Compound Interest with Deposits
Model a savings or investment account where a lump sum grows and you add the same amount every month. The output separates what you put in from what compounding added.
Balance with monthly deposits
Enter values.
How this is calculated
We use monthly compounding on the running balance. Each month: balance grows by annual rate ÷ 12, then your monthly deposit is added at month-end.
Future value = starting × (1+r)n + payment × ((1+r)n − 1) ÷ r, with r = monthly rate and n = months.
Use this tool for
- Checking whether $300/month plus an opening balance reaches a house-down-payment target.
- Comparing “lump now” vs “smaller lump + DCA” when markets are volatile.
- Teaching how even a modest rate changes the curve over a decade.
Common questions
Beginning vs. end of month deposits?
Deposits are modeled at month-end after interest accrues—common for spreadsheet templates; a few dollars differ from “beginning” timing.
Taxes or fees?
Not included—lower the return field to approximate after-fee performance.
