Compound Interest

See what a starting amount, a rate and a regular contribution add up to over the years.

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What this tool does

Compound Interest steps the balance forward through the term instead of applying a single closed-form formula, which is what lets the compounding frequency and the contribution schedule stay independent of each other. Interest can compound yearly, half-yearly, quarterly, monthly, weekly or daily; a contribution can be paid every month or once a year, at the start or the end of each period. What comes back is a table of every year — start balance, contributions, interest earned, end balance — closed by a Total row, with a one-line summary of the final balance underneath it. The simulation always takes at least twelve steps a year, so a contribution paid mid-year starts earning straight away even when you have chosen yearly compounding.

What it does not model is the part worth reading before you rely on a figure. The rate is held constant for the whole term, so this is arithmetic about a fixed return, not a forecast of an investment that rises and falls. Nothing is deducted for inflation, tax or platform charges, and the contribution never escalates with prices or earnings, so every number is nominal. It also carries no currency: amounts are formatted as plain numbers in your locale, so the units are whatever you had in mind when you typed them. The effective annual rate is worked out internally but never reaches the screen — the table is the whole of the rendered result — so the cost of a lower compounding frequency shows up in the balances rather than as a figure you can read off.

The calculation happens in this browser tab and nothing is uploaded, which is the reason a projection of your own savings growth does not end up in anybody's request log. The settings panel is the quick way in; the optional text box accepts "field: value" lines — principal, rate, years, compounding, contribution — when you would rather keep a scenario as text you can edit, save and paste back. Aliases such as amount, interest, term and monthly are understood, and a field it does not recognise is rejected by name rather than quietly ignored.

How to use it

  1. Enter the starting amount and the rate. The rate is annual and nominal, entered as a percentage. Anything from -100 up to 1000 is accepted, so a shrinking balance can be modelled as easily as a growing one.
  2. Choose how often interest compounds. Yearly through to daily. The same nominal rate compounded more often earns more over a year, but no effective rate is printed for you, so run the term twice if you want to see the size of the gap.
  3. Set the contribution, its frequency and its timing. Monthly or yearly, paid at the start or the end of each period. Start-of-period contributions earn a period more interest. Leave it at zero for a lump sum left alone.
  4. Read the year-by-year table. Each row shows how much of that year's growth came from money you added and how much from interest, and the final row totals contributions and interest against the closing balance.

Processing & privacy

Runs on your device. Your file is processed directly in this browser.

Common use cases

Valuing a savings habit

Set the starting amount to zero and enter only the monthly contribution to see how much of the final balance is interest rather than deposits.

Comparing two headline rates

A rate compounded daily is not the same offer as the same rate compounded yearly. Run the same rate at each frequency and compare the two closing balances.

Sizing a fixed-term deposit

A bond or fixed-rate account has a stated rate and term, which is exactly the fixed-rate assumption this makes, so the projected maturity value is a fair one.

Testing a target by trial

There is no solver for the contribution you would need, so raise the contribution and re-run until the closing balance clears the number you are aiming at.

Questions

Does anything I type get uploaded?
No. The projection is computed in this browser tab, so the amounts, the rate and the term stay on your machine. No request is made to a server at any point in the calculation.
Does compounding more often actually change the answer?
Yes. The rate you enter is nominal and annual, and compounding it more than once a year means interest starts earning interest before the year is out, so 5% compounded monthly is worth about 5.116% over a full year. That effective figure is not displayed anywhere, so the difference reaches you only as a larger closing balance.
Does it account for inflation, tax or fees?
No. Every figure is in nominal terms before any deduction. If you want a real-terms view, subtract your inflation assumption from the rate before entering it and read the answer as today's money.
Can it tell me what to invest to reach a target?
Not directly — it projects forward from the inputs rather than solving backwards for one of them. Adjusting the contribution or the term and running it again gets to the same place in a few passes.
Why is the table yearly when I chose monthly compounding?
The internal simulation runs at least twelve steps a year, and more when you compound weekly or daily, but the rows summarise each full year. A term that ends part-way through a year gets its own final row.
Is this the right tool for a loan or mortgage?
No. Debt repayment amortises a balance downwards against a fixed payment, which is a different calculation. Use Loan Calculator for repayments, interest and total cost.
What are the limits on the inputs?
The term can run up to 200 years, and the starting amount and contribution cannot be negative. A job with both the starting amount and the contribution set to zero is refused, since there would be nothing to grow.

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