Running totals?

I'm trying to get a formula to calculate based on a cell being populated for a monthly tracking sheet.

My example:
Column A has dates- 1-Jun thru 5-Jun in cells A1:A5

Column B has order quantities

Column C has revenue totals

I want my formula to calculate my revenue per day, but only when cells in column B are entered- that way the formula stays up to date. Right now I have to take the sum of revenue in column C and divide by the number of days in the week or month to get my number. I have to update that day number each day- hence why I'd like it to calculate once data is entered in column B.

I hope all of this makes sense... thanks in advance!

The counta function was what I needed- thanks!