Change from 2 chain rings to single chain ring on a MTB. What size should I take? My current drive train is pretty much used up and needs replacement. I thought with this occasion, I could replace my curret drive train (2 chain rings, 10 speed cassette) with a singe chain ring and 11 speed cassette.

Currently I've got the following parts:

Chainset
Brand: Race Face
Type: Turbine
Sprocket 1: 26 Teeth
Sprocket 2: 36 Teeth

Cassette
Brand: Shimano
Type: Deore XT / CS-M8000
Ration chainring: 11-40 Teeth


I would like to get a single chain ring so I can remove a shifter and also gain more space to the ground. It did happen to me quite a few times that the larger chain ring caught a rock or a tree.

What size of chain ring (teeth) would I need so I'd have about the same transmission as with the current setup (especially regarding climbing)?
Currently the gear ratio of your lowest gear is:

26/40 = 0.65

The Shimano XT cassette with the largest range is 11-46.
To get the same lowest gear with this cassette you would need an chain ring with 46*0.65 = 29.9 teeth

You would need a 11-46 cassette and an chain ring with 30 teeth to get the same lowest gear with an 1x setup.

But your highest gear will be lower.
At the moment the gear ratio of your highest gear is:

36/11 = 3.27

With the setup from above it would be:

30/11 = 2.73

This is approximately the same ration as the second largest gear you have at the moment with

36/13 = 2.77