One official way to estimate the strength of a user selected password such as "Tr0ub4dor&3" is to look at NIST recommendations. Granted that this is now deprecated, but the relevant publication was NIST Special Publication 800-63 Version 1.0.2, Electronic Authentication Guideline.
Table A.1 (reproduced below in case of link rot):-
The reasoning behind this table is within the document at $\S$ A.2.1 Guessing Entropy Estimate. NIST therefor estimates that the entropy is 33 bits if we interpolate for 11 characters and use dictionary and composition rules.
The difficulty of assessing the entropy of short sequences, particularly human produced ones is the take away from this question. The two current answers diverge in strength by a factor of 32. If we compare NIST's estimate to Blafasel's original query on 50 bits, the entropy diverges 131,072 times. NIST says of the above, "Readers are cautioned against
interpreting the following rules as anything more than a very rough rule of thumb method". True.
Another take away is that very few sites will allow the stronger and easier to remember technique of choice from a word list, such as "correcthorsebatterystaple". The on-line version of the UK government doesn't, no bank I'm aware of does, and stackexchange.com doesn't.