Savings Interest Calculator
Project how a starting balance and regular monthly deposits can grow under an assumed APY. APY is an effective annual yield you supply — not a quoted bank rate.
Your results will appear here
Enter your assumptions and calculate to see the estimated result and supporting details.
How this calculator works
Enter starting balance, monthly deposit, APY, and duration in whole years.
Results show ending balance, total deposits, interest earned, an annual projection table, and a growth chart.
Formula / methodology
rm = (1 + APY)^(1/12) − 1 Each month: balance = balance × (1 + rm) + deposit (end-of-month deposits)
- Equivalent closed-form growth uses (1 + APY)^elapsedYears for each cash flow under the same end-of-month timing.
- With no deposits, ending balance equals starting × (1 + APY)^years because APY is effective annual.
- With 0% APY, ending balance equals starting balance plus all deposits.
Worked example
Starting $1,000 with $100 monthly deposits for 2 years at 0% APY ends at $3,400.
Starting $10,000 with no deposits for 10 years at 5% APY ends at about $16,288.95.
Important assumptions
- APY is a user-supplied effective annual yield.
- Monthly deposits occur at the end of each month.
- Duration uses whole years (1–100).
- Taxes and fees are not modeled.
Common questions
APY is already an effective annual yield. The equivalent monthly rate is (1 + APY)^(1/12) − 1 so twelve months compound back to the APY.
Yes. Ending balance equals starting balance plus all deposits.
Under the same APY, balances, years, and end-of-month contributions, Savings Interest should reproduce a Savings Goal Mode A target within cents-level tolerance.