Split the household costs without rebuilding the formulas
One spreadsheet, and only one, that applies the pro rata method. You enter two net incomes and your shared expenses; it returns what each person owes. Nothing to install, nothing to pay.
How it works out the shares
It adds the two net incomes, works out each person's share of that total, then applies the share to every expense line. On €2,400 and €1,600 the shares come to 60% and 40%, and each line is cut in that proportion.
The sheet arrives pre-filled with the worked example: €2,400, €1,600 and €1,620 of shared costs. Both people then spend 40.5% of their income on those costs. Replace the numbers with yours and everything below follows.
The last row is a check. It compares the share of income each person puts in and showsOK when the two match. That is the check that takes longest by hand, and the one that catches a mistyped cell.
What it does not do
It calculates the current month and forgets the rest. It keeps no history, it does not track who paid for the groceries, and it does not handle personal accounts alongside a joint one.
That is the point: one sheet, one job. If the pro rata split is all you need, it will stay enough. If you want to know, in June, how the split has moved since January, you need something else.
The full method, the formulas to copy and the cases where it gets awkward are written up inthe guide on pro rata splitting, which is in French.