# Compound interest calculator with monthly contributions

> Simulate how your savings grow with compound interest and monthly contributions: final balance, interest earned, value after tax and inflation.

- Interactive version: https://simulkit.com/en/compound-interest
- Other languages: [Français](https://simulkit.com/fr/interets-composes.md)

Compound interest makes your interest earn interest in turn. This calculator shows how savings grow from a starting balance and monthly contributions, after fees, tax and inflation.

## Parameters

### Savings

| Parameter | URL key | Default | Range | Description |
|---|---|---|---|---|
| Starting balance | `initial` | $10,000 | $0 to $50,000,000 | Amount invested on day one. |
| Monthly contribution | `monthly` | $500/month | $0 to $1,000,000 | Added at the end of each month. |
| Duration | `years` | 25 years | 1 year to 60 years | Number of years invested. |
| Yearly contribution increase | `contributionGrowth` | 0.0% | 0.0% to 20.0% | To keep up with your income: the monthly contribution grows by this percentage every year. |

### Return and fees

| Parameter | URL key | Default | Range | Description |
|---|---|---|---|---|
| Annual return | `annualReturn` | 7.0% | -10.0% to 25.0% | Average return before fees and tax. It can be negative. |
| Yearly fees | `feesRate` | 0.20% | 0.00% to 5.00% | Management fees charged every year, as a share of the balance. |
| Inflation | `inflationRate` | 2.5% | 0.0% to 15.0% | Used to express the result in today's money. |
| Tax on gains | `taxRate` | 15% | 0% to 60% | Tax paid on gains when you withdraw. Use 0 for a tax-free account. |

## Results with the default parameters

- Final balance: $430,971
- Total contributed: $160,000
- Interest earned: $270,971
- After tax: $390,325
- In today's money: $210,538
- Doubling time: 11 years

## How the math works

Every month, the balance grows at the chosen return, net of yearly fees, then the month's contribution is added.

At the end, gains are taxed once. The value in today's money divides the result by cumulated inflation.

## Why time matters so much

Growth is exponential: the last years add far more than the first ones. Starting 5 years earlier often matters more than contributing more.

Fees matter too: 1% a year can cost a large share of the final balance over 30 years. Compare two scenarios to see it.

## Frequently asked questions

### What is the difference between simple and compound interest?

With simple interest, only the starting balance earns interest. With compound interest, interest already earned earns interest too, which speeds up growth over time.

### Which return should I use?

It depends on the investment. A savings account pays little but carries no risk. Stocks have historically returned more over long periods, with sometimes deep drops. Test several assumptions.

### What is the rule of 72?

A shortcut for the doubling time: 72 divided by the annual return. At 6% a year, money doubles in about 12 years. The calculator shows the exact value.

### Are my numbers sent anywhere?

No. The math runs in your browser. Nothing is sent until you click Save or Share.

## Open it with your own numbers

Every parameter can be passed in the query string, using the keys above. For example:

https://simulkit.com/en/compound-interest?initial=12500&monthly=630