I bought this stuff at Home Depot - all they carry at ours:
Kleen Strip Green Muriatic Acid.
There is no indication of strength on the bottle or the sds other than "<30%"
Many of the Q&A answers say 20% (and go on to reference TFP and pool math - awesome!).
Is there any way to find out for sure, and if 20% is the number I'll give ya' a nickel if you add that to pool math! (That's a lie - but there is a "thanks" in it for you!)