What resistor for an unknown LED?

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

I use this tool:

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

It has info on what stats common colours carry.

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.

Sounds about right.

Could always just measure the led resistance with a multimeter

Quote:

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Originally Posted by typhoon_driver

Could always just measure the led resistance with a multimeter

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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?