How to decide if you need a manager (as an actor)

I went to the United States earlier this year, and I went with very little understanding of what a manager actually is, and I must admit I was a little naive just thinking they were the same thing with a different name, they are not! Therefore I will share with you what I learnt. And why I have a manager AND an agent.

Firstly I would recommend you understand exactly what an agent does, if you are currently working in the UK yet want to work in the States also you must must must know the difference and know what a manager does and what an agent does, they like it when you know what your talking about :) .

An agent here in the UK, will represent you, represent you as a product, and will sell you and your qualities for each role they feel is suitable, they manage your invoices, negotiate each role on your behalf , that is why it is essential to get to know your agent, make sure your agent knows the direction you want to go with your career.

A agent in the US will do most of the above, yet will diary manage, will liaise with you about what roles they feel you are suitable for and will get you the best money possible, an agent also negotiates money.
Your agent in the US will work extremely closely with your manager, your US manager is NOT able to submit you for jobs, that is only allowed by your agent (they have laws which restrict them in the US) . So hence the reason the two work closely together. Your manager will build up your profile, will organise your diary with your agent and will be your best friend and companion to screenings/premiers etc. Your manager is the one who will give you career advice, organise classes for you to attend and make sure the press know who you are, the manager will advise you on how to dress and behave. The manager will organise the best accommodation for you, your dietary requirements on set, your insurances, health care, travel and most importantly they will be your visa sponsor and work with the lawyers on your behalf, they tell the press where you are and what your doing so they can take quick snaps, they organise dinners and charity functions etc So basically, they organise your life.
Therefore unless you have an agent there really isn't any need to have a manager. A manager will take around 10-15% and the same goes for an agent.

In the UK, a manager (as far as im aware) is the same as an agent, however I do not know of any 'managers' in the UK, I would research the company thoroughly before going with a 'manager'.

If you are not planning on working anywhere except the UK, I really don't think you need a manager. If you are just starting out in the US or the UK with no to minimal credits again I don't think you need a manager.
If you are working solidly in Tv and Film in the UK and worldwide, I would start researching into a manager, they will build your profile, which in turn will get your more work and more meetings, speak to your current agent and ask which managers they can recommend and if they feel you are ready for one. Most of the bigger UK agents have managers they work closely with in the United States, so I would advise always speaking with your agent BEFORE you approach any managers.

I hope this has helped and good luck