Meaning of add up

 Add up 

Meaning - to calculate a total sum or to increase by small amounts until there is large total. 

Examples 

1) When you are feeding a family of six, the bills soon add up.

2) Now add up the number of calories you have eaten.

If  a set of facts does not add up , you do not believe it is correct because it does not match other information you already have.

Example - There is something about this case that just does not add up.

If something adds up , it makes sense or is reasonable.

Examples -

1) Her story does not add up , I think she is lying.

2) Watson claimed he was at home at the time of murder but police said his story didn't add up.