Should I tell my manager I’m freelancing?

It probably pays to let your boss know that you are freelancing for the sake of smooth working relations, if only to reassure them that it won’t have any negative impact on your performance or their business. To avoid misunderstandings and mishaps, you should:   

Check your contract. Some companies write non-competition clauses into their contracts to avoid competitors making use of your skills (that your employer is paying you for, and potentially helped you develop). If you are in breach of contract, you could face the sack, so you will need to weigh up the risks of any 'moonlighting' carefully. 

Be upfront about it. The professional worlds we work in are small places and getting smaller thanks to social media, as well as good old-fashioned office gossip. Save yourself the embarrassment of your manager finding out about your bit on the side from someone else. Set up a meeting to explain to them that they have nothing to worry about. You don’t have to ask their permission or go into details about the projects you are working on, but your manager should come away confident that: there is no conflict of interest (e.g. you’re not doing the same job for a direct competitor); it isn’t going to prevent you giving your day job your undivided attention; and your freelancing will happen out of hours and off the premises.