Whether you are taking out a mortgage, financing a car, or borrowing for a business expense, every loan comes down to the same question: how much will this actually cost? The monthly payment number your bank quotes is only part of the story. The total interest paid over the life of the loan — which can equal or exceed the original amount borrowed — is what determines the true cost. This guide explains how that number is calculated and what levers you have to reduce it.
The monthly payment for a fixed-rate amortising loan is calculated with this formula:
M = P × (r(1+r)ⁿ) ÷ ((1+r)ⁿ − 1)
Where:
This looks more intimidating than it is. Take a practical example: a $25,000 car loan at 7% interest over 5 years (60 months).
That is, you pay nearly $4,700 in interest on a $25,000 loan — about 18.8% of the original amount. Use Criply's loan calculator to run any scenario instantly without the arithmetic.
Amortization is the process of paying off a loan with regular equal payments where the split between principal and interest changes over time. In the early months of a loan, most of your payment goes to interest because the balance is high. As you pay down the principal, the interest portion shrinks and more goes to principal each month.
Here is a simplified view of the first three months on that same $25,000 car loan:
By month 60, almost the entire payment goes to principal. The amortization schedule shows every single month's breakdown and helps you understand exactly where your money is going.
Because interest is charged on the outstanding balance, reducing the principal faster saves a compounding amount of interest over the life of the loan. Even a modest extra payment early in a long loan has an outsized effect.
On a 30-year $250,000 mortgage at 6.5%, the standard monthly payment is around $1,580. Adding just $200 extra to principal each month:
The mechanism is straightforward: every extra dollar of principal reduces the balance that future interest is charged on. Earlier payments have a larger compounding effect. A $200 extra payment in month 1 saves more total interest than the same $200 paid in month 300 — because it reduces the balance for all the months in between.
This is why financial advisers consistently recommend paying extra toward the mortgage principal when cash flow allows, particularly in the first decade of a long loan.
Mortgages are typically the largest loans most people take, with terms of 15 to 30 years. Because of the long repayment period, a relatively small difference in interest rate has a dramatic effect on total cost. On a $300,000 mortgage, the difference between a 6% and 7% rate over 30 years is approximately $70,000 in total interest. Fixed-rate mortgages lock in the rate; variable (adjustable) rate mortgages start lower but can increase. For stability, most first-time buyers choose fixed.
Personal loans are typically unsecured (no collateral) and come with higher interest rates than secured loans — often 8–24% depending on credit score. Terms are usually 1–7 years. Because rates are high and terms are shorter, total interest as a percentage of principal can be significant. A $10,000 personal loan at 15% for 3 years costs around $2,480 in interest — nearly 25% of what was borrowed.
Auto loans are secured against the vehicle. Rates are lower than personal loans (typically 4–12% depending on credit and whether the car is new or used) with terms of 3–7 years. A key consideration with auto loans is depreciation: a new car loses 20–30% of its value in the first year. Borrowing more than the car's resale value — being "underwater" on the loan — can create problems if the car is written off or you need to sell early.
When comparing loan offers, look at:
Criply's loan calculator lets you model any combination — change the rate, term, or amount and see the monthly payment, total interest, and full amortization schedule update in real time. You can also download the schedule as a CSV for further analysis.
7 practical ways to work with PDFs faster. Free, instant download.
Use our free Loan Calculator tool — works in your browser, nothing to install.
Loan Calculator — Free