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Build an amortization schedule and download an Excel file with every payment split into interest and principal.
An amortization schedule lists every payment on a loan and splits each one into interest and principal. Early payments are mostly interest because the balance is large; late ones are mostly principal because it is not. The schedule is how you see where the money actually goes.
Monthly payment
$1,896.20
Payments
360
Total interest
$382,636.71
Clears
2056-08
The download is a real Excel workbook with two tabs, not a comma separated list with a spreadsheet extension. Every row of the schedule is a formula against the row above it.
1. Schedule
Every payment, numbered and dated, split into interest and principal with the balance carried down. Interest is the previous balance times the monthly rate, principal is the payment minus that, and the balance is the previous balance minus the principal. Edit a payment and everything below follows.
2. Summary
The loan as entered, the scheduled payment, the date it clears, the total interest and the total paid. If you are overpaying, it also carries the months and the interest that saves.
Header rows are bold on a yellow fill, the header row stays frozen while you scroll, and the column widths are set so nothing arrives as a row of hash symbols.
| # | Date | Payment | Interest | Principal | Balance |
|---|---|---|---|---|---|
| 1 | 2026-09-01 | $1,896.20 | $1,625.00 | $271.20 | $299,728.80 |
| 2 | 2026-10-01 | $1,896.20 | $1,623.53 | $272.67 | $299,456.13 |
| 3 | 2026-11-01 | $1,896.20 | $1,622.05 | $274.15 | $299,181.98 |
| 4 | 2026-12-01 | $1,896.20 | $1,620.57 | $275.63 | $298,906.35 |
| 5 | 2027-01-01 | $1,896.20 | $1,619.08 | $277.12 | $298,629.23 |
| 6 | 2027-02-01 | $1,896.20 | $1,617.57 | $278.63 | $298,350.60 |
| 359 | 2056-07-01 | $1,896.20 | $20.40 | $1,875.80 | $1,890.67 |
| 360 | 2056-08-01 | $1,900.91 | $10.24 | $1,890.67 | $0.00 |
The first rows and the last two, out of 360. Early payments are almost all interest and late ones almost all principal, which is the thing an amortization schedule exists to show. The file contains every row, each one a formula against the row above it.
Excel
Double click the download. If Excel shows a yellow protected view bar, click Enable Editing so the formulas calculate.
Google Sheets
Upload it to Drive and open it, or use File then Import then Upload inside an existing sheet. Formatting and formulas both carry across.
Numbers on a Mac
Open it directly. Numbers converts the workbook and keeps the totals working, though the yellow header fill may shift slightly.
Formula
Payment = P x r / (1 - (1 + r)^-n) | Interest = balance x r | Principal = payment - interestP = The amount borrowed
r = The monthly rate, which is the annual rate divided by twelve
n = The number of monthly payments, which is the term in years times twelve
Worked Example
300,000 at 6.5 per cent over 30 years
Did you know? On a thirty year loan at 6.5 per cent, the halfway point in time is nowhere near the halfway point in the balance. After fifteen years of payments roughly two thirds of the original amount is still owed, because the early instalments are mostly interest. That gap is the single clearest argument for overpaying early rather than late.
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