Are you trying to calculate a requirement or find out capacity of an existing pump?
Existing pump needs manufacturer and model no and then you can find most pump data on the vendors website, even small domestic pumps. You could test it yourself, but that seems a bit extreme...
New pump head will vary depending if this is a once through end point delivery (a tap or tank) when you need to add static head to friction losses or a circualtion mode (radiators or HXs) when it is friction losses in the pipes and equipment only which is pressurised.
Posts above give you some good guidance, but your post is a bit vague so gets a vague response I'm afraid....
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way