How to set radius about 100km by buffer? (QGIS 3.0.1)

Question

I am trying to create a buffer with radius about 100km (from the previous version of QGIS 2.12 it should be equal buffer distance 150000 with EPSG3857 (Pseudo Mercator) - fixed distance buffer. I tried to use the same for buffer in the latest version of QGIS but its creating one big buffer around all the way or buffers which are less then 2km in radius. Could you please give me an advice what number should I set up please and in which format? The other way which I am trying is used to fixed distance buffer as well but when start to run the analyse the the following error apears Error:

WKT: unknown projection (Mercator_Auxiliary_Sphere)

Answer 1

First advice: Buffer only using UTM-projections, since these are based on metric distances. Second advice: Resave your layer to UTM, buffer it, then - if truly necessary - resave again to your original CRS.

As to which UTM-strip you should use: Consider where your area of interest is, and then simply google it.