Medium or long cage derailleur?

[kiwifruit]

Hi all, I need to change my rear derailleur on my mountain bike. Is a Sram GX 2.1 10 speed, running on 30T front and 11-40T cassette. Bit confused, do I need a medium or long cage? Thanks

 

[Cycleops]

If your present derailleur doesn’t have the 40t capability you can buy a “Wolftooth” range extender which drops the derailleur away from the cassette, costs about £20 but non branded ones are around a fiver.

 

[Ajax Bay]

Just look at the specs of the possible replacement RDs: they will give the 'wrapping' capacity (in the OP's case 29t) and the largest sprocket size (in this case 40t). Increasing population of 1x drivetrains mean that the above requirement is pretty typical. It's not a question of medium v long.