Simple Interest vs Compound Interest: 5 Key Differences

Simple interest vs compound interest comes down to one question: is interest charged only on the original amount, or also on interest that has already built up? That single difference is small over a year and very large over decades, and it works in your favour on savings and against you on unpaid debt.

Updated September 2026. General information, not financial advice. Figures are illustrative and ignore tax, fees and inflation.

The two methods in plain terms

Simple interest is calculated on the principal only. The formula is principal times rate times time. Borrow or save 10,000 at 5 percent simple interest and you earn or owe 500 every year, no matter how long it runs.

Compound interest is calculated on the principal plus any interest already added. The US Consumer Financial Protection Bureau describes it as earning interest on your savings and on the interest earned along the way. The formula for the final amount is principal times (1 + rate per period) raised to the number of periods.

The CFPB gives a simple illustration: 1,000 at 5 percent a year grows to 1,050 after one year and to 1,102.50 after two. The extra 2.50 in year two is interest on the first year of interest.

Simple interest vs compound interest: 5 key differences

1. What interest is charged on. Simple interest uses the original principal throughout. Compound interest uses a base that grows each time interest is added.
2. The shape of growth. Simple interest grows in a straight line, by the same amount each period. Compound interest grows by a percentage of an increasing total, so the yearly gain gets bigger over time.
3. How often interest is added matters. With simple interest the frequency makes no difference. With compound interest, adding interest monthly or daily produces more than adding it yearly at the same stated rate, which the CFPB lists as one way savings grow faster.
4. Sensitivity to time. Over a short period the two give nearly the same result. Over long periods the gap widens sharply, as the example below shows.
5. Who benefits. On savings and investments compounding helps the saver. On debt where unpaid interest is added to the balance, it helps the lender.

A worked example over 30 years

Take an illustrative 10,000 at 5 percent a year, with nothing added or withdrawn:

• After 1 year: simple 10,500.00; compounded yearly 10,500.00; compounded monthly 10,511.62.
• After 10 years: simple 15,000.00; compounded yearly 16,288.95; compounded monthly 16,470.09.
• After 20 years: simple 20,000.00; compounded yearly 26,532.98; compounded monthly 27,126.40.
• After 30 years: simple 25,000.00; compounded yearly 43,219.42; compounded monthly 44,677.44.

Doing the sums yourself

Each figure comes from a short calculation you can repeat on any calculator:

• Simple, 10 years: interest is 10,000 × 0.05 × 10 = 5,000, so the total is 15,000.
• Compounded yearly, 10 years: 10,000 × 1.05¹⁰ = 16,288.95.
• Compounded monthly, 10 years: the monthly rate is 0.05 ÷ 12, applied 120 times, so 10,000 × (1 + 0.05 ÷ 12)¹²⁰ = 16,470.09.

The only change between the second and third lines is how often interest is added. The stated rate is the same, yet monthly compounding produces 181.14 more over ten years, because each month’s interest starts earning straight away.

In the first year the methods are almost identical. By year 30, yearly compounding has produced 18,219.42 more than simple interest. Money compounding yearly at 5 percent doubles in a little over 14 years, while simple interest at the same rate takes 20 years to add the original amount again.

Rates move over time, which changes these outcomes. Our explainer on how interest rates are set covers why.

Where you meet each method

Simple interest

Many loans that are repaid in regular instalments use a simple-interest method. The CFPB explains that most US car loans calculate interest daily or monthly on the balance actually outstanding, so extra payments reduce future interest. Standard repayment mortgages work the same way: each month interest is charged on the current balance, and because every payment covers that interest in full, it never builds on itself. Our guide to reading an amortization schedule shows this month by month.

Compound interest

Savings accounts that add interest to the balance, and investments where returns are reinvested, compound. On the borrowing side, compounding bites when payments do not cover the interest due, for example when only part of a credit card balance is paid and interest is then charged on a larger balance the following month.

The same loan can behave either way depending on how it is repaid. A loan where every payment covers the interest due never compounds in practice. Miss a payment, or make one smaller than the interest charged, and the unpaid interest may be added to the balance, so the next charge is calculated on a bigger number. That is why falling behind on debt tends to get more expensive over time rather than staying flat.

Why simple interest vs compound interest matters for your money

• For savers: start early and leave interest in the account. Time does most of the work, so the years matter more than small differences in rate.
• Compare like with like: two accounts at the same headline rate can pay different amounts if one adds interest more often. Look for an annual figure that already includes compounding, often labelled AER or APY.
• For borrowers: pay at least the interest due each period so it cannot be added to the balance, and pay extra towards principal where the loan allows it.
• Check the loan method: the CFPB notes that precomputed car loans, which add all the interest at the start, give little benefit for paying early, unlike simple-interest loans.

To see how much interest a loan costs over its life, our loan repayment calculator shows the monthly payment and total interest for any amount, rate and term. If a lender quotes both a rate and an APR, our guide to APR vs interest rate explains which to compare.

Common questions

What is the main difference between simple and compound interest? Simple interest is charged only on the original principal. Compound interest is charged on the principal plus interest already added, so the amount it is calculated on keeps growing.

Which is better, simple or compound interest? It depends which side you are on. Compound interest is better for savers because balances grow faster. Simple interest is usually better for borrowers because interest does not build on itself.

Do mortgages use simple or compound interest? Standard repayment mortgages charge interest on the outstanding balance each period, and each payment covers that interest, so it does not compound as long as payments are made in full.

How does compounding frequency change the result? At the same stated rate, more frequent compounding gives a slightly higher total. 10,000 at 5 percent for 30 years reaches 43,219.42 compounded yearly and 44,677.44 compounded monthly.

Sources and further reading

Where the figures and rules above come from, so you can check them:

• How compound interest works, with the 1,000 at 5 percent example: US CFPB
• Simple interest compared with precomputed interest on car loans: US CFPB
• How interest and principal are applied on a mortgage: US CFPB

Photo credits: Stacks of Coins by Kevin Schneider, CC0, via Wikimedia. Piggy Bank by Artsy Crafty, CC0, via Stocksnap.