Why the spreadsheet always ends badly

Almost everyone starts with a spreadsheet. It is the reasonable choice: it is free, you already know how to use it, and a column of dates and amounts looks like more than enough.

And it works. For a few months.

The research is not encouraging

Ray Panko at the University of Hawaii has spent years collecting field audits of spreadsheets in real operational use. Across 55 spreadsheets from five high-methodology studies, 91% contained at least one error. Across a broader set of 163 spreadsheets from nine studies, the weighted average error rate was 84%.

Panko's own caveat matters: those figures likely understate the problem, because human inspectors catch only around 60% of the errors that are actually there.

The honest reading is not "spreadsheets are always wrong." Errors per cell are rare — a percent or two. But a spreadsheet that grows will very probably contain at least one wrong final number, and those are extraordinarily hard to spot by looking.

Where a loan spreadsheet specifically breaks

Interest accrues daily; you open the file when you remember. That is the structural problem. The number you are looking at is the number from the last time you updated it, not today's.

Partial repayments require a decision on every row. Interest first, or straight to principal? You have to decide, and decide the same way every time. The first time you think about it. By the fifth you quietly resolve it differently, and the sheet is now internally inconsistent.

If interest compounds, each row depends on the one above it. A mistake in March contaminates everything after it, and you will not notice until something fails to reconcile in September.

The other person cannot see it. So it stops being an agreement and becomes your version of events. When a discrepancy appears, what you have is your file against their memory.

The cost you do not measure

It is not the hour spent building it. It is remembering to update it, doubting the last figure, and that small awkward moment of checking the numbers before messaging someone.

And there is a quieter cost: the loans you stopped tracking. The second one got less care. The third lives in a WhatsApp thread. At that point you do not have a system, you have fragments.

What a purpose-built tool changes

Not arithmetic Excel cannot do. It does it without you remembering.

Interest accrues every day whether the app is open or not. Partial repayments are split by a consistent rule, and the split is shown. The history is kept day by day, so when something looks wrong you can go to the exact date. And the other person can see the same thing you do, which means there are no longer two versions.

If your spreadsheet works, there is no rush. But if you have ever opened it and not been quite sure about the last number, more formulas will not fix that.