The one-sentence difference
Simple interest is calculated only on your original principal. Compound interest is calculated on your principal plus all the interest you've already earned — so you earn interest on your interest.
That small distinction is the engine behind nearly all long-term wealth building. Let's break each one down.
How simple interest works
With simple interest, you earn the same fixed amount every period because it's always based on the same starting balance. The formula is:
Interest = Principal × Rate × Time
Say you deposit $10,000 at 5% simple interest. Every year you earn a flat $500 (5% of $10,000) — no more, no less. After 10 years you've earned $5,000, for a total of $15,000. The growth is perfectly linear: a straight line.
How compound interest works
Compound interest recalculates your balance each period and pays interest on the new, larger total. The formula is:
A = P × (1 + r/n)n·t
where P = principal, r = annual rate, n = times compounded per year, t = years.
Take that same $10,000 at 5%, compounded annually. Year one you earn $500 — identical to simple interest. But year two you earn 5% of $10,500, which is $525. Year three, 5% of $11,025, which is $551.25. Each year the gain is a little bigger because the base keeps growing. After 10 years you'd have about $16,289 — roughly $1,289 more than simple interest, with no extra effort.
Side by side over time
Here's $10,000 at 5%, simple vs. compound (annually):
- After 5 years: Simple $12,500 · Compound ≈ $12,763
- After 10 years: Simple $15,000 · Compound ≈ $16,289
- After 20 years: Simple $20,000 · Compound ≈ $26,533
- After 30 years: Simple $25,000 · Compound ≈ $43,219
At 30 years the compound balance is nearly double the simple-interest balance. The longer the time horizon, the wider the gap — which is why time is compound interest's best friend.
Where you'll meet each one in real life
Where interest usually compounds (in your favor)
- Savings accounts and CDs — interest is typically compounded daily or monthly.
- Investment accounts — reinvested dividends and gains compound over time.
- Long-term retirement savings — reinvested earnings can compound over decades.
Where simple interest often applies
- Many auto loans and some personal loans — interest is often calculated on the principal balance.
- Most bonds — coupon payments are usually based on the bond's face value.
The warning label: compounding can work against you
Compound interest is a wonderful servant but a terrible master. On credit card debt, interest typically compounds — often daily — on a balance that already includes unpaid interest. At a 22% APR, an unpaid balance can balloon frighteningly fast, the same snowball effect working in reverse. The takeaway: let compounding work for you in savings and investments, and pay down high-interest debt before it compounds against you.
See it on your own numbers
Want to watch compounding pull ahead of simple interest with your own principal, rate, and contributions? Our free compound interest calculator shows the exact growth — including a year-by-year breakdown.