I .
( informal )
1.
(especially in negative sentences) to seem reasonable; to make sense :
His story just doesn't add up.
2.
(not used in the progressive tenses) to increase by small amounts until there is a large total :
When you're feeding a family of six the bills soon add up.
II .
add up | ˌadd sth ↔ ˈup
to calculate the total of two or more numbers or amounts :
The waiter can't add up.
Add up all the money I owe you.