Simple vs. Compound Interest Explained
Both types of interest make your money grow (or make your debt more expensive), but they work very differently over time. Here's a clear, no-jargon breakdown of each — with the formulas, real examples, and a quick-reference comparison table.
1. What is simple interest?
Simple interest is calculated only on the original principal — the amount you initially deposited or borrowed. It doesn't change over time because past interest is never added back into the calculation.
Simple interest formula
I = P × r × t
A = P + I
I = total interest earned
P = principal (starting amount)
r = annual interest rate (as a decimal)
t = time in years
A = total amount (principal + interest)
Example
$5,000 at 4% for 10 years: I = 5,000 × 0.04 × 10 = $2,000.
Total amount: $5,000 + $2,000 = $7,000.
2. What is compound interest?
Compound interest is calculated on the principal plus all previously accumulated interest. Each compounding period, your balance grows and the next interest calculation uses the new, larger balance. This creates an exponential growth curve.
Compound interest formula
A = P × (1 + r/n)n×t
n = number of times interest compounds per year
All other variables are the same as simple interest.
Example (same numbers, compounded annually)
$5,000 at 4% for 10 years, compounded annually:
A = 5,000 × (1.04)10 = $7,401.22.
Interest earned: $7,401.22 − $5,000 = $2,401.22.
3. Side-by-side comparison
Here's the same $5,000 at 4% — simple versus compound — at different time horizons:
| Years | Simple interest | Compound interest | Difference |
|---|---|---|---|
| 1 | $5,200 | $5,200 | $0 |
| 5 | $6,000 | $6,083 | $83 |
| 10 | $7,000 | $7,401 | $401 |
| 20 | $9,000 | $10,956 | $1,956 |
| 30 | $11,000 | $16,217 | $5,217 |
Notice the pattern: in year 1 they're identical. By year 10 the gap is meaningful. By year 30 compound interest has earned you almost $5,217 more — and that's with a modest 4% rate and no additional contributions.
4. 10-year growth comparison
Here's what the year-by-year growth looks like for $5,000 at 4%:
| Year | Simple balance | Simple interest earned | Compound balance | Compound interest earned |
|---|---|---|---|---|
| 0 | $5,000 | — | $5,000 | — |
| 1 | $5,200 | $200 | $5,200 | $200 |
| 2 | $5,400 | $200 | $5,408 | $208 |
| 3 | $5,600 | $200 | $5,624 | $216 |
| 5 | $6,000 | $200 | $6,083 | $234 |
| 7 | $6,400 | $200 | $6,580 | $253 |
| 10 | $7,000 | $200 | $7,401 | $284 |
With simple interest, you earn a flat $200 every single year. With compound interest, the annual earnings start at $200 but grow steadily — by year 10, you're earning $284 per year instead of $200. That accelerating growth is the hallmark of compounding.
5. When does each type apply?
Simple interest
- Short-term personal loans
- Auto loans (many use simple interest)
- Some student loans
- Treasury bonds and certain government securities
- Interest-only commercial loans
Compound interest
- Savings accounts and CDs
- Credit cards (compounds daily!)
- Mortgages
- Most investment accounts
- Business loans and lines of credit
- Student loan interest after capitalization
💡 Pro tip for borrowers
Compound interest works against you on debt. Credit card debt at 24% APR compounded daily is one of the most expensive forms of borrowing. Paying off compound-interest debt should be a top priority.
6. What it means for borrowers
When you're earning interest, compound interest is your best friend — it accelerates your wealth. But when you're paying interest on debt, compound interest works against you:
Credit card example
A $5,000 credit card balance at 24% APR compounded daily, with only minimum payments ($100/month):
- Time to pay off: ~9 years
- Total interest paid: ~$5,840
- Total paid: ~$10,840 — more than double the original balance
This is why financial advisors stress paying off high-interest debt before investing. The “return” you get by eliminating 24% compound interest on debt is guaranteed — no investment can reliably match that.
7. Key takeaways
Simple interest = interest on the original principal only. Predictable, flat, linear growth.
Compound interest = interest on principal + accumulated interest. Exponential growth that accelerates over time.
Time is the key variable. Short-term, the difference is small. Long-term, compound interest dramatically outperforms.
For savers/investors: compound interest is your superpower. Start early and let time work for you.
For borrowers: compound interest is your biggest risk. Prioritize paying off high-interest debt.
8. Calculate it yourself
Plug your own numbers into our free calculators and see the difference instantly — with year-by-year breakdowns and interactive charts.