Try

=SUMPRODUCT(--(B1:B20-A1:A20<4))/COUNT(B1:B20)

"James A." wrote:

Let's say column A is my project start date and column B is my project finish
date. I have my formula which calculates how many days it took me to
complete each project which ranges anywhere from 1 day to 7 days. I had 20
projects last month and my contract is to complete each project within 3
days.

I need excel to calculate how many projects were completed within 3 days and
the total percentage.

Any help is greatly appreciated!