=sum(iif([company],yes,[TripCost]))

[kenmn]

If yes then sum total of all trip costs for that category.

On the form is millage start, millage end, 4 tick boxes one for each category.

total millage

Trip cost (millage x 40)

Not getting the desired result for some reason despite trying several permutations

[Seth Schrock]

Try replacing Yes with True.

[Rabbit]

Your expression doesn't make any sense, basically what you're doing is:

if company
   then yes
if not company
   then tripcost

[kenmn]

Thank you for your reply I kind of guessed it was something like that. The problem I have is what to change. I basically want the expression to total where 'yes' is ticked. Can you help?

[Rabbit]

What you want to change is that if company is yes/true, then return the trip cost, otherwise return 0.