Two bank accounts both advertise "5% interest." One pays simple interest, the other compounds monthly. Ten years later, the balances differ by hundreds of dollars — on the same rate, same deposit. The words simple and compound describe completely different growth engines, and knowing which one you're getting (or paying) is worth real money.
For the full story on compounding's snowball effect, read the power of compound interest — then run your own comparison with Utilo's free compound interest calculator.
The two formulas, side by side
Simple interest: A = P(1 + rt) — interest is computed only on the original principal, every period, forever. Growth is a straight line.
Compound interest: A = P(1 + r/n)nt — interest is computed on principal plus all previously earned interest. Growth curves upward.
Concrete example — $10,000 at 5% for 20 years:
- Simple: 10,000 × (1 + 0.05 × 20) = 10,000 × 2 = $20,000
- Compound (annual): 10,000 × 1.0520 = $26,533
Same rate, same time — a $6,533 gap, and it widens every year. Over 40 years the gap explodes: $30,000 simple vs $70,400 compound.
When the difference barely matters (and when it dominates)
Over one year, the gap is tiny: $10,000 at 5% gives $10,500 simple vs $10,511 compound (monthly). That's why short-term decisions — a 6-month CD, a 90-day loan — can safely ignore the distinction. The difference is a creature of time: negligible under a year, noticeable at five, decisive at twenty.
Compounding frequency matters too, but less than people think. At 5% over 20 years: annual compounding → $26,533; monthly → $27,126; daily → $27,171. Monthly vs. annual is worth about $600 on $10k — real money, but dwarfed by the rate and the timespan.
Where you'll meet each type
- Compound: savings accounts, CDs, money market accounts, credit cards, mortgages (amortized — functionally compound), investment returns.
- Simple: many auto loans, some personal loans, US Treasury I-bonds' fixed component mechanics, and most "interest-only" arrangements.
The pattern: institutions pay you compound when they must compete for deposits, and charge you compound on revolving debt. Simple interest shows up where regulations or product design keep things, well, simple. Always check the disclosure: look for APY (includes compounding) vs. APR (may not), and the compounding frequency in the fine print.
Which is better? It depends which side you're on
Saving or investing: compound wins, always. Borrowing: simple is cheaper — a simple-interest auto loan costs less than an equivalent compound one. The golden rule: you want your assets compounding and your debts simple (or better yet, gone).
One more asymmetry worth knowing: inflation compounds against your cash. Money sitting at 0% while prices rise 3% yearly loses ~26% of purchasing power per decade. Doing nothing is itself a compounding decision — just in the wrong direction. That's the real argument for putting long-term money to work, which you can model with Utilo's free financial calculators.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest applies only to the original principal; compound interest applies to principal plus all previously earned interest, so it accelerates over time.
Which is better, simple or compound interest?
For savers and investors, compound. For borrowers, simple is cheaper. The gap is tiny short-term and enormous over decades.
Do banks use simple or compound interest?
Most deposit accounts and credit cards compound (usually daily or monthly). Some auto and personal loans use simple interest — check the terms.
How do I calculate both?
Simple: A = P(1 + rt). Compound: A = P(1 + r/n)^(nt). Or use Utilo's free compound interest calculator for instant answers with a year-by-year table.