Simple vs Compound Interest: The Difference That Costs (or Makes) You Thousands
Simple and compound interest sound similar but produce wildly different results over time. Here is how each works, with a clear side-by-side example.
Two accounts, same deposit, same interest rate, same number of years, and yet one ends up with thousands more than the other. The only difference is whether the interest is simple or compound. Understanding that distinction is one of the highest-value things you can learn about money.
Simple interest: a flat charge
Simple interest is calculated only on your original amount (the principal). It never changes, because it ignores the interest you have already earned.
Simple interest = P × r × t
Deposit $10,000 at 6% and you earn a flat $600 every year. After 20 years that is $12,000 of interest, for a total of $22,000. Predictable, but it leaves money on the table.
Compound interest: a snowball
Compound interest is calculated on your principal plus all the interest earned so far. Each year's interest joins the balance, so the next year earns a little more. The growth accelerates.
That same $10,000 at 6%, compounded annually, reaches about $32,071 after 20 years, more than $10,000 extra, from the identical deposit and rate.
Seeing the gap
Notice too that compounding frequency matters a little: monthly compounding edges out annual, because interest is added, and starts earning, twelve times a year instead of once.
How the gap widens over time
| Years | Simple (6%) | Compounded yearly (6%) |
|---|---|---|
| 5 years | $13,000 | $13,382 |
| 10 years | $16,000 | $17,908 |
| 20 years | $22,000 | $32,071 |
| 30 years | $28,000 | $57,435 |
At 5 years the difference is minor. By year 30 compound interest has produced roughly double the earnings of simple interest. Time is what separates them.
Credit cards and many loans compound interest against you. An unpaid balance grows the same way savings do, which is why minimum payments can keep you stuck for years. Treat compounding debt as an emergency.
Where you meet each type
- Compound interest (works for you): savings accounts, investments, retirement funds.
- Simple interest (often on debt): some car loans, short-term personal loans, certain bonds.
- Compound interest (works against you): credit cards and revolving debt.
Watch compounding in action. See how a deposit plus monthly contributions grows over any time horizon.
Open the Compound Interest Calculator →The bottom line
Simple interest is a flat fee on your original money; compound interest earns on itself and accelerates. Put your savings where it compounds, keep your debts simple and short, and let time do the heavy lifting on the right side of the ledger.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so the amount earned each period stays constant. Compound interest is calculated on the principal plus all previously earned interest, so the amount grows every period.
What is the simple interest formula?
Simple interest = P × r × t, where P is the principal, r is the annual rate as a decimal, and t is the time in years.
Is compound interest always better?
For money you are saving or investing, yes, it grows faster. For money you owe, compound interest works against you, which is why high-interest debt can snowball.