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Simple interest is paid only on the original amount. Compound interest is paid on the original amount plus any interest already added. Over one year the two can be identical, but the gap widens every year after that: $8,000 at 5% earns $12,000 in simple interest over 30 years, while compound interest turns it into $34,575.54, more than $14,500 extra.
As a saver, you want compound interest working for you. As a borrower, you want simple interest, or better still, no interest at all. The examples below show exactly where the difference comes from and where each type shows up in real accounts and loans.
The two formulas
With simple interest, every year earns the same dollar amount: P × r. With compound interest, each year's interest is calculated on a bigger balance than the year before.
Worked example: $8,000 at 5%
Take $8,000 at 5% a year. For the compound column, interest is added once a year.
| Years | Simple interest balance | Compound interest balance | Difference |
|---|---|---|---|
| 1 | $8,400.00 | $8,400.00 | $0.00 |
| 5 | $10,000.00 | $10,210.25 | $210.25 |
| 10 | $12,000.00 | $13,031.16 | $1,031.16 |
| 20 | $16,000.00 | $21,226.38 | $5,226.38 |
| 30 | $20,000.00 | $34,575.54 | $14,575.54 |
The simple interest balance rises by exactly $400 every year, in a straight line. The compound balance curves upward. After 30 years, simple interest has added 1.5 times the original deposit; compound interest has added more than 3.3 times it. You can reproduce the compound column in the compound interest calculator by entering $8,000, 5%, no monthly contribution and yearly compounding.
Why the gap starts at zero
In year one, both methods charge 5% on $8,000, so both pay $400. From year two, compound interest is calculated on $8,400, not $8,000:
| Year | Compound interest earned | Balance at year end |
|---|---|---|
| 1 | $400.00 | $8,400.00 |
| 2 | $420.00 | $8,820.00 |
| 3 | $441.00 | $9,261.00 |
| 4 | $463.05 | $9,724.05 |
| 5 | $486.20 | $10,210.25 |
Simple interest would pay $400 in every one of those years. The extra $20, $41, $63.05 and $86.20 are interest on interest. They look trivial at first, which is why people underestimate compounding.
Key idea After 20 years, the compound account has earned $13,226.38 in interest. Of that, $8,000 is the "simple" part and $5,226.38, or 39.5%, is interest earned on earlier interest. That share keeps rising the longer you leave the money alone.
Higher rates widen the gap faster
The rate matters as much as the time. Here is the interest earned on $8,000 over 25 years at three illustrative rates, compounded yearly:
| Rate | Simple interest earned | Compound interest earned | Compound ÷ simple |
|---|---|---|---|
| 3% | $6,000 | $8,750 | 1.46× |
| 6% | $12,000 | $26,335 | 2.19× |
| 9% | $18,000 | $60,985 | 3.39× |
Tripling the rate triples simple interest, but it multiplies compound interest roughly sevenfold. That is the same math behind the Rule of 72: at 9%, compound money doubles about every 8 years, while simple interest takes 11.1 years to add even one full doubling.
Where you'll see simple interest
Interest paid out instead of reinvested
Many CDs and bonds can pay interest out to you rather than adding it to the balance. When that happens, you earn simple interest on the original deposit.
Say you put $20,000 in a five-year CD at an illustrative 4%:
- Interest paid out monthly: $66.67 a month, $4,000.00 over five years.
- Interest left in and compounded monthly: $4,419.93 over five years.
Taking the income costs you $419.93 in growth. That is fine if you need the money to live on; if you don't, leave the interest in.
Simple interest loans
Many car loans and personal loans are described as "simple interest" loans. That means interest is charged only on the principal you still owe, not on unpaid interest. The loan still amortizes: each payment covers that period's interest first, and the rest reduces the balance.
On an $18,000 balance at an illustrative 6.9%, interest accrues at about $3.40 a day, or $102.08 over a 30-day month. Pay an extra $1,000 toward principal and the daily charge falls to about $3.21, saving roughly $5.67 in the very next 30 days and more every month after. That is why simple interest loans reward early or extra payments; see paying off a loan early for the full effect, or test payments in the loan calculator.
Timing works the other way too. If a loan charges interest daily and you pay five days late, about $17.01 of extra interest builds up on that $18,000 balance, so less of your payment reaches the principal. Ask your lender whether interest accrues daily and set up payments to arrive on or before the due date.
Flat-rate loans: simple interest that costs more
A flat-rate loan charges simple interest on the original amount for the whole term, even as you pay it down. A $12,000 loan at a 6% flat rate over three years costs 12,000 × 0.06 × 3 = $2,160 in interest, for payments of $393.33 a month. Because you are paying interest on money you've already repaid, that is equivalent to about 11.08% on a normal reducing balance. The flat rate vs reducing balance guide shows how to spot it.
When compound interest works against you
Credit cards compound. If you carry a balance, interest is added to what you owe, and next month you pay interest on that interest.
To isolate the effect, imagine a $3,000 card balance at an illustrative 22% APR with no payments and no fees for two years (in reality you'd be charged late fees and the account would go delinquent, so this is purely to show the math):
| Method | Interest after 2 years |
|---|---|
| Simple interest | $1,320.00 |
| Compounded monthly | $1,639.60 |
| Compounded daily | $1,657.50 |
Compounding adds more than $300 on top of the simple figure in just two years. The credit card interest guide explains how issuers calculate the daily charge.
Does the compounding frequency matter?
A bit, but much less than the rate and time. Our $8,000 at 5% grows to $34,575.54 in 30 years with yearly compounding and $35,741.95 with monthly compounding. The jump from simple to compound interest is worth $14,575.54; moving from yearly to monthly adds a further $1,166.41. For the details, read daily vs monthly compounding.
Quick reference
| Situation | Usually | Good or bad for you? |
|---|---|---|
| Savings account, interest left in | Compound | Good |
| CD or bond with interest paid out | Simple | Fine if you need the income |
| Car or personal loan (amortizing) | Simple, on remaining balance | Better for borrowers |
| Flat-rate loan | Simple, on original amount | Costlier than it looks |
| Credit card balance carried over | Compound | Bad |
Frequently asked questions
Is simple interest ever better for savers?
Not for growth. Simple interest only makes sense when you want regular income from your savings, such as monthly interest payments to cover bills.
How do I know which one my account uses?
For savings, compare the stated rate with the APY: if the APY is higher, interest compounds. For loans, the agreement states how interest is calculated. Ask the lender directly whether interest is charged on the remaining balance or the original amount.
Can I turn simple interest into compound interest myself?
Yes. If a CD or bond pays interest out, you can deposit those payments into a savings account or reinvest them. You get compound growth as long as you put the interest back to work.
Why does compound interest feel slow at first?
Because interest on interest starts tiny. In our example, it adds just $20 in year two. Its effect builds slowly and then accelerates, so the payoff depends on staying invested for years, not months.
How to use this
- Saving: choose accounts that compound and leave the interest in. Compare them by APY.
- Investing: reinvest dividends and interest unless you need the income.
- Borrowing: prefer loans that charge interest on the remaining balance, and avoid flat-rate deals unless you know the equivalent rate.
- Carrying card debt: treat it as urgent, because compounding is on the lender's side.
To see the difference with your own numbers, enter your balance, rate and timeframe in the compound interest calculator. Then work out the simple figure by hand (principal × rate × years) and compare the two. The year-by-year table in the calculator shows exactly when interest on interest starts to pull ahead.
This article is for general education and is not financial advice. Rules, rates and products vary by lender and country. See our disclaimer.
