The Google Sheets formula for splitting bills by income

To split a bill in proportion to two incomes, work out one person's share of the combined income and multiply the bill by it. In Google Sheets, with take-home pay in B1 and B2 and the bill in B4, that is =B4*($B$1/($B$1+$B$2)) for the first person and =B4-$C$4 for the second. Everything below is why it works and where it goes wrong.

What each part of it does

The inner bracket, $B$1/($B$1+$B$2), is the only piece doing any thinking. It answers 'what fraction of the money coming into this household comes from this person', and it produces a decimal between 0 and 1. Format that cell as a percentage while you are building the sheet, because seeing 57% written out is a much better check on your own arithmetic than seeing 0.5714285714.

The multiplication then applies that fraction to whatever the bill is. And the second person's share is deliberately the bill minus the first share, rather than a second percentage calculation, so that rounding can never produce two halves that fail to add up to the whole.

🔑The dollar signs are not decoration. $B$1 stays pointing at B1 when you drag the formula down a column; B1 without them slides to B2, B3, B4 and gives you a sheet full of numbers that look plausible and are wrong.

A worked example

Take-home pay of $4,200 and $3,100, and rent of $2,400. Combined income is $7,300. The first person's share is 4,200 divided by 7,300, which is 0.575, or 57.5%. Applied to the rent that is $1,381. The second person pays the remaining $1,019.

Both people are now handing over about 33% of their take-home pay for the same apartment. Under an even split, it would have been 29% and 39%. That ten-point gap is the whole argument, and writing it in a cell is usually more persuasive than saying it at the kitchen table.

The three ways this breaks

• An irregular income. If one person is freelance, a single month is the wrong input. Use a rolling three or six month average in its own cell and point the formula at that instead, and agree in advance how often you recalculate it.
• Take-home versus gross. Use what actually lands in the account. Gross pay produces a split that neither person recognises, because their tax situations are not the same.
• Someone changes B1 without saying so. This is the common one. A raise gets typed in, every share on the sheet moves, and the other person finds out through the numbers. Decide up front whether the split follows income automatically or gets renegotiated, because doing that in the moment is much harder.

What it does not answer

A proportional split tells you what each person owes. It does not tell you what each person actually paid, which is a different column and the one that causes the arguments. Somebody always covers more than their share in a given month, usually by accident, and unless the sheet is keeping that running difference somewhere it disappears.