Compound Interest vs. Simple Interest: How They Differ and Why It Matters

Simple Interest

Simple interest is calculated only on the original principal. Formula: Interest = Principal × Rate × Time. Example: $10,000 at 5% for 10 years = $10,000 × 0.05 × 10 = $5,000 in interest. Total = $15,000. The interest amount is the same each year ($500).

Compound Interest

Compound interest is calculated on principal plus previously earned interest. You earn "interest on interest." Formula: A = P(1 + r/n)^(nt) where P = principal, r = annual rate, n = compounding frequency, t = years. Same example ($10,000 at 5%, compounded annually, 10 years): A = $10,000 × (1.05)^10 = $16,288. Extra $1,288 from compounding.

Compounding Frequency Matters

Annual compounding: $10,000 → $16,288 in 10 years
Monthly compounding: $10,000 → $16,470 in 10 years
Daily compounding: $10,000 → $16,487 in 10 years

Compounding frequency matters most over long periods. The difference between monthly and daily compounding is minimal; the difference between no compounding and daily compounding over 30+ years is enormous.

The Rule of 72

Quick mental math for compounding: 72 ÷ interest rate = years to double. At 6%, money doubles every 12 years. At 8%, every 9 years. At 10%, every 7.2 years. This rule helps quickly evaluate investment growth potential and illustrates why starting early compounds dramatically more than starting later.

Compound Interest Working Against You

Credit card debt compounds (usually daily). Credit card at 22% APR: $5,000 balance doubles to $10,000 in roughly 3.3 years without payments. Understanding compounding explains both the magic of long-term investing and the trap of high-interest debt.

Frequently Asked Questions

How often does a savings account compound interest?

Most high-yield savings accounts and money market accounts compound daily and credit interest monthly. The difference between daily and monthly compounding on a savings account is negligible for most balances — focus on finding the highest APY rate rather than optimizing compounding frequency at current interest rates.

Is simple interest ever better than compound interest?

For borrowers, yes — simple interest loans (some car loans, personal loans) cost less than equivalent compound interest loans. For savers, compound interest is always better. Understanding which type applies to each financial product helps you compare costs and returns accurately.