How loan amortization works
Why a 30-year mortgage can cost more in interest than you borrowed, how every payment is split, and the few decisions that change the total by tens of thousands.
In short
- An amortized loan is repaid in equal monthly payments. Each payment covers that month's interest first; the rest reduces the balance.
- Because interest is charged on the balance, early payments are mostly interest. On a 30-year mortgage at 6.5%, 85% of the first year's payments go to interest.
- The term matters as much as the rate. The same $320,000 costs $408,142 in interest over 30 years and $181,758 over 15.
- Extra payments go straight to principal. An extra $100 a month on that mortgage saves $61,698 and ends the loan almost 4 years sooner.
What amortization means
Amortization is the way most loans are paid back: in equal installments over a fixed term, until the balance reaches exactly zero on the last payment. Mortgages, car loans, personal loans and most student loans work this way.
The word comes from the old French amortir, "to kill off". Each payment kills off a little of the debt. What surprises most borrowers is how unevenly that happens. The payment stays the same every month, but what it pays for changes completely over the life of the loan.
The three numbers behind every payment
- Principal: the amount you borrow. For a home, that is the price minus your down payment.
- Interest rate: the yearly cost of borrowing, quoted as a percentage. Lenders divide it by 12 to get a monthly rate.
- Term: how long you have to repay, usually in years. A 30-year mortgage has 360 monthly payments.
Change any one of these and both the monthly payment and the total interest change. The rest of this guide shows by how much.
How amortized loans differ from other loans
Not every loan amortizes. With an interest-only loan you pay only the interest for a set period, so the balance doesn't fall at all. A balloon loan has small payments and one large final payment. Credit cards have no fixed term: the minimum payment changes with the balance, which is why card debt can last for decades. Amortized loans are the predictable kind: you know the exact payment and the exact payoff date on day one.
How the monthly payment is calculated
Lenders use one standard formula, the amortization formula. It finds the single payment that, repeated every month, pays all the interest and the full principal by the end of the term:
M = P × r(1 + r)n ÷ ((1 + r)n − 1)
- M = monthly payment
- P = principal (amount borrowed)
- r = monthly interest rate (yearly rate ÷ 12, as a decimal)
- n = number of monthly payments
Worked example, step by step
Take a $400,000 home with a 20% down payment, financed for 30 years at 6.5%.
- Principal: $400,000 − $80,000 down = $320,000.
- Monthly rate: 6.5% ÷ 12 = 0.541667%, or r = 0.0054167 as a decimal.
- Number of payments: 30 years × 12 = n = 360.
- Growth factor: (1 + r)n = 1.0054167360 ≈ 6.9918.
- Top of the formula: 320,000 × 0.0054167 × 6.9918 ≈ 12,119.12.
- Bottom of the formula: 6.9918 − 1 = 5.9918.
- Payment: 12,119.12 ÷ 5.9918 ≈ $2,022.62 a month.
That figure covers principal and interest only. Property tax, home insurance and any mortgage insurance are added on top, which is why the amount a lender quotes as your total monthly payment is usually higher.
Checking the first month by hand
You can verify any amortization schedule with simple arithmetic. In month one, interest is the balance times the monthly rate: $320,000 × 0.0054167 = $1,733.33. The rest of the payment, $2,022.62 − $1,733.33 = $289.28, reduces the balance to $319,710.72. Month two repeats the process on the new, slightly smaller balance: $1,731.77 of interest and $290.85 of principal.
Why early payments are mostly interest
Interest is always charged on the balance you still owe. At the start, the balance is as high as it will ever be, so the interest portion of each payment is at its largest. Only what is left over reduces the debt.
In the first year of our example you pay $24,271. Of that, $20,695 is interest and only $3,577 reduces the loan. Each month the balance falls a little, so the interest falls a little and the principal portion grows. This pattern is called front-loaded interest, and it isn't a trick by the lender; it is simply what happens when interest is calculated on a large balance.
| Year | Interest paid | Principal paid | Balance at year end |
|---|---|---|---|
| 1 | $20,695 | $3,577 | $316,423 |
| 10 | $17,861 | $6,410 | $271,284 |
| 20 | $12,014 | $12,257 | $178,129 |
| 30 | $833 | $23,438 | $0 |
The crossover point
At some point, the principal portion of the payment becomes larger than the interest portion. On this loan, that happens in month 233, about 19 years in. After the crossover, the balance falls quickly: the final payment is $2,011.72 of principal and just $10.90 of interest.
Front-loading is also why refinancing or selling early can disappoint. After 10 years of payments on this loan you have paid $242,714 in total, yet only $48,716 of that has reduced the balance. The other $193,998 was interest.
How the balance falls over time
Plot the remaining balance and you get a curve, not a straight line. It sags slowly for years and then drops steeply at the end.
On this loan you don't owe less than half of the original $320,000 until month 257, about 21 years and 5 months into a 30-year term. That matters if you expect to sell or move: your home equity in the early years comes mostly from your down payment and any rise in the home's value, not from your monthly payments.
How the loan term changes the total cost
A longer term lowers the monthly payment because the principal is spread over more payments. But every extra year is another year of interest on a large balance. Here is the same $320,000 at 6.5% over three terms:
| Term | Monthly payment | Total interest | Total repaid |
|---|---|---|---|
| 15 years | $2,787.54 | $181,758 | $501,758 |
| 20 years | $2,385.83 | $252,600 | $572,600 |
| 30 years | $2,022.62 | $408,142 | $728,142 |
Going from 30 to 15 years raises the payment by about $765 a month, or 38%, but cuts total interest by $226,384, more than half. Shorter terms often come with lower rates too, which widens the gap further.
The right term depends on your budget, not only on the math. A longer term gives you a lower required payment and more flexibility. A middle path many people use is to take the longer term and voluntarily pay more each month, which keeps the safety net while cutting interest.
How the interest rate changes the total cost
On a long loan, small rate differences turn into large sums because they apply to a big balance for many years. The same $320,000 over 30 years:
| Rate | Monthly payment | Total interest |
|---|---|---|
| 5.5% | $1,816.92 | $334,093 |
| 6.5% | $2,022.62 | $408,142 |
| 7.5% | $2,237.49 | $485,495 |
Each percentage point is worth about $210 a month here, and roughly $75,000 in total interest. That is why it pays to compare at least three lenders, improve your credit score before applying, and ask whether paying discount points up front lowers the rate enough to be worth it over the years you expect to keep the loan.
What extra payments really do
Any amount you pay above the scheduled payment goes straight to principal. That lowers the balance on which every future month's interest is calculated, so the effect compounds in your favor.
- $100 extra a month saves $61,698 in interest and pays the loan off 3 years and 10 months early.
- $300 extra a month saves $138,446 and pays it off 8 years and 10 months early.
- A one-time $10,000 lump sum after the first year saves $50,537 and ends the loan 2 years and 5 months early.
Monthly extras or a lump sum?
The earlier extra money arrives, the more interest it saves, because it removes balance that would otherwise be charged interest for decades. A lump sum early in the loan is powerful; small monthly extras are easier to keep up and add up to more over time. Many borrowers do both: a fixed monthly extra plus any bonus or tax refund.
Check these before you pay extra
- Prepayment penalties. Some loans charge a fee for paying early. Read your contract or ask the lender.
- Where the money goes. Tell the lender to apply extra payments to principal, and choose a shorter term rather than a lower payment if you're given the option.
- Higher-interest debt first. Paying off a credit card at 22% beats prepaying a mortgage at 6.5%.
- Your emergency fund. Money paid into a loan is hard to get back. Keep a cash cushion first.
Car loans and personal loans
Shorter loans amortize the same way, but front-loading is much milder because there are fewer years of interest. A $25,000 car loan at 8.5% over 5 years costs $512.91 a month. In month one, $177.08 is interest and $335.83 is principal, so you are already paying down the loan faster than you are paying interest.
Over the first year, interest makes up 32% of your payments, compared with 85% on the 30-year mortgage. The total interest over the 5 years is $5,775. Stretch the same loan to 7 years and the payment drops to $395.91, but total interest rises to $8,257, and you may owe more than the car is worth for longer.
Before signing, compare offers by APR rather than the interest rate. APR includes the lender's fees, so it shows the true yearly cost and makes offers comparable.
How to read an amortization schedule
An amortization schedule is the table of every payment over the life of the loan. Your lender must be able to give you one, and the calculators on this site build one for you instantly. Each row usually shows:
- Payment number or date: which month the row describes.
- Payment: the fixed amount you pay, sometimes with tax and insurance added.
- Interest: the previous balance × the monthly rate.
- Principal: the payment minus the interest.
- Remaining balance: the previous balance minus the principal.
Two quick checks catch most errors: interest in any row should equal the previous balance times the monthly rate, and the balance in the final row should be zero. If your lender's schedule differs slightly from a calculator, it is usually because of a different first-payment date, fees or daily interest.
Common mistakes borrowers make
- Comparing loans by monthly payment. The lowest payment is often the most expensive loan. Compare total cost.
- Ignoring the term. Five extra years on a loan can cost more than a slightly higher rate.
- Assuming early payments build equity. In the first decade of a 30-year mortgage, most of what you pay is interest.
- Refinancing into a new 30-year loan late in the term. It restarts front-loaded interest. Keep the remaining term the same or shorter.
- Prepaying while carrying expensive debt. Pay the highest rate first.
Common questions
Does amortization mean I pay more interest at the beginning?
Yes. The interest rate stays the same, but interest is charged on the balance, which is highest at the start. So the interest share of each payment starts high and falls every month.
Can I calculate a loan payment without a calculator?
Yes, with the formula M = P × r(1 + r)n ÷ ((1 + r)n − 1). A phone calculator with a power key is enough. The worked example above shows each step.
Is it better to choose a 15-year or a 30-year mortgage?
A 15-year mortgage costs far less in total interest but has a much higher payment. A 30-year mortgage is more flexible. If you can afford the higher payment comfortably and still save for emergencies, the shorter term usually wins on cost.
What happens to amortization if I make extra payments?
Extra payments reduce the principal immediately, so every later month is charged less interest. Your regular payment stays the same, but more of it goes to principal, and the loan ends earlier.
Do variable-rate loans amortize?
Yes, but the schedule is recalculated whenever the rate changes. The payment is set so the remaining balance is repaid over the remaining term at the new rate.
Amortization comes down to one idea: interest follows the balance. Bring the balance down faster with a bigger down payment, a shorter term or extra payments, and the total interest falls with it. Put your own numbers into the calculators below and see the year-by-year split before you sign.