Reckonary / Finance / Simple vs Compound Interest
Simple vs compound interest: why the gap explodes over time
Put $10,000 in an account at 5% and leave it for 30 years. If the interest is simple, you end with $25,000. If it compounds, you end with $43,219. The whole $18,219 difference is interest that went on to earn more interest. Same deposit, same rate, same three decades — the only thing that changed is what the interest was charged on. That snowball is why saving advice worships compounding, and it's also why a credit-card balance you carry gets away from you.
What's the difference between simple and compound interest?
Simple interest is charged on the original deposit and nothing else. Compound interest is charged on the deposit plus all the interest that has already piled up. That's the entire distinction, and everything surprising about compounding falls out of it.
Watch one year at a time. Year one, both pay 5% on your $10,000, so both add $500 — they're identical. Year two is where they split: simple pays 5% on the original $10,000 again ($500), while compound pays 5% on the $10,500 you now have ($525). It's only a $25 gap. But simple keeps charging the same $500 forever, and compound keeps charging a bigger number every year, so the small early gaps stack into a large one.
Simple interest grows your money in a straight line. Compound grows it on a curve that bends upward. Over a year or two you can barely tell them apart; over decades the curve leaves the line far behind.
Why does compound interest pull so far ahead over time?
Because the base it charges on keeps getting bigger. Simple interest always reckons from the same $10,000, so it adds a flat $500 a year. Compound reckons from a balance that grows, so the amount it adds grows too — this year's interest becomes next year's principal.
Here's the same $10,000 at 5%, tracked both ways. The last column is the interest-on-interest: the part compound earns that simple never does.
| After… | Simple interest | Compound interest | Interest on interest |
|---|---|---|---|
| 1 year | $500 | $500 | $0 |
| 5 years | $2,500 | $2,762.82 | $262.82 |
| 10 years | $5,000 | $6,288.95 | $1,288.95 |
| 20 years | $10,000 | $16,532.98 | $6,532.98 |
| 30 years | $15,000 | $33,219.42 | $18,219.42 |
| 40 years | $20,000 | $60,399.89 | $40,399.89 |
Look at the last two rows. By year 30 the interest-on-interest alone ($18,219) is worth more than everything simple interest earned in the whole 30 years ($15,000). By year 40 it's double that — $40,400 of interest-on-interest against $20,000 of simple interest. The snowball isn't a rounding detail you can ignore — given enough time, it becomes the main event.
Drag the sliders below through the same story. Pull Years down to 1 and the two bars line up; stretch it out and the teal bar climbs away from the gray one. The space between them is the snowball.
Set a deposit, a rate, and a number of years. The gray bar is where simple interest leaves you; the teal bar is where compound interest does. The space between them is interest earning interest:
Over 30 years, compound ends $18,219.42 ahead of simple on the same $10,000 at 5%. That gap is pure interest-on-interest — money the simple version never charges, because it always keeps counting from the original deposit.
Two things feed the curve: time and rate. Time we just watched. Rate matters because a higher rate means bigger interest to compound on next year, so the bend gets steeper — the same reason the gap between an APR and its APY widens most on a high-rate credit card.
How long does it take to double your money?
This is where the two really part ways. At 5%, compound interest doubles your money in a bit over 14 years. Simple interest takes 20 — nearly six years longer at the exact same rate.
The simple case is easy arithmetic: to double, you need 100% of your deposit back in interest, and 5% a year gets there in 100 ÷ 5 = 20 steps, no shortcuts. Compound gets there faster because each year's interest joins the pile and starts pulling its own weight — the shortcut for estimating it is the Rule of 72, which says 72 ÷ 5 ≈ 14.4 years. The honest figure is 14.2, so the rule is close.
| Rate | Simple: years to double | Compound: years to double |
|---|---|---|
| 2% | 50 | 35 |
| 4% | 25 | 17.7 |
| 5% | 20 | 14.2 |
| 8% | 12.5 | 9 |
Simple doubling is just 100 divided by the rate. At any rate you'd actually earn or pay, compound doubling comes in below it — roughly 70% of the time simple needs. In plain years, that head start is wide when rates are low (15 years at 2%) and narrow when they're high (about 3.5 years at 8%), but the proportion barely moves.
Do loans use simple or compound interest?
Mostly simple — and that's quietly in your favor. A standard mortgage, a car loan, and a federal student loan all charge simple interest on the balance you still owe. You pay that interest as you go, so there's no pile of unpaid interest sitting around to compound. As you knock down the principal, the interest shrinks with it.
One catch with student loans: leave the interest unpaid during school or a pause and it can be folded onto your balance — after that, you pay interest on it too. Cover the interest as it accrues and the loan stays simple.
The exception is the one that hurts: a credit-card balance you carry month to month compounds, usually daily. The same engine that turns a retirement account into a snowball turns card debt into one too, just aimed the wrong way. If you're carrying a balance, it's worth seeing how long paying only the minimum actually takes.
| Where the money is | How interest works | Who it favors |
|---|---|---|
| Standard mortgage | Simple, on the balance | You, the borrower |
| Auto loan | Simple, on the balance | You, the borrower |
| Federal student loan | Simple, accrues daily | You, the borrower |
| Credit card you carry | Compound, daily | The lender |
| Savings account or CD | Compound | You, the saver |
| Retirement fund | Compound | You, the saver |
So which one do you actually want?
It flips depending on which side of the interest you're standing on. Compounding is the same math whether it's building your savings or building someone's claim on you, so "compound interest is good" is only half a sentence. It's good when you're earning it and rough when you're paying it.
When you're growing money, you want it to compound, and you want as many years as possible for the curve to bend — starting early beats starting big. When you're borrowing, the friendly setup is simple interest on a balance you pay down, and the trap is a compounding debt you let ride.
Watch a balance compound year by year with your own numbers:
Open the compound interest calculator →If you only need the flat, no-snowball version — a fixed rate on a fixed deposit — the simple interest calculator does that side. The difference between the two answers, on the same inputs, is the snowball this whole guide is about.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is always charged on the original deposit and nothing else, so it adds the same amount every year. Compound interest is charged on the deposit plus every bit of interest already added, so each year's interest is a little bigger than the last. That second part — interest earning interest — is the whole difference, and it's why compound pulls away over time while simple grows in a straight line.
How much more does compound interest earn than simple interest?
It depends entirely on how long the money sits. On $10,000 at 5%, simple interest earns $15,000 over 30 years while compound earns $33,219 — more than double. The extra $18,219 is interest-on-interest, and by year 30 it's larger than the entire simple total. Short horizons barely differ: over the first year the two are identical.
How long does it take to double your money at 5%?
With compound interest, a bit over 14 years — the Rule of 72 estimates 72 ÷ 5 = 14.4, and the real figure is 14.2. With simple interest it takes 20 years exactly, because you need 100% of the deposit back in interest and 5% a year gets there in 100 ÷ 5 = 20 steps. Same rate, nearly six extra years, entirely because simple interest never builds on itself.
Do loans use simple or compound interest?
Most loans built to be paid down use simple interest on the balance: standard mortgages, auto loans, and federal student loans charge interest on what you still owe, and once you pay it there's no leftover interest to compound. A credit-card balance you carry is the opposite — it compounds daily, which is why card debt snowballs the same way a retirement account does, just against you.
Is compound interest always better than simple interest?
Only when you're the one earning it. At the same rate and term, compound produces more interest than simple — great when it's landing in your savings, painful when it's stacking onto a debt. On money you owe, simple interest is the cheaper structure for you; on money you're growing, compound is the one you want.
Last reviewed July 2026. Compound figures use annual compounding to isolate the difference from simple interest; compounding more often (monthly or daily) lifts the compound totals slightly, which is a separate effect. Rates shown are examples for the math, not offers from any bank or lender.