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Tool / long-term maths
Compound interest and contribution calculator
This compound interest calculator models a recurring monthly contribution with a constant annual return assumption. It is useful for understanding time, contributions and compounding. It is not a forecast: real returns are uneven, fees and taxes matter, and losses are possible.
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local only
How to use it
Numbers are clearer when the assumptions are visible.
Start with the default example. Change one input at a time and notice which part of the result moves. Then read the explanation below before using any real-world number.
- Use your account currency consistently.
- Include costs and uncertainty outside the simple model.
- Verify the product’s own contract or fee rules.
Quick answer
The short answer
This tool converts your assumed annual return to a monthly rate and models end-of-month contributions. The result is a constant-rate scenario, not a forecast.
Input reference and starting values
| Input | Meaning | Starting value |
|---|---|---|
| Starting amount | Account currency | 1000 |
| Monthly contribution | Account currency | 100 |
| Assumed annual return | Percent | 6 |
| Time horizon | Years | 10 |
What the model assumes
The calculation compounds a monthly rate derived from the annual assumption and adds the contribution at the end of each month. It shows a mathematical path under a steady rate—not what a market will actually deliver. Real returns can be negative, lumpy and affected by fees, taxes, inflation and contribution timing.
Try a lower return, no contribution and a shorter horizon. Comparing scenarios is more educational than focusing on one attractive outcome.
Working modelFuture value = contributions + growth under the chosen constant-rate assumption
Time does not remove investment risk
A longer horizon can give compounding more time to work, but it does not guarantee a positive result. The asset, allocation and sequence of returns still matter. A high assumed return can make the output look precise while hiding uncertainty.
Use a range of assumptions and keep emergency savings, debt costs and liquidity needs outside the growth illustration.
In practice
A constant rate is a scenario
A monthly growth assumption produces a smooth projection, while real asset returns can be negative and uneven. Run more than one assumption and compare the end result with the amount you contributed.
The model assumes a particular contribution timing and does not include inflation, tax, fees or changes in rate. Treat its result as a mathematical illustration, never an advertised return.
Primary references
Continue with original sources.
These links provide definitions and current context. Product rules, regulation and network details can change, so verify time-sensitive information at the source.
Common questions
Before you move on
Does this account for inflation?+
No. The output is nominal. Compare it with an inflation assumption separately if you need a real-value estimate.
Does monthly contribution timing matter?+
Yes. This version assumes contributions at the end of each month. Other timing produces a different result.
Can markets return the same percentage every year?+
No. The constant rate is a teaching simplification, not a realistic guarantee.
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