I've used the calculators many times for 555 Astable circuits often with good results.
This morning I ran into a 40Hz problem where all calculators agree on the 3 components, including even a hand calculation and I have to use a capacitor that is way different from what the calculations say.
To get about 42 Hz, I have in the circuit R1 6.8K, R2 22K C 4.7uf
All calculations say this give a frequency of 6Hz
Holding the resistors constant, to get 42 Hz, the calculators say C should be .68 uf.
Almost 1 order of magnitude difference.
What am I not understanding?