What resistor for an unknown LED?

[limpet]

I want to run an led on 18v for power indication but I don't know the led specs.
Can anyone give me a rough idea of the resistance needed?
Thanks

[PJ-27]

I use this tool:

http://led.linear1.org/1led.wiz

It has info on what stats common colours carry.

[limpet]

Thanks, I put some average specs in and it said 820ohm, so I've got 4x 200ohm that I had lying around and it seems to be ok.

[PJ-27]

Sounds about right.

[typhoon_driver]

Could always just measure the led resistance with a multimeter

[limpet]

Could always just measure the led resistance with a multimeter

I can do that easy enough and it would make it easier for unknown LEDs, how can I work out the required resistor from that though is there a formula?