How Loan Interest Is Calculated: Amortization Explained With Real Numbers

When you take out a personal loan or a car loan, the lender gives you one number to focus on: the monthly payment. But inside that payment, the split between interest and principal changes every single month. Understanding how that works makes it much easier to compare offers, decide on a loan term, and know when extra payments are worth it.

We’ll use one example throughout: $25,000 borrowed at 7.5% a year, repaid over 5 years. You can follow along in our loan calculator.

Step 1: Turn the yearly rate into a monthly rate

Loan rates are quoted per year, but payments are monthly, so lenders divide the yearly rate by 12:

Each month, the interest you owe is simply your current balance multiplied by that monthly rate.

Step 2: Work out the fixed payment

An amortizing loan has the same payment every month, set so the balance reaches zero with the final payment. The standard formula is:

With P = $25,000, r = 0.00625 and n = 60 payments, the result is $500.95 a month. You don’t need to do this by hand — but it helps to know the payment is fixed, while what it pays for is not.

If you want to try it yourself, our guide on how to calculate a loan payment works through the formula by hand and in Excel.

Step 3: Split each payment into interest and principal

Every month the lender takes that month’s interest out of your payment first. Whatever is left reduces the balance:

In month 1, interest is $25,000 × 0.00625 = $156.25. The remaining $344.70 of the payment lowers the balance to $24,655.30. In month 2, interest is calculated on that new, smaller balance, so it drops slightly to $154.10 — and a little more goes to principal. That shift continues all the way to the end.

Add it all up and you pay $5,056.92 in interest over the five years: $1,729.81 in the first year, but only $237.25 in the last.

Interest-only vs. amortizing loans

Some loans let you pay only the interest for a set time. On our example, an interest-only payment would be $25,000 × 0.00625 = $156.25 a month. That is far lower than $500.95, but none of it reduces the balance. After any number of interest-only months, you still owe the full $25,000.

Say the loan had 2 interest-only years, then 3 years to repay at the same 7.5%:

• Months 1–24: $156.25 a month, all of it interest ($3,750 in total).
• Months 25–60: $777.66 a month, the same as a 3-year loan.
• Total interest: $6,745.60, which is $1,688.68 more than the plain 5-year loan.

The low start makes the loan look cheap, and then the payment jumps by $621.41. Interest-only periods are common on some lines of credit, construction loans and investment property loans. If you take one, plan for the higher payment from day one.

Interest rate vs. APR

The interest rate is the cost of the money itself. The APR (annual percentage rate) also includes most required fees, such as an origination fee, expressed as a yearly rate. Two loans with the same interest rate can have different APRs if one charges higher fees, so the APR is the better number for comparing offers side by side.

Here is what a fee does to our example. Say the lender charges a 3% origination fee ($750) and takes it out of the money you receive. You still pay $500.95 a month for 60 months, but only $24,250 reaches your account. That raises the APR from 7.5% to about 8.79%. Our guide to APR vs. interest rate shows how to compare offers like this.

Watch out for “flat rate” quotes

Some lenders and dealers quote a flat or add-on rate. Instead of charging interest on the shrinking balance, they charge it on the full original amount for the whole term:

That is $4,318 more interest than the amortizing loan at the “same” 7.5%. Measured properly, a 7.5% flat rate over five years is equivalent to an APR of about 13.3%. If an offer doesn’t state an APR, ask for one before you sign.

Daily simple interest on car loans

Many car loans, and some personal loans, use daily simple interest. Instead of one twelfth of the yearly rate, the lender charges a daily rate on the balance for each day since your last payment:

On a $25,000 balance, that is $143.84 for a 28-day gap, $154.11 for 30 days and $159.25 for 31 days. The monthly method charges $156.25 however many days pass. If you pay on the same day each month, the two methods cost almost the same over a year.

The difference shows up when you pay early or late. Pay 10 days late on a balance of $24,655.30, and about $50.66 of extra interest builds up. That comes out of your payment before principal, so the balance falls more slowly, even if no late fee applies. Paying a few days early has the opposite effect. Credit cards use a similar daily rate; see how credit card interest is calculated. You can test car loan terms in our car loan calculator.

How extra payments cut your interest

Because interest is charged on the remaining balance, anything that lowers the balance sooner lowers every future interest charge. On our example loan:

A one-time lump sum works the same way, and timing matters. An extra $2,000 paid with payment 12 clears the loan in 55 months and saves $660.75 in interest. The same $2,000 paid with payment 48 saves only $133.13, because by then most of the interest has already been charged.

Two things to check first: whether your loan has a prepayment penalty, and whether your lender applies extra money to principal (what you want) or treats it as an early payment of next month’s bill.

How the term changes the total cost

A longer term spreads the balance over more months, so each payment is smaller — but interest is charged for longer:

• 3 years: $777.66 a month, $2,995.60 total interest
• 5 years: $500.95 a month, $5,056.92 total interest
• 7 years: $383.46 a month, $7,210.38 total interest

The 7-year loan is $117 a month cheaper than the 5-year loan, but more than $2,150 more expensive overall. If you can comfortably afford the shorter term, it nearly always wins.

Common mistakes

• Shopping by monthly payment. A longer term always lowers the payment. The 7-year loan above costs $2,153.46 more in interest than the 5-year loan.
• Ignoring fees. A 3% fee on our example adds about 1.3 points to the true yearly cost.
• Assuming extra money goes to principal. Some lenders hold it for next month’s bill. Ask, and check your next statement.
• Paying late on a daily-interest loan. Even with no late fee, the extra days of interest slow your payoff.
• Comparing a flat rate with an APR. A 7.5% flat rate is closer to a 13.3% APR than to a 7.5% one.

A quick checklist before you sign

1. Compare offers by APR, not by monthly payment.
2. Confirm the loan is amortizing, not flat-rate.
3. Ask about origination fees and prepayment penalties.
4. Choose the shortest term you can afford with a safety margin.
5. Run the numbers yourself in the loan calculator, including any extra payments you plan to make.

Frequently asked questions

Why is so much of my early payment interest?

Interest is charged on the balance, and the balance is highest at the start. On our example, the first payment is $156.25 interest and $344.70 principal, while the last payment has only $3.11 of interest.

Does paying extra lower my monthly payment?

Usually not. On most personal and car loans, extra payments shorten the loan and cut total interest, but the required payment stays the same. Some lenders will recalculate the payment on request, so ask yours if a lower payment matters more to you than an earlier payoff.

Is daily simple interest worse than monthly interest?

Not if you pay on time. Over a year, both methods charge about the same; daily interest costs more only when you pay late, and it saves you a little when you pay early.

How do I find my rate if I only know the payment?

Use the RATE function in Excel or Google Sheets: =RATE(number of payments, −payment, loan amount) × 12. For our example, =RATE(60, -500.95, 25000) × 12 returns about 7.5%.

This article is for general education and is not financial advice. Rules, rates and products vary by lender and country. See our disclaimer.