

If you choose to continue with a custom voltage and current selection, then once you have completed selecting the bulb type, then select the rated bulb voltage and current from the respective drop down controls.  Hit the "calculate" button determine the correct resistor value for you selections.   The next highest "standard" resistor value will be displayed in the "ohms" box, the actual calculated wattage will be displayed in "actual wattage" box and the standard wattage values that the resistors are manufactured to will be displayed in "standard wattage" text box. 

To make another selection, hit the "reset" button to restore the default values.


  Select your bulb type

Decoder common voltage [5.5 to 15.0] volts

bulb or LED voltage [1.5 to 12.0] volts

bulb or LED current rating [12 to 60] milliamps


actual wattage

standard wattage


Created 06/29/2007 by Scott Kurzawski
Updated 02/11/2020