Most homeowners know they have a mortgage payment due each month. Far fewer know what is actually happening inside that payment — why so much of it goes to interest for years before meaningfully reducing what they owe. That is not a flaw in the system. It is just arithmetic. And once you see the arithmetic clearly, some things that seemed abstract become very practical.
This guide explains how mortgage amortization works — not in abstract terms, but by building a real schedule from scratch and tracing exactly where each dollar or pound goes.
Run Your Own Numbers
See your full amortization schedule month by month — or model savings from extra payments:
1. The Core Principle: Fixed Payment, Shifting Split
A fixed-rate mortgage has one defining feature: the monthly payment amount never changes. What does change — every single month — is how that payment divides between interest and principal.
Interest is always calculated on the current outstanding balance. Start with a large balance and you owe a lot of interest that month. Pay some of it off, the balance drops, and next month's interest charge is slightly lower. Because the total payment stays the same and interest fell by a few dollars, a few extra dollars go to principal. Repeat 360 times for a 30-year mortgage.
The Amortization Formula
M = P × [r × (1 + r)^n] ÷ [(1 + r)^n − 1]
2. A Full Worked Example: $320,000 at 6.75% for 30 Years
Using the same loan as our Mortgage Calculator default: a $400,000 home, 20% down, $320,000 borrowed at 6.75% for 30 years.
M = 320,000 × (0.005625 × 7.5332) ÷ (7.5332 − 1)
M = 320,000 × 0.042375 ÷ 6.5332 = $2,076 / month
3. The First 6 Months of Your Schedule
Watch what happens inside each of those $2,076 payments:
| Month | Opening Balance | Interest (0.5625%) | Principal | Closing Balance |
|---|---|---|---|---|
| 1 | $320,000 | $1,800 | $276 | $319,724 |
| 2 | $319,724 | $1,798 | $277 | $319,447 |
| 3 | $319,447 | $1,797 | $279 | $319,169 |
| 6 | $318,607 | $1,792 | $283 | $318,323 |
| 12 | $316,883 | $1,782 | $293 | $316,590 |
| Year 1 Total | — | $21,496 | $3,410 | $316,590 |
After 12 payments totalling $24,906, your balance has only dropped by $3,410. That is not a scam or a trick. It is simply how interest on a large balance works: 86% of year one's payments went to interest. The ratio steadily improves from here — just slowly.
4. The Crossover Point — When Principal Overtakes Interest
On this example, the month when the principal portion of your payment finally exceeds the interest portion arrives around Month 238 — approximately Year 19.8. For roughly the first two-thirds of a 30-year mortgage, interest is the dominant cost in each payment.
| Year | Balance at Period End | Monthly Interest Portion | Monthly Principal Portion |
|---|---|---|---|
| Year 1 | ~$316,590 | ~$1,782 | ~$293 |
| Year 5 | ~$300,402 | ~$1,692 | ~$384 |
| Year 10 | ~$272,963 | ~$1,538 | ~$537 |
| Month 238 (crossover) | ~$182,856 | ~$1,034 | ~$1,041 |
| Year 25 | ~$105,445 | ~$601 | ~$1,474 |
| Year 30 (final payment) | $0 | ~$12 | ~$2,064 |
5. What Extra Payments Actually Do
Every extra dollar you pay beyond the scheduled amount goes entirely to principal — no interest portion. This matters because it has a compounding benefit: reduce the principal today, and every future month's interest calculation starts from a lower base.
$200/month extra
- Reduces the 30-year term to 280 months (about 23.3 years)
- Saves roughly $112,000 in total interest
- No change to required monthly payment — you simply stop early
One extra full payment per year
- Reduces the 30-year term to 290 months (about 24.2 years)
- Saves roughly $97,000 in total interest
- Practical trick: divide your monthly payment by 12 (about $173) and add it each month — that pays off in 288 months and saves about $101,000
One caveat: some mortgage agreements, particularly in the UK and some European markets, include early repayment charges (ERCs) during fixed-rate periods — typically 1–5% of the amount overpaid beyond an annual allowance. Always check the terms before making large lump-sum overpayments.
6. Amortization vs. Interest-Only Mortgages
Interest-only mortgages — common in the UK buy-to-let market and historically popular before the 2008 crisis — require only the interest payment each month. The principal never decreases during the term and is repaid in full at the end, typically from the sale of the property.
The monthly payment is lower on an interest-only basis, but the total cost is dramatically higher because interest accrues on the original full balance throughout. On the same $320,000 / 6.75% example: an interest-only mortgage costs $1,800/month indefinitely, with $320,000 still owed at the end. A fully amortizing mortgage costs $2,069/month and arrives at zero.