How to use Compound Interest
Project monthly compounding and regular contributions.
- Choose the operation and enter every labeled input, keeping units and the time period consistent.
- Calculate, compare the worked example below and inspect any warnings.
- Change one assumption to compare another scenario; keep the model’s limits in view.
Formula & calculation method
Monthly rate r = annual nominal rate ÷ 1200. Future balance = P(1 + r)^n + D[(1 + r)^n − 1] ÷ r, with contributions D at month end. At r = 0, balance = P + Dn.
Input reference · 4 fields
These labels describe the calculation’s base units. Supported measurements can switch through the header’s region settings; follow the current workspace labels. Prefilled values are examples.
- Starting balance
- Enter a plain number with a decimal point if needed. The workspace shows an illustrative value, not a recommendation.
- Monthly contribution
- Enter a plain number with a decimal point if needed. The workspace shows an illustrative value, not a recommendation.
- Annual nominal rate (%)
- Enter a plain number with a decimal point if needed. The workspace shows an illustrative value, not a recommendation.
- Term (years)
- Enter a plain number with a decimal point if needed. The workspace shows an illustrative value, not a recommendation.
A quick example
At zero interest, 1,000 plus 100 per month for one year becomes 2,200.
Assumptions, edge cases & limits
- Monthly deposits are assumed at the end of each month.
- Returns are constant assumptions; taxes, fees, inflation and investment losses are excluded.
- Assumes a constant rate. Returns are not guaranteed; taxes, fees, and inflation are excluded.
Questions about Compound Interest
Are monthly contributions made at the start or end of the month?
This model adds them at the end of each month. A beginning-of-month contribution would earn one more month of growth and can produce a different result.
Is the displayed rate an annual effective yield?
No. It is a nominal annual assumption divided into monthly rates. Check how a provider quotes its rate before comparing a projection with an account or investment product.
Are my files or inputs uploaded?
Calculation and file processing run in your browser without a processing upload. Downloads go to your device. Choosing a next-step transfer creates a temporary local copy for that workflow. Region preferences are stored separately from tool inputs. See our privacy details.
Can I use this tool for free?
Yes. No signup, subscription or payment is required to use the tool or download its output. File tools have device and size limits; a smaller batch may work better on a phone.
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