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Simple vs Compound Interest -- When Each Applies
Published May 1, 2026
Simple vs Compound Interest — When Each Applies
Interest is the cost of borrowing money or the reward for saving it. Whether that interest is calculated simply or with compounding has a dramatic effect on the final amount over time — and choosing the wrong mental model leads to surprises in both directions.
The formulas
Simple interest:
Interest = Principal × Rate × Time
Final = Principal × (1 + Rate × Time)
Compound interest:
Final = Principal × (1 + Rate/n)^(n×t)
Where n is the number of compounding periods per year and t is time in years.
A direct comparison
$10,000 invested at 6% annual rate for 10 years:
| Method | Compounding | Final value | Interest earned |
|---|---|---|---|
| Simple | — | $16,000 | $6,000 |
| Compound | Annually | $17,908 | $7,908 |
| Compound | Monthly | $18,194 | $8,194 |
| Compound | Daily | $18,221 | $8,221 |
The same 6% rate produces 32% more interest under daily compounding than simple interest over a decade. The gap widens dramatically at longer time horizons.
Quick comparison
| Simple interest | Compound interest | |
|---|---|---|
| Growth shape | Straight line | Accelerating curve |
| Typical use | Short-term loans, some bonds, car finance | Savings accounts, mortgages, investments, credit cards |
| Easier for mental math | Yes | No |
| Matches most bank accounts | No | Yes |
When simple interest is used
- Car loans and personal loans from many lenders — interest accrues on the original balance, not the remaining balance (reducing the total interest compared to compound)
- Short-term bridge loans — simple interest is straightforward for a 30- or 60-day note
- US savings bonds (Series I) — technically compound but displayed as simple in some communications
- Short time horizons — for periods under one year, compounding makes little practical difference
When compound interest dominates
- Savings accounts and CDs — interest earned in each period is added to the balance and earns more interest
- Mortgages — most jurisdictions compound monthly, meaning unpaid interest capitalises into the principal
- Credit cards — daily compounding on the carried balance is why high-APR balances grow so fast
- Investment portfolios — returns reinvested generate returns on returns (this is the principle behind long-term wealth building)
- Student loans in some countries — unpaid interest may capitalise at the end of a grace period
The rule of 72
A useful mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money with compound interest.
- 6% → 72/6 = 12 years to double
- 9% → 72/9 = 8 years to double
- 12% → 72/12 = 6 years to double
Use the Interest Calculator to compare simple and compound scenarios side by side at any rate and time horizon, and read What Is Compound Interest? for the full formula walkthrough.