Compound Interest Calculator
Model real savings growth, with regular contributions applied on the schedule you actually use.
Runs in your browser. Nothing you enter here is uploaded to a server.
How to use the compound interest calculator
Step 1: Enter the starting amount and rate
The rate is the nominal annual rate. Choose how often it compounds: daily, monthly, quarterly, semiannually or annually.
Step 2: Set the time horizon
Enter a duration in years or months. Longer horizons are where compounding separates itself from simple interest.
Step 3: Add recurring contributions if you make them
Choose the amount, how often you contribute and whether each contribution lands at the beginning or the end of the period. Beginning-of-period contributions earn one extra period of interest.
Step 4: Read the yearly table and export it
Each row shows the opening balance, contributions, interest earned and closing balance for that year. Download the whole table as CSV.
Worked example
10,000 invested at 5% with 300 added every month for ten years
Monthly compounding, contributions at the end of each month.
- Starting amount
- 10,000.00
- Total contributions
- 36,000.00
- Total interest earned
- 17,054.78
- Ending balance
- 63,054.78
Interest accounts for about 27% of the final balance. The same 46,000 sitting in a zero-interest account would still be 46,000.
How the projection is calculated
Rather than applying an annual formula and approximating contributions, the calculator steps through each compounding period in turn. That is what makes contribution timing and mismatched frequencies come out right.
periodic rate = annual rate ÷ 100 ÷ periods per year
each period: balance = balance × (1 + periodic rate)
beginning-of-period contributions are added before interest is applied
end-of-period contributions are added after interest is applied
lump sum only: A = P(1 + r/n)^(n × t)- A contribution frequency that differs from the compounding frequency is handled by placing each contribution in the period it falls into, not by averaging it across the year.
- A 0% rate is valid and simply returns the sum of the starting amount and the contributions.
- Negative rates below −100% per period are rejected, because a balance cannot compound through zero.
Limitations and privacy
What this tool does not do
- This is a projection at a constant rate, not a forecast. Real investment returns vary year to year and can be negative.
- Tax on interest or gains, platform fees and fund charges are all excluded. They reduce real returns, sometimes substantially.
- Inflation is not applied. The ending balance is in nominal terms, so its purchasing power will be lower than today’s equivalent amount.
- Contributions are assumed to be made in full and on time for the entire period.
Balances, rates and contribution amounts are calculated in your browser and never transmitted.
Frequently asked questions
Does compounding frequency make much difference?
Less than people expect at ordinary rates. Taking 10,000 at 5% for ten years, annual compounding ends at 16,288.95 and daily compounding at 16,486.65 — a difference of 197.70, or about 1.2%. Most of that gain arrives by the time you reach monthly compounding, which lands at 16,470.09. Rate and time matter far more than frequency.
Should contributions be at the beginning or the end of the period?
Beginning-of-period means each contribution earns interest for one extra period, so the ending balance is slightly higher. If you are modelling a salary deduction that lands on payday, end-of-period is usually the closer match.
What is the rule of 72?
A quick estimate: divide 72 by the annual percentage rate to get the approximate years for money to double. At 6%, that is about twelve years. It is a mental shortcut, and this calculator gives the exact figure.
Why is the total interest so much larger in later years?
Because interest is applied to a bigger balance each period, and in this model that balance includes all previously earned interest. The yearly table makes it obvious: the interest column grows even when the contribution column does not.
Can I model a loan with this?
No — a loan is paid down rather than built up, and the payment structure is different. Use the loan calculator or the mortgage calculator for borrowing.
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