Simple vs Compound Interest: The Simple Math
- chrispeck8
- Jul 13
- 4 min read
When it comes to loans, the type of interest you pay doesn't just change how much you pay to the bank, but what you don't pay to yourself to create financial freedom. Many people don’t realize that compound interest bearing loans like credit cards and mortgages, even if they seem manageable, can actually cost more than loans with simple interest i.e. car loans, personal loans, etc. We want to explain how that happens and show you with clear examples why you should always prioritize the money you can save in the long run vs what may be on autopilot today.
Understanding Simple Interest and Compound Interest
Interest is the cost of borrowing money. But not all interest works the same way. There are two main types: simple interest and compound interest.
Simple interest is calculated only on the original amount you borrowed, called the principal. The interest does not grow on itself. For example, if you borrow $10,000 at 15% simple interest, you pay 15% of $10,000 every year until the loan is paid off. This does NOT mean you pay $1,500 in total interest. It means you will pay 10% of the balance every month and the rest goes towards the principal.
Compound interest is calculated on the principal plus any interest that has already been added. Sometimes it even accrues daily instead of monthly or annually. This means the interest grows over time, making the total cost higher. Credit cards and many mortgages use compound interest.
Why Smaller Credit Cards and Mortgages Can Cost More
Credit cards and mortgages often use compound interest, which means the interest you owe can grow quickly if you only make minimum payments. Even if the interest rate looks lower than a car loan, the way interest is calculated can make the total cost much higher.
For example, a $10,000 credit card balance with a 15% interest rate compounded monthly can end up costing you TWICE as much in interest than the exact same simple interest car loan at 10%. This happens because the credit card interest compounds daily, and if you only pay the minimum, the balance grows.
Mortgages, even though they are large loans, also use compound interest, making them the costliest option. The adage is when you buy a house, be prepared to pay for two. The largest mistakes we see people make is coming into a little money and deciding to pay off their car at 9.99% interest vs their house (5-10x their car loan) at 4.9%. A half that rate, putting that $20k towards principal on the house saves you twice the interest you will still pay on that car.
Why Credit Cards Can Trap You in Debt
Credit cards use compound interest and often have high rates. The average rate nationally was closer to 23 just last month. If you only pay the minimum, most of your payment goes to interest, and the principal barely decreases. This causes your balance to grow, and you end up paying much more over time.
Even a small balance on a credit card can become a big problem if you don’t pay it off quickly. The compounding effect means the interest builds on itself, making it harder to escape debt.
Mortgages and Compound Interest
Mortgages are usually large loans with long terms, often 15 to 30 years. They use compound interest, which means the interest accumulates on the principal and the unpaid interest. While mortgage rates are often lower than credit cards, the long term means you pay a lot of interest overall.
Smaller mortgages and home equity loans often have higher rates or fees as well, making them more expensive than simple interest loans. If you can afford to pay more than the minimum, you can reduce the interest paid and shorten the loan term. But if you only pay the minimum, the interest compounds and costs add up.
Real Products That Show These Differences
To make this clearer, I want to mention two types of loans that illustrate these points well.
Car Loan: This is a simple interest loan with fixed payments.
$10,000 car loan at 15% interest at 6 years
Monthly Payment - $218
Total interest - $5442
Credit Card:
$10,000 car loan at 15% interest at 8.5 years
Monthly Payment - $223
Total interest - $12,211
If you pay the same monthly amount on both loans, you will pay off the car loan faster and save over $1k a year. Plus the remaining 18 months payments to pay off the credit card so a total of just at $10k over 6 years. I don't know about you, but that's an electricity bill, or internet, or utilities, or savings every month.
This shows how important it is to understand the type of interest on your loans.
How to Use This Knowledge to Your Advantage
Knowing the difference between simple and compound interest helps you make smarter borrowing decisions.
Choose simple interest loans when possible, especially for large purchases like cars or personal loans.
Avoid carrying balances on credit cards. Pay them off quickly to avoid compounding interest.
If you have a mortgage, try to pay extra when you can to reduce the principal and interest.
Work with trusted financial advisors who can help you find the best money savings options for your situation.
If you want to explore loan options that fit your needs, consider checking out our debt management or counseling options offered by Peck Family Financial. They provide clear terms and support to help you make the best financial decisions for your family and your future.
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