Google-sheets – How to have Google Spreadsheets calculate certain cells in increments

There is no built in function to calculate the weighted average so you have to write a custom function if you want to avoid using too many temporary cells. So here is how you can achieve your goal.

Go to Tools > Scripts > Script editor..., Copy/Paste the bellow code and save it:

function weightedAverage(v, w) {
  var sum_v = 0;
  var sum_w = 0;

  if (v[0].length != w[0].length) {
    return "#ERROR: Incorrect number of values and weights";  
  }

  for (var c = 0; c < v[0].length; ++c) {
    sum_v += v[0][c] * w[0][c]; 
    sum_w += w[0][c]; 
  }

  return sum_v/sum_w;
}​

Use it like:

=weightedAverage(B3:D3,$B$2:$D$2) 

Where B3:D3 are your values and $B$2:$D$2 your weights. It's not error proof (the only check is to make sure both arrays have the same length) but it will do the trick.

In the above example I'm not trying to extract the weights from the title of the attribute, but I'm reading them from the second row (B2:D2) to make our life easier and clearer. The $ is not changing the result of the formula. It's only affecting what's happening when you're copying the formula in another cell. The part of the cell reference following the $ will not change (link for more details). Write the formula once in the cell E3 and Copy it to the rest of the rows to see it in action.

You can use the SUMIF function to calculate the total sum for each person.

If you enter the following formula in a cell you like, it will give you the total amount spent by "Me":

=SUMIF(A2:A6,"=Me",B2:B6)

where A2:A6 is the Range, "=Me" is the Criteria, and B2:B6 is the Sum_Range.