How to read an amortization schedule (and beat it with extra payments)
Somewhere in your closing packet is a table with 360 rows that nobody ever reads. That table is the honest biography of your loan — including the part where your first payment is 86% interest, and the part where a modest extra payment quietly deletes six figures. Here's how to read it.
The one rule that generates the whole table
A fixed-rate loan has exactly one moving part: each month, you're charged interest on whatever you still owe, and everything else in your payment reduces what you owe. That's it. Every row of an amortization schedule is that sentence, applied once.
Take a $350,000 loan at 6.5% over 30 years. The fixed monthly payment works out to $2,212.24. In month one you owe the full $350,000, and the monthly interest rate is 6.5% ÷ 12 ≈ 0.5417%. So:
- Interest for month 1: $350,000 × 0.5417% = $1,895.83 — gone, that's the cost of borrowing this month.
- Principal for month 1: $2,212.24 − $1,895.83 = $316.41 — the only part that actually reduces your debt.
- New balance: $349,683.59.
Next month the interest is charged on $349,683.59 instead of $350,000, so it's slightly smaller ($1,894.12), which leaves slightly more for principal ($318.12). And so on, 360 times:
| # | Payment | Interest | Principal | Balance after |
|---|---|---|---|---|
| 1 | $2,212.24 | $1,895.83 | $316.41 | $349,683.59 |
| 2 | $2,212.24 | $1,894.12 | $318.12 | $349,365.47 |
| 3 | $2,212.24 | $1,892.40 | $319.84 | $349,045.63 |
| 12 | $2,212.24 | $1,876.46 | $335.78 | $346,087.93 |
| 360 | $2,210.12* | $11.91 | $2,198.21 | $0.00 |
*The final payment self-adjusts down so the balance lands on exactly zero.
Notice what the schedule is telling you: the payment never changes, but its composition flips over the life of the loan — from 86% interest at the start to under 1% interest at the end. Total interest over the full 360 months: about $446,404. On a $350,000 loan, you pay roughly $796,404 all-in. The schedule doesn't editorialize; it just shows you.
Why the flip happens so slowly
People are often shocked that after five full years of payments on this loan, the balance has only dropped to about $327,600 — you've paid roughly $132,700 and own about $22,400 more of your house. That's not a trick; it's just the rule above. When the balance is big, the interest charge is big, and the leftover principal sliver is small. The schedule speeds up on its own — but only in the back half, decades from now.
Which is exactly why extra payments are so much more powerful than intuition suggests.
The extra-payment math, honestly
An extra payment has a special property: 100% of it is principal. The regular payment's interest charge is already covered, so every extra dollar goes straight at the balance — and every dollar of balance you remove stops generating interest for every remaining month of the loan.
Same loan, plus $200 extra each month:
| As scheduled | + $200/month | |
|---|---|---|
| Monthly payment | $2,212.24 | $2,412.24 |
| Payoff | 30 years | 23 years 10 months |
| Total interest | ≈ $446,404 | ≈ $338,308 |
| Interest saved | — | ≈ $108,096 |
Six years and two months of payments deleted, and about $108,000 kept, for $200 a month. (Numbers computed month-by-month with the exact method above — not a rule-of-thumb estimate. Your loan's figures will differ; run your own numbers before deciding anything.)
The calculator that shows your whole schedule
Type four numbers — amount, rate, term, start date — and get your payment, total interest, payoff date, and the full month-by-month schedule. Add an extra payment and watch the payoff date and interest-saved readouts recalculate instantly. Pure formulas, works in Excel and Google Sheets.
Three things to check on any schedule
- The crossover month — the first row where principal exceeds interest. On the example loan it doesn't arrive until month 233, well into year 20. If that number offends you, that's the extra-payment argument in one row.
- Interest-to-date at year 5 — what the loan costs if you sell or refinance early. Most people never look; it's the number that actually applies to them, since few 30-year loans live 30 years.
- The final row — a correct schedule lands the balance on exactly $0.00 with a slightly-adjusted last payment. If a spreadsheet leaves $3.17 dangling in month 360, the formulas are sloppy — and if it's sloppy there, don't trust the totals either.
One honest caveat to close: whether extra payments are your best move depends on your rate, your other debts, and what the money could earn elsewhere — that's a judgment call, not spreadsheet output. What the schedule gives you is the true price of each option, to the cent, so the judgment is at least made with real numbers.