Interest Calculator
See how savings grow with compound or simple interest — with regular deposits, any compounding frequency and a year-by-year breakdown.
- Compound interest
- Regular deposits
- Yearly table
- Private
More options
Details
How to calculate interest
Starting amount, rate and time.
- 1
Enter the basics
Starting amount, yearly interest rate and how long the money stays invested.
- 2
Choose the compounding
Pick compound or simple interest and how often interest is added: yearly, monthly, daily or continuously.
- 3
Add deposits
Optionally add a regular deposit and watch the balance, interest and chart update.
Watch the snowball grow
Everything that moves a savings balance.
Compound or simple
Compare interest on interest with plain simple interest on the same numbers.
Any compounding
Once a year, twice, quarterly, monthly, daily or continuous — with the matching effective annual rate.
Regular deposits
Add a contribution every week, month, quarter or year, at the start or end of the period.
Growth chart
See deposits and balance diverge as interest accumulates.
Year by year
A table of deposits, interest and balance for every year, downloadable as CSV.
Link to your calculation
Copy a link that reopens the calculator with your numbers filled in — handy for sharing or saving a scenario.
Made for phones
Large fields, the right keyboard for numbers and a layout that fits small screens.
Private by design
Everything is calculated in your browser. Your numbers are not sent to a server.
Simple vs. compound interest
Simple interest is paid on the starting amount only: interest = P × r × t. $10,000 at 5 % for 10 years earns $5,000. Compound interest is paid on the starting amount and on the interest already earned: A = P × (1 + r ÷ n)ⁿᵗ, where n is the number of compounding periods per year. The same $10,000 grows to $16,289 with yearly compounding — $1,289 more.
How often interest is added matters less than people think. With monthly compounding the balance after 10 years is $16,470, with daily compounding $16,487. Time and rate matter far more: after 30 years at 5 % the $10,000 has become $43,219. The rule of 72 gives a quick estimate — money doubles in about 72 ÷ rate years, so roughly 14 years at 5 %.
The effective annual rate (APY or AER) converts any compounding frequency into the equivalent once-a-year rate, which makes accounts comparable. 5 % compounded monthly is an APY of 5.116 %. Remember that interest may be taxable and that inflation reduces what the final balance will buy.
10,000 at 5 % with yearly compounding
| After | Compound interest | Simple interest |
|---|---|---|
| 10 years | 16,289 | 15,000 |
| 20 years | 26,533 | 20,000 |
| 30 years | 43,219 | 25,000 |
| 40 years | 70,400 | 30,000 |
Tips
- Planning for old age? The Retirement Calculator adds inflation and a retirement income.
- Find out what your balance will really be worth with the Inflation Calculator.
- Borrowing instead of saving? The Loan Calculator shows what interest costs you.
- Start early rather than big: ten extra years of compounding usually beat doubling the monthly deposit.
Frequently asked questions
Can’t find your answer? Contact us — we reply quickly.
What is the compound interest formula?
A = P × (1 + r ÷ n)ⁿᵗ, with P the starting amount, r the yearly rate as a decimal, n the compounding periods per year and t the years. With continuous compounding it becomes A = P × e^(r × t).
What is the difference between APR and APY?
APR is the nominal yearly rate without compounding. APY (or AER) includes compounding and is the rate you actually earn in a year. APY is always equal to or higher than APR.
How does the rule of 72 work?
Divide 72 by the interest rate to estimate the years needed to double your money. At 6 % it takes about 12 years; at 3 % about 24.
Does daily compounding make a big difference?
Not much. At 5 %, daily instead of yearly compounding adds about 0.13 percentage points a year. The interest rate and the length of time are much more important.
Are regular deposits made at the start or the end of the period?
At the end by default, as with most savings plans. Switch on “Deposit at the start of each period” if you pay in at the beginning; each deposit then earns interest one period longer.
Are my numbers stored or sent anywhere?
No. The calculation runs in your browser. Your last inputs are remembered on your own device only, so the form is still filled in when you come back; use Reset to clear them.
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