Tracking mortgage overpayments in a spreadsheet
A spreadsheet tracks overpayments perfectly well until two people are paying into it. What breaks is not the arithmetic — it is that a spreadsheet has one copy, no record of who paid, and no memory of what it looked like last month.
Most people who decide to overpay start with a spreadsheet, and that is a reasonable place to start. The maths is not hard: a balance, a monthly interest figure, a payment, and a new balance. A column each and you have a tracker.
What the spreadsheet has to get right
Four things, and only the first is obvious.
- Interest before payment, not after. A repayment mortgage charges interest on what you owed at the start of the month, then takes the payment. Applying them the other way round makes every row slightly too flattering, and the error compounds over a term.
- The payment date, not the date you typed it. Payments are made in arrears — the payment at the end of month one covers the interest that accrued through month one. A row dated when you happened to open the file drifts out of step with the lender's.
- Nothing assumed. A month with no payment recorded is a month where interest accrued and nothing came off. A spreadsheet that auto-fills the standing order tells you the balance fell when it did not.
- The total paid, not the extra. Recording £1,400 as an overpayment when £1,200 was the contractual payment turns a £200 extra into £1,400 of extra, and the projection is wrong from that row onward.
Where it stops working
Not the arithmetic. The record-keeping.
- There is one copy. Whoever has it open has it. The other person is looking at a version from last Tuesday, or at a screenshot.
- It does not record who paid. If one of you covers a month, the row looks the same as any other. A year later there is no way to reconstruct it, and the file is the only record there ever was.
- It has no history. A cell that gets overtyped is gone. There is no version of a spreadsheet that tells you what a figure used to be.
- It lives on one machine. The most common way a five-year overpayment record ends is a laptop, not a decision.
If you are building one anyway
Then build it with these columns, because they are the ones people leave out and then need: the date the payment was made, who made it, the total paid rather than the extra, and the balance before that month's interest. The last one is what lets you check a row against a statement rather than taking it on trust.
What we built instead
Mortgage Meadow is the same arithmetic with the four record-keeping problems fixed: two people, each payment attributed to whoever made it, an encrypted backup so a lost phone is not a lost history, and nothing ever assumed paid. It exports to CSV, so it is still your file — how we work it out sets out the model in full, including where it is deliberately simpler than a lender's.
Questions people ask
Can I keep using a spreadsheet alongside it?
Yes. Mortgage Meadow imports a CSV and exports one, so a spreadsheet you already keep can be brought in, and everything in the app can be taken back out.
What should a mortgage overpayment spreadsheet include?
The date the payment was made, who made it, the total paid rather than just the extra, and the balance before that month's interest. The last one is what lets you check a row against a statement.