Simple Interest vs Compound Interest
The difference is a straight line versus a curve. Simple interest is paid only on your original principal, so it grows by the same amount every year — principal × rate × time. Compound interest is paid on the principal plus the interest already earned, so it grows exponentially: interest starts earning interest. On $10,000 at 7%, both add $700 in year one — but after 30 years simple interest reaches $31,000 while compound interest reaches about $76,123. Time turns a small edge into a huge one.
The short answer
Both add interest to your money — the difference is what the interest is calculated on:
- Simple interest is paid only on the original principal. It adds the same amount every year, so the balance grows in a straight line: principal × rate × time.
- Compound interest is paid on the principal plus all the interest already earned. Because interest starts earning its own interest, the balance grows exponentially.
For a single year at the same rate they are identical. Over decades they are not close — compound interest can end up more than double the simple result.
Try both: the future value calculator and the compound interest calculator project real balances at any rate and term.
The two at a glance
| Simple interest | Compound interest | |
|---|---|---|
| Charged on | Principal only | Principal + interest earned |
| Growth shape | Straight line | Exponential curve |
| Formula | P × (1 + r × t) | P × (1 + r)t |
| Common on | Some car and short-term loans, certain bonds | Savings, investments, mortgages, credit cards |
The middle row is the whole story: a line that rises steadily versus a curve that bends upward and keeps steepening.
A side-by-side example
Take $10,000 at 7% a year and follow both methods over time (compound interest here uses annual compounding):
| Years | Simple interest | Compound interest | Gap |
|---|---|---|---|
| 1 year | $10,700 | $10,700 | $0 |
| 5 years | $13,500 | $14,026 | $526 |
| 10 years | $17,000 | $19,672 | $2,672 |
| 20 years | $24,000 | $38,697 | $14,697 |
| 30 years | $31,000 | $76,123 | $45,123 |
Year one is a tie — both add $700. From there the paths split, slowly at first and then dramatically. By year 30 the compound balance is more than double the simple one, and the $45,123 gap came entirely from interest earning interest.
Why the gap grows
Simple interest always works from the same $10,000, so it adds exactly $700 a year, forever. Compound interest works from a growing balance: 7% of $10,000 in year one, but 7% of $10,700 in year two, 7% of $11,449 in year three, and so on. Each year the base is bigger, so each year adds a little more than the last.
That is why the compound line curves upward while the simple line stays straight. The effect is small early and enormous late — the same snowball described in how compound interest works. Compounding more often than once a year pushes it further still: at monthly compounding, that same $10,000 reaches about $81,165 after 30 years instead of $76,123.
Where you meet each one
- Simple interest tends to appear on many car loans, some personal and short-term loans, and certain fixed-coupon bonds — places where a predictable, evenly spread charge is wanted.
- Compound interest runs almost everything long-term: savings accounts, CDs, index funds and retirement accounts, mortgages, and credit cards.
- The takeaway: aim to earn compound interest on your savings and, where you can, pay down compounding debt fast — because on a balance you owe, the same curve works against you.
The formulas
Simple interest grows the balance in a straight line:
Compound interest raises the growth factor to a power, which is what bends the curve:
Here P is the principal, r the annual rate as a decimal, and t the number of years. When interest compounds more than once a year, r is divided by the number of periods and t multiplied by it — monthly compounding uses (1 + r/12) raised to 12t, which grows faster still. The future value formula covers the compounding version in full.
Assumptions
- A single lump sum. The example grows one $10,000 deposit with no extra contributions or withdrawals.
- A fixed 7% rate. Both methods use the same constant rate so the comparison is like-for-like; real rates vary.
- Compound figures use annual compounding unless noted. Monthly or daily compounding grows faster, as flagged above.
- No fees or taxes. Both columns are gross of costs, which would lower real returns.
Frequently asked questions
The bottom line
Simple interest is a straight line; compound interest is a curve that keeps steepening. They match for a single year, but over 30 years at 7% the same $10,000 grows to $31,000 with simple interest and about $76,123 with compound — more with monthly compounding. Earn compound interest wherever you can, and treat compounding debt as the same force working in reverse.
See it for yourself with the future value calculator or the compound interest calculator, or read how compound interest works.
Disclaimer: This page is for general educational purposes only and is not financial advice. The figures are illustrative and assume a constant rate; real results vary with rates, compounding frequency, fees and taxes. Consider speaking with a qualified financial professional before making decisions about your own money.