I recently had a 100 amp subpanel installed in a basement. The electrician ran aluminum wire from a 100 amp breaker to the subpanel. The subpanel has room for 6 breakers; 4 are installed (20 amp each). I know that aluminum wire carries only 61% of conductivity that same size copper carries. How can I compare the size of the aluminum wire that was installed to a copper wire?

All SER wire is Aluminum now. What does the wire say? Depending on AWG, Aluminum can is only 30% under and that is why the AWG number is lower(more ampacity). The wire should be at least a #1. For long distances, copper wire cost is prohibitive.

Brian,
Did you mean to say 1/0?

I recently had a 100 amp subpanel installed in a basement. The electrician ran aluminum wire from a 100 amp breaker to the subpanel. The subpanel has room for 6 breakers; 4 are installed (20 amp each). I know that aluminum wire carries only 61% of conductivity that same size copper carries. How can I compare the size of the aluminum wire that was installed to a copper wire?Why do you need to make this comparison? Do you not trust the electrician who did the installation?

I will say, in my area #2AL SER is typical. Those who say this is not a legal installation can save it. It's what's done around here, and I agree with it. Simple as that.
If #2AL is appropriate for a 100A service it's fine for a 100A sub-panel.

Some electrical supply houses sell a gauge which actually matches up to the wire diameter as a reference. But as far as your requirements and depending on the wire covering, #1 is fine for a 100 amp subpanel. Can you read anything at all on the wire?
Also, there are several charts available which tell you the diameter of any given wire size... you can try that, if you can do it "safely" with the power to the panel turned off.