Put $10,000 in an account at 6% and leave it alone for 30 years. Counted as simple interest, it earns $18,000. Counted as compound interest, it earns $47,435 — from the same deposit at the same rate. That $29,435 gap is entirely the difference between the two ways of counting, and nothing else.
The one difference that matters
Simple interest is calculated on your original amount and nothing else. Compound interest is calculated on your original amount plus every bit of interest already added. Whether past interest earns its own interest — that single rule is the whole story.
The formulas make it concrete:
- Simple:
Interest = Principal × rate × years - Compound:
Total = Principal × (1 + rate ÷ n)^(n × years), wherenis how many times a year it compounds.
Simple interest grows in a straight line — the same dollar amount every year. Compound interest curves upward, because the base it's calculated on keeps getting bigger. Early on the two look almost identical. The distance between them is a late-game phenomenon.
Same money, same rate, side by side
Take $10,000 at 6% for 5 years.
Simple: 10,000 × 0.06 × 5 = $3,000 in interest. You end with $13,000.
Compound, once a year: 10,000 × 1.06⁵ = $13,382. That's $3,382 in interest — only $382 more than simple.
At five years the gap is small enough to ignore. The point of compounding is that it doesn't stay small. Watch the same deposit stretch out:
- 10 years: simple earns $6,000; compound earns $7,908. Gap: $1,908.
- 30 years: simple earns $18,000; compound earns $47,435. Gap: $29,435.
Compound earned more than 2.5 times the interest over 30 years, with no extra deposits. The curve does all its work at the end: interest credited in year 30 is calculated on a balance that has already grown past $54,000, so a single year adds more than $3,000 — more than the entire first five years of simple interest combined.
A quick way to feel the curve without a calculator is the Rule of 72: divide 72 by the rate to estimate the doubling time. At 6%, compound money doubles in about 12 years (72 ÷ 6), and 1.06¹² does work out to 2.01. Simple interest at 6% doesn't double your money from interest alone until year 17.
Where you'll actually run into each
Simple interest shows up more than most people expect:
- Most U.S. auto loans are simple-interest loans. Interest accrues on the outstanding principal balance, so paying early genuinely lowers what you owe — there's no interest-on-interest waiting to catch up.
- Bonds pay simple interest on face value. A coupon doesn't compound unless you take the cash and reinvest it yourself.
- Short-term personal notes and many fixed installment loans use it too.
Compound interest runs the accounts that grow — or grow against you:
- Savings accounts, CDs, and money-market accounts compound, often daily.
- Credit cards compound daily on any balance you carry.
- Mortgages charge interest monthly on the balance you still owe, so a 6% stated rate works out to roughly 6.17% once you're paying it twelve times a year.
One honest caveat on mortgages: as long as you pay on schedule, you never stack interest-on-interest the way a growing savings balance does. Each month's interest is billed and paid, not rolled into next month's balance — so a current mortgage behaves a lot like simple interest on a shrinking principal. It only compounds against you if you miss payments and let unpaid interest capitalize.
The same force, working for you and against you
Here's the credit-card version of the exact same math. A $5,000 balance at 22% APR that you don't touch for a year does not cost you $1,100. Cards compound daily, so the effective annual rate is closer to 24.6% — about $1,230. That extra $130 is interest charged on interest that was itself charged days earlier. Carry the balance longer and the gap widens the same way a savings account would, just pointed in the wrong direction.
That's the asymmetry worth internalizing: compounding is the best feature of a savings account and the worst feature of a credit card, and it is the identical formula. The only thing that flips is who's holding the balance that keeps growing.
When the simple calculation is right — and when it lies to you
The Simple Interest Calculator is the correct tool when interest genuinely doesn't compound: a car loan, a bond coupon, a short-term note, or a quick "what does this roughly cost per year" estimate. It runs entirely in your browser — nothing you type gets uploaded or stored — so it's fine to plug in real loan numbers.
It's the wrong tool for anything that compounds. Point it at a savings account or a credit-card balance and it will understate the figure, and the error grows with time. For those, reach for compound math instead.
Three more edges worth knowing before you trust any single number:
- Rate beats structure over short spans. A simple-interest loan at 4% costs less than a compound one at 8%. "Simple" does not mean "cheaper" — check the actual dollars.
- Compounding frequency matters. 6% compounded daily beats 6% compounded annually. When a bank advertises APY instead of a plain interest rate, the compounding is already baked into that number.
- These calculators ignore taxes, inflation, fees, and extra payments. A 6% return with 3% inflation is closer to 3% in real purchasing power, and a loan's APR can sit above its stated interest rate once fees are folded in.
The takeaway
Before you sign a loan or park savings anywhere, run one comparison: the same principal, rate, and term through both a simple and a compound calculation. On a short loan the two answers nearly match, and the Simple Interest Calculator tells you what you need. On anything you'll hold for years — savings you're building, a mortgage you're paying, a card balance you keep rolling — run the compound number too and read the gap between them. That gap is the real money, and the only choice you get is which direction it flows.