How Loan Amortization Works, and What Extra Payments Really Save
Money ·
Take out a 30-year mortgage and look at your first statement, and something seems wrong. You’ve paid almost $1,900, but the amount you owe has barely moved. Most of the money went to interest.
That isn’t a trick by the bank. It’s how an amortized loan works: the same payment every month, split differently between interest and the loan itself as the years go by. Once you see how the split works, it’s clear why the early years cost so much, why the rate matters as much as it does, and why a small extra payment can take years off the loan.
All the numbers below come from the site’s mortgage and loan calculator, so you can put in your own loan and follow along.
What amortization means
To amortize a loan is to pay it off gradually, in regular payments that cover both the interest and part of the amount borrowed (the principal), so that the last payment brings the balance to exactly zero. The word comes from the Old French amortir, “to deaden” or “to kill”: the debt is killed off bit by bit. (Mortgage shares the root: it means “dead pledge”, usually explained as a pledge that dies once the loan is paid off, or is lost if it isn’t.)
Most mortgages, car loans and personal loans work this way. Credit cards don’t: you choose how much to pay each month, so there’s no schedule that ends on a set date.
How each payment is split
Every month, two things happen:
- Interest is charged on what you still owe. The yearly rate is divided by 12, so a 6.5% loan charges about 0.54% a month.
- The rest of your payment goes to the principal, and the balance goes down by that much.
Next month, the interest is worked out again on the new, slightly lower balance. Because the balance is a little lower, the interest is a little less, so a little more of the same payment goes to the principal. That small shift repeats every month for the whole loan.
Here’s what that looks like for a $300,000 loan at 6.5% over 30 years. The monthly payment is $1,896.20.
| Payment | Interest | Principal | Still owed |
|---|---|---|---|
| Month 1 | $1,625.00 | $271.20 | $299,728.80 |
| Month 2 | $1,623.53 | $272.67 | $299,456.13 |
| Month 12 | $1,608.40 | $287.80 | $296,646.88 |
| Month 233 | $946.51 | $949.69 | $173,791.39 |
| Month 360 | $10.24 | $1,890.67 | $0.00 |
The first payment is 86% interest. After a full year of payments, totaling about $22,750, the balance has gone down by just $3,353. It takes until month 233, more than 19 years in, before more of each payment goes to the loan than to interest. And the balance doesn’t fall below half the amount borrowed until year 22.
By the last payment, it’s the other way around: just $10 of interest and almost all the rest going to the loan.
Over the full 30 years, the loan costs $382,636.71 in interest, more than the $300,000 borrowed. Altogether you’d pay $682,636.71.
Where the monthly payment comes from
The payment is set so that, with this monthly split, the balance lands on exactly zero at the final payment. The formula is:
payment = P × r / (1 − (1 + r)^−n)- P: the amount borrowed
- r: the monthly interest rate (the yearly rate ÷ 12, as a decimal: 6.5% → 0.065 ÷ 12)
- n: the number of monthly payments (30 years → 360)
You don’t need to work it out by hand, but the formula shows the three things that set the payment: how much you borrow, the rate, and how long you take to pay it back. Lenders round the result to the cent, which can leave a few cents to settle in the final payment. That’s why the last payment in the table above is slightly different from the rest.
Why the rate matters so much
Because interest is charged on the whole balance, and the balance stays high for years, a small change in the rate makes a big difference to the total. Here’s the same $300,000 over 30 years at different rates:
| Rate | Monthly payment | Total interest |
|---|---|---|
| 5.5% | $1,703.37 | $313,210 |
| 6.0% | $1,798.65 | $347,515 |
| 6.5% | $1,896.20 | $382,637 |
| 7.0% | $1,995.91 | $418,524 |
| 7.5% | $2,097.64 | $455,155 |
Each half a percentage point adds about $100 to the monthly payment and around $35,000 of interest over the life of the loan. That’s why shopping around for a lower rate, even a slightly lower one, is often worth the effort.
Shorter terms cost far less
The other lever is time. The same $300,000 at 6.5% over 15 years instead of 30:
- The monthly payment rises to $2,613.32, about $717 more.
- The total interest falls to $170,398, less than half the 30-year loan’s $382,637.
The balance shrinks much faster from the very first month, so there’s less for interest to be charged on. In real life, 15-year mortgages also tend to come with lower rates than 30-year ones, which widens the gap further. The trade-off is a payment that’s harder to afford each month and less room in the budget if something goes wrong.
Fixed and variable rates
Everything above assumes a fixed rate: the same rate, and so the same payment, for the whole loan. You know from day one exactly what you’ll pay and when the loan ends.
With a variable (or adjustable) rate, the rate can change during the loan, usually following a published benchmark rate plus a fixed margin. Many variable mortgages start with a few years at a fixed introductory rate, then adjust at set intervals. When the rate changes, the lender works out a new payment so the loan still ends on time: the same calculation as before, using what’s still owed, the new rate, and the months left.
To see what that can mean, take the same loan and suppose the rate changes after 5 years. By then, $280,833 is still owed, with 25 years to go:
- If the rate rises to 8.5%, the payment goes up from $1,896 to $2,261, about $365 more each month.
- If it falls to 4.5%, the payment drops to $1,561.
Variable loans usually have caps that limit how much the rate can change at each adjustment and over the life of the loan, so check what those are before you sign. A variable rate can start lower than a fixed one, but you take on the risk that rates go up. A fixed rate costs a bit more for that certainty.
The calculator works with a fixed rate. To see what a rate change would do, enter the balance left, the new rate and the years left as a new loan.
What extra payments save
An extra payment works differently from a regular one: all of it goes to the principal. None of it goes to interest, because that month’s interest is already covered by the regular payment. And because the balance is now lower, every month after that charges a little less interest, so more of each regular payment goes to the loan too. The effect builds on itself, the same way compound interest builds savings.
Here’s what paying extra every month does to the same $300,000 loan at 6.5%:
| Extra each month | Paid off in | Time saved | Interest saved |
|---|---|---|---|
| None | 30 years | – | – |
| $100 | 26 years | 4 years | $60,996 |
| $200 | 23 years 1 month | 6 years 11 months | $103,450 |
| $500 | 17 years 6 months | 12 years 6 months | $179,762 |
An extra $100 a month, $31,200 over the 26 years, saves $61,000 in interest and takes four years off the loan. Extra payments do the most good early, when the balance and the interest on it are at their highest. So a little extra in the first few years is worth more than the same amount near the end.
The biweekly trick
You may have heard of paying every two weeks instead of once a month. Paying half the monthly payment every two weeks means 26 half payments a year, which adds up to 13 full payments instead of 12. It works because of the extra payment, not the timing.
You can get almost the same effect by adding a twelfth of your payment to each monthly one. For this loan, that’s an extra $158.02 a month, which pays it off in 24 years 2 months and saves about $87,000 in interest.
Before you pay extra
- Tell your lender it’s for the principal. Some lenders put extra money toward next month’s payment instead of the balance, which doesn’t save any interest. Check your statement to see where it went.
- Check for prepayment charges. Some loans charge a fee for paying off too much early, especially in the first years or during a fixed-rate deal. Your loan documents will say whether yours does, and how much you can overpay without a charge.
- Keep a cushion first. Money paid into the loan is hard to get back out if you need it. Many people build up some emergency savings, and pay off higher-interest debt like credit cards, before paying a mortgage down early.
- Compare it with other uses for the money. Paying off a 6.5% loan early is like earning 6.5% on that money, with no risk. Whether that’s better than saving or investing it depends on your rate and your situation. The compound interest calculator shows how the same money would grow if you saved it instead.
The payment isn’t the whole monthly cost
The payment in all these examples covers only principal and interest. A home also comes with property tax and home insurance. They’re often paid through the lender as part of a single monthly bill, but they don’t reduce the loan. Depending on the loan and the deposit, there may also be mortgage insurance, HOA fees, or service charges. The calculator can add tax and insurance to show the full monthly cost, and the budget planner shows how that fits next to everything else you spend.
Try it with your own loan
Open the mortgage and loan calculator, enter your loan amount, rate and term, then try a few extra monthly payments. The chart shows the interest and principal paid each year, and the table shows exactly what you’d still owe at the end of every year.
This article explains how loans work in general. It isn’t financial advice. Loan terms, fees and rules vary by lender and country, so check the details of your own loan before making a decision.