Interest Calculator
Project how a balance grows with compound interest and regular monthly contributions.
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What Interest Calculator does
Compound interest means earning interest on interest already earned, which is why a balance grows slowly at first and then increasingly quickly.
This calculator runs the growth month by month, adding your contributions and crediting interest at the compounding frequency you choose, so the effect of annual, quarterly and monthly compounding can be compared directly.
- Starting principal, annual rate and time in years
- Optional monthly contribution
- Annual, quarterly or monthly compounding
- Split between deposits and interest earned
How to use Interest Calculator
- 1
Enter your starting amount
The balance you are beginning with, which may be zero.
- 2
Set the rate and period
Use the annual rate. Longer periods show the effect of compounding most clearly.
- 3
Add a monthly contribution
Regular contributions usually dominate the outcome over a short period; compounding dominates over a long one.
- 4
Compare compounding frequencies
More frequent compounding produces a slightly higher result at the same nominal rate.
Limits and known behaviour
- A constant rate of return is assumed. Real investments fluctuate, and a sequence of poor early years produces a materially worse outcome than an average suggests.
- Inflation is not applied, so the result is in nominal terms. At 3% inflation, money loses roughly half its purchasing power over 24 years.
- Tax on interest or gains is not modelled.
- Fees are not deducted, and an annual management charge of even 1% compounds against you significantly over decades.
- Amounts are calculated and displayed in US dollars using US lending conventions. The arithmetic is currency-independent, so the results hold for another currency if you read the symbol as your own.
- Figures are estimates for comparison and planning. They are not financial advice, not a quote, and not an offer of credit.
Privacy and data handling
Runs entirely in your browser
- Every figure you enter stays in this page. What you type about your savings is not transmitted, logged or stored anywhere.
- There is no account, no saved history and no autosave: reloading the page clears the form.
Site-wide data handling, including analytics and advertising, is described in the privacy policy.
Frequently asked questions
What does compounding frequency actually change?
It changes when interest is credited and starts earning interest itself. At the same nominal rate, monthly compounding yields slightly more than annual — for example 5% compounded monthly gives about 5.12% effective over a year.
Should I trust a 7% annual return?
It is a common long-run figure for broad stock market indices before inflation, and it is an average over decades, not something any particular year delivers. Test a lower rate to see how sensitive your plan is.