What a debt payoff spreadsheet needs to get right

The interest formula has to run before the payment subtracts

The order of operations matters. Each month's interest is the current balance times the annual rate divided by twelve, and that amount gets added to the balance before the payment is subtracted. A spreadsheet that subtracts payment first and adds interest after will understate how much you actually owe.

Rollover needs its own formula, not a manual note

Once a debt hits zero, its minimum payment should automatically flow into the extra payment on the next debt in your chosen order. If your spreadsheet handles this with a manual note to "remember to add $40 next month," it's one missed update away from being wrong. A formula that checks whether the prior debt's balance is zero and adds its minimum to the next debt's payment column keeps this automatic.

A date column turns a month count into something checkable

"Paid off in month 18" is harder to verify against real life than "paid off around March 2028." Adding a date column, calculated from your start date plus the month number, lets you compare the spreadsheet's projection against an actual calendar and catch drift early.

Building in a check that catches your own mistakes

A spreadsheet does exactly what its formulas say, including formulas with an error in them.

1. Add a column that flags any balance that went up when it should have gone down.
2. Add a check for any balance that's negative, which signals a formula error, not a paid-off debt.
3. Spot-check one month's interest by hand against the formula's output.
4. Confirm the rollover triggers on the month a debt actually reaches zero, not one month early or late.
5. Compare the spreadsheet's current-month balance against your real statement at least once a quarter.

Common questions

Do I need a separate row for interest and payment, or can they combine?

Either works as long as the order is right: interest calculated on the current balance, then payment subtracted. Some people prefer separate columns so mistakes are easier to spot.

How do I handle a variable interest rate in a spreadsheet?

Give the rate its own column you can update, rather than typing it into the interest formula directly, so a rate change only requires one edit instead of hunting through every month's formula.

What if my spreadsheet and my statement disagree?

Trust the statement for your actual balance and check the spreadsheet's formulas for where the drift started, usually a payment date mismatch or a rate that changed without being updated.

Is a spreadsheet as accurate as a dedicated debt app?

It can be, if the formulas are built correctly and kept updated. The tradeoff is that you're responsible for catching your own formula errors, where a dedicated app has that logic built in.