Provided there aren't totals at the bottom of these tabs, something like this would work in Tab 27, B2: =AVERAGE(Sheet1:Sheet26!B2:B4), where B4 refers to the last cell on the tab with the most data. Weight lost could be modified to go off of column C. As a caveat, note that there would be a difference between modifying the above for % Lost and simply dividing B3 by B2. It will likely be small, but averaging the averages will assign them all the same weighting (no pun intended) whereas taking average weight lost divided by average starting weight will give more weight to percentages lost for higher starting weights.