Every month you make the same mortgage repayment, yet in the early years your balance barely seems to move. That is not a trick by your lender. It is how amortization works, and once you understand it you can make much smarter decisions about extra repayments, refinancing and choosing a loan term.
This guide walks through the maths in plain English, shows a full year-by-year example, and explains why money you put in early is worth so much more than money you put in late.
What amortization actually means
Amortization is the process of paying off a debt with a series of equal, scheduled repayments. Each repayment does two jobs:
- It pays the interest that has built up since your last repayment.
- It reduces the principal, which is the amount you still owe.
The repayment amount is fixed so that, if you make every payment on time and the rate does not change, the balance lands exactly on zero with the final payment. In the US this is usually called a fully amortizing loan. In Australia and New Zealand it is called a principal and interest (P&I) loan. In the UK it is a repayment mortgage, as opposed to an interest-only one.
The key detail is that interest is always charged on the balance that is left. When the balance is large, the interest portion is large. As the balance shrinks, interest shrinks, and more of the same fixed repayment goes towards principal. That gradual shift is the whole story of amortization.
The repayment formula
Lenders and calculators, including our mortgage calculator, use the standard annuity formula to work out the regular repayment:
M = P × r(1 + r)n ÷ ((1 + r)n − 1)M is the repayment per period, P is the amount borrowed, r is the interest rate per period (the annual rate divided by the number of repayments per year) and n is the total number of repayments.
Take a $360,000 loan at 6.5% a year over 30 years, repaid monthly:
- P = 360,000
- r = 0.065 ÷ 12 = 0.0054167
- n = 30 × 12 = 360
Plug those in and the monthly repayment comes out at $2,275.44. Over 360 payments you hand over $819,160 in total, which means $459,160 is interest, more than the original loan.
The formula looks intimidating, but you rarely need to use it by hand. What matters is what it implies: for a given rate, the repayment is set so the loan finishes on time, and every payment is split between interest and principal according to the balance at that moment.
Inside a single payment: how the split shifts
Here is the first payment on that $360,000 loan:
- Interest for the month: $360,000 × 0.065 ÷ 12 = $1,950.00
- Principal repaid: $2,275.44 − $1,950.00 = $325.44
- New balance: $359,674.56
So about 86% of your very first repayment is interest. Next month the interest is calculated on $359,674.56 instead of $360,000, so it is slightly lower, and the principal portion is slightly higher. That tiny change repeats 360 times.
By the final payment the picture has flipped completely. The last repayment is $12.26 of interest and $2,263.19 of principal.
The crossover point. On this loan, the principal part of your repayment does not overtake the interest part until payment 233, in year 20. You are well past the halfway mark in time before most of each repayment is actually paying down the house.
The higher the rate and the longer the term, the later that crossover arrives. On a lower rate or a shorter loan, principal takes over much sooner.
A year-by-year amortization example
The table below summarizes the same loan: $360,000 at 6.5% over 30 years, with monthly repayments of $2,275.44, or $27,305 a year. Figures are rounded to the nearest dollar.
| Year | Interest paid that year | Principal paid that year | Share of repayments that is interest | Balance at end of year |
|---|---|---|---|---|
| 1 | $23,282 | $4,024 | 85% | $355,976 |
| 2 | $23,012 | $4,293 | 84% | $351,683 |
| 3 | $22,725 | $4,581 | 83% | $347,102 |
| 5 | $22,090 | $5,215 | 81% | $337,000 |
| 10 | $20,094 | $7,211 | 74% | $305,194 |
| 15 | $17,333 | $9,972 | 63% | $261,213 |
| 20 | $13,516 | $13,789 | 50% | $200,395 |
| 25 | $8,237 | $19,068 | 30% | $116,295 |
| 30 | $938 | $26,368 | 3% | $0 |
A few things stand out:
- After five years you have paid about $136,500 in repayments, but the balance has only fallen by $23,000.
- After ten years you still owe about $305,000, roughly 85% of what you borrowed.
- You owe half the original loan only after about 21 and a half years.
- The last ten years repay more than half the principal, because by then interest is a small slice of each payment.
This is why people who sell or refinance after five or seven years are often surprised by how little equity their repayments have built. Most of their equity usually comes from the deposit and any rise in the property’s value, not from principal repaid.
Why extra payments early matter most
Because interest is charged on the outstanding balance, every extra dollar you pay off principal stops attracting interest for the rest of the loan. A dollar paid off in year 1 avoids interest for up to 29 more years. A dollar paid off in year 25 avoids it for five years at most.
Here is the effect of adding an extra $200 a month to the same loan, depending on when you start:
| Extra $200 a month starting | Total extra paid | Interest saved | Time saved | Interest saved per $1 of extra |
|---|---|---|---|---|
| Year 1 | $57,200 | $108,917 | 6 years 1 month | $1.90 |
| Year 6 | $49,400 | $69,891 | 4 years 4 months | $1.41 |
| Year 11 | $40,800 | $41,098 | 2 years 11 months | $1.01 |
| Year 16 | $31,200 | $21,155 | 1 year 11 months | $0.68 |
Starting in year 1 saves nearly two dollars of interest for every dollar of extra repayment. Wait until year 16 and each extra dollar saves less than 70 cents.
One-off lump sums follow the same pattern. A single $10,000 payment:
- at the end of year 1 saves about $51,058 in interest and 2 years 2 months
- at the end of year 10 saves about $25,153 and 1 year 3 months
- at the end of year 20 saves about $8,749 and 8 months
Try it yourself. Put your own balance, rate and extra amount into the extra repayments calculator to see your new payoff date. For a full list of practical strategies, read our guide on how to pay off your mortgage faster.
Before you send extra money to your lender, check that it will be applied to principal rather than held as a prepayment of future instalments, and look for any prepayment penalties or caps on overpayments. Some fixed-rate loans, especially in Australia, the UK and Canada, limit how much you can repay early without a fee.
Amortization with fortnightly or weekly repayments
The same logic applies whatever your repayment frequency, but there are two very different ways to repay more often, and they produce very different results.
True fortnightly or weekly repayments use the same formula with more, smaller periods. For our $360,000 loan, a true fortnightly repayment works out to $1,049.71 over 780 fortnights, and a true weekly repayment to $524.75. Total interest falls only slightly, to about $458,774 fortnightly and $458,609 weekly, because you pay the same amount per year and interest is reduced a little more often.
Accelerated or “half-monthly” repayments are where the real saving comes from. You pay half your monthly repayment every fortnight: $1,137.72. Because there are 26 fortnights in a year, you make the equivalent of 13 monthly repayments instead of 12. On this loan that one extra payment a year clears the mortgage in about 24 years 2 months and saves about $105,700 in interest.
| Repayment style | Amount | Paid per year | Approximate payoff | Total interest |
|---|---|---|---|---|
| Monthly | $2,275.44 | $27,305 | 30 years | $459,160 |
| True fortnightly | $1,049.71 | $27,292 | 30 years | $458,774 |
| Half-monthly every fortnight | $1,137.72 | $29,581 | 24 years 2 months | $353,414 |
The lesson is that frequency on its own makes little difference. What shortens the loan is paying more per year.
In the US, this is sold as a biweekly mortgage. Some third-party biweekly programs charge setup or processing fees and only pass the money to your lender monthly, so check the terms or simply make an extra principal payment yourself. In Canada, fixed-rate mortgage interest is usually compounded semi-annually rather than monthly, so the same 6.5% rate produces a slightly lower monthly payment of about $2,255 on this loan.
What changes your amortization schedule
Your schedule is a forecast, not a promise. It changes when any of the inputs change:
- Rate changes. On a variable or adjustable-rate loan, the lender recalculates the repayment or the term when the rate moves. A higher rate means more of each payment goes to interest. Our guide to fixed vs variable rates covers how to stress-test this.
- Interest-only periods. During an interest-only period no principal is repaid at all. When it ends, the full balance must be amortized over a shorter remaining term, so repayments jump.
- Term length. A 25-year loan has higher repayments than a 30-year loan, but principal takes over much sooner and total interest is far lower.
- Refinancing. Moving to a new 30-year loan restarts the schedule, putting you back at the interest-heavy start. Keeping your old repayment amount after refinancing avoids this.
- Fees and insurance. Your amortization schedule covers principal and interest only. Taxes, insurance and fees sit on top, and they also affect the true cost of the loan. See APR vs interest rate for how fees are captured in comparison figures.
How to use your own schedule
Ask your lender for your amortization schedule, or generate one with our loan calculator, which shows every payment’s interest and principal split. Then look for three things:
- Your current position. Find this month’s row. The interest portion tells you what the loan is really costing you right now.
- The crossover point. If it is many years away, early extra repayments will have an outsized effect.
- Your remaining interest. Add up the interest column from today to the end. That is the figure extra repayments and a better rate can shrink.
Understanding amortization will not change your rate, but it will tell you exactly where your money goes, and where an extra dollar does the most good. For definitions of terms like principal, P&I and amortization schedule, see our financial glossary.
This guide is general information, not personal financial advice. Rules and products vary by country and lender.