How to Sum Squares in Excel with SUMSQ

SUMSQ adds up squared values — the building block of forecast error, RMSE and variation.

You will learn to

• Add squared forecast errors
• Turn them into RMSE
• Measure total variation around an average

How it works

SUMSQ(number1, [number2], …) squares every value and adds the results: =SUMSQ(3,4) is 9 + 16 = 25. Squaring makes every error positive and punishes big misses much more than small ones, which is why the sum of squared errors is the standard way to judge a forecast.

Divide by the number of values and take the square root to get RMSE, an average error in the same units as your data: =SQRT(SUMSQ(D2:D8)/COUNT(D2:D8)). SUMSQ also accepts calculated arrays such as C2:C8-AVERAGE(C2:C8), giving the total variation around the mean.

=SUMSQ(D2:D8)