Simple vs Compound Interest -- When Each Applies

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.