Compound Interest vs Simple Interest: The Real Difference
Simple interest is linear — you earn the same amount every year. Compound interest is exponential — you earn interest on your interest. Here's exactly how big the difference is over time.
The Core Distinction
Simple interest is calculated only on the original principal. Every period, you earn the same fixed amount.
Compound interest is calculated on the principal plus all accumulated interest. Every period, you earn a little more than the last — because your interest balance is now part of the base.
The formulas:
- Simple: A = P × (1 + r × t)
- Compound: A = P × (1 + r/n)^(n×t)
Where P = principal, r = annual rate, t = years, n = compounding periods per year.
Use our Compound Interest Calculator to model any scenario instantly.
A $10,000 Example Over 20 Years at 7%
| Year | Simple Interest Balance | Compound Interest Balance |
|---|---|---|
| 1 | $10,700 | $10,700 |
| 5 | $13,500 | $14,026 |
| 10 | $17,000 | $19,672 |
| 15 | $20,500 | $27,590 |
| 20 | $24,000 | $38,697 |
At year 20, compound interest delivers $38,697 vs simple interest's $24,000 — a difference of $14,697 on the same $10,000 investment at the same 7% rate. No additional money added.
Why Compounding Accelerates Over Time
In year 1, both methods produce $700. But in year 10, the compound account earns $1,297 (on a ~$18,500 balance) while the simple account still earns just $700. By year 20, the compound account is earning over $2,500 per year — still at the same 7% rate.
This acceleration is why Einstein (apocryphally) called compound interest the "eighth wonder of the world." The longer the timeline, the more dramatic the gap.
How Compounding Frequency Affects Returns
Compounding doesn't just happen annually. The more frequently interest compounds, the faster money grows:
| Compounding frequency | $10,000 at 7% after 20 years |
|---|---|
| Annual | $38,697 |
| Quarterly | $39,296 |
| Monthly | $39,497 |
| Daily | $39,543 |
The difference between annual and daily compounding is modest (~$846) at 7%. The frequency matters more at higher rates and over longer time horizons.
Where You Encounter Each Type
Simple interest applies to:
- Most car loans and some personal loans (interest calculated on original balance)
- Short-term bonds
- Some savings accounts that compound but reset periodically
Compound interest applies to:
- Savings accounts and high-yield savings accounts (daily or monthly compounding)
- Investment accounts (returns compound continuously)
- Credit card debt (daily compounding — working against you)
- Mortgages (monthly compounding amortization)
The Danger of Compound Interest on Debt
Compound interest works in reverse when you owe money. A $5,000 credit card balance at 22.99% APR, compounding daily, grows to roughly $6,297 after just one year with no payments — $1,297 in interest on a $5,000 balance in 12 months.
Over 5 years with no payments: ~$15,400. That's the same math that builds wealth in investments, working against you at 3× the typical investment return rate.
The Rule of 72
A useful shortcut: divide 72 by your interest rate to estimate how many years it takes to double your money with compound interest.
- At 6%: 72 ÷ 6 = 12 years to double
- At 8%: 72 ÷ 8 = 9 years to double
- At 10%: 72 ÷ 10 = 7.2 years to double
This rule only works for compound interest. With simple interest at 8%, doubling takes exactly 12.5 years (100% ÷ 8% = 12.5).
Conclusion: Key Takeaways
- Simple interest grows linearly; compound interest grows exponentially
- On $10,000 at 7% over 20 years, compounding delivers $14,697 more than simple interest
- Compounding frequency (daily vs annual) matters, but less than the rate and time horizon
- Compound interest is an ally in investments and a powerful adversary in credit card debt
- The Rule of 72 estimates doubling time: divide 72 by your rate
Calculate compound growth on your savings →
Also see: The 50/30/20 Budget Rule for how to allocate income toward savings that benefit from compounding.