# Compound Interest Calculator

> Work out what a starting balance plus regular monthly saving grows into, and how much of that growth is your own money rather than interest. The formula, its assumptions and its sources are published below, and the year-by-year table downloads as a spreadsheet.

Use it: https://tessalor.com/en/money/compound-interest

This tool runs entirely in the browser. Nothing entered into it is uploaded.

## Formula

```
monthlyFactor = (1 + r/n) ^ (n/12)

For each of the 12 x years months:
  balance = balance x monthlyFactor + contribution

where
  r  = annual return, as a decimal
  n  = compounding periods per year (1, 4, 12 or 365)

Present value, when an inflation rate is given:
  real = balance / (1 + i) ^ years
```

## Assumptions

- The return is the same every year. Real returns vary, sometimes a great deal, and a steady average will overstate a bad decade and understate a good one.
- Contributions arrive at the end of each month, so a payment does not earn interest during the month it is made. This is the conventional and slightly conservative treatment.
- Contributions never change. Saving that rises with your income produces a materially larger figure than this shows.
- Tax, platform charges and fund fees are not deducted. A 0.5% annual fee is roughly a 0.5 percentage point cut to the return you enter.
- Compounding frequency is converted to a monthly growth factor so that contributions and compounding can differ without changing the method.

## Inputs

| Name | Label | Type | Default | Range |
| --- | --- | --- | --- | --- |
| `principal` | Starting amount | currency | 10000 | 0 to 100000000 |
| `monthly` | Added each month | currency | 250 | 0 to 1000000 |
| `rate` | Annual return | percent | 5 | -20 to 30 |
| `years` | For how long | integer | 20 | 1 to 60 |
| `compounding` | Interest is added | select | 12 |  |
| `inflation` | Inflation | percent | 0 | 0 to 20 |

## Outputs

- `finalBalance` — Balance at the end (currency), primary
- `totalContributed` — Total you put in (currency)
- `totalInterest` — Interest earned (currency)
- `realBalance` — Worth in today's money (currency)

## Questions

### How long does it take to double my money?

Divide 72 by the annual return. At 6% that is roughly 12 years, at 8% roughly 9. The rule of 72 is an approximation that holds well between about 4% and 12%; outside that range it drifts, so use the year-by-year table rather than the shortcut.

### Does compounding daily rather than monthly make much difference?

Less than most people expect. At 5% over 20 years on a lump sum, daily compounding beats monthly by well under 1%. The frequency matters far less than the rate, the amount and the number of years.

### Should I enter a return before or after inflation?

Enter the nominal return, the headline number, and put inflation in the separate field. The tool then shows both the future balance and what it would be worth in today's money, which is usually the figure that actually answers the question.

### Why is the interest so small in the first few years?

Because early on almost all of the balance is money you paid in. Interest compounds on a base that takes years to build, which is why the year-by-year table is worth reading: the curve is nearly flat at the start and steepens later.

## Sources

- [Compound Interest Calculator](https://www.investor.gov/financial-tools-calculators/calculators/compound-interest-calculator) — Investor.gov, U.S. Securities and Exchange Commission, applies to the regulator's own calculator, used to check this one. Retrieved 2026-07-30.
- [Principles of Finance, section 8.2: Annuities](https://openstax.org/books/principles-finance/pages/8-2-annuities) — OpenStax, Rice University, applies to the future value of an ordinary annuity, and the end-of-period convention. Retrieved 2026-07-30.

## Variants

- [What does saving 500 a month grow into?](https://tessalor.com/en/money/compound-interest/on-500-a-month)
- [How long does it take to double my money?](https://tessalor.com/en/money/compound-interest/double-my-money)

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Estimate, not advice. See https://tessalor.com/en/disclaimer.
Machine-readable catalogue: https://tessalor.com/api/tools.json