Three quick steps
How to use this calculator
- 1Enter what you have and how long it can grow.
Use whole years plus optional months. A zero-length term keeps the starting balance unchanged.
- 2Add a hypothetical rate and its compounding frequency.
Choose nominal if the quoted rate compounds at the selected frequency; choose APY if the rate is already effective.
- 3Describe deposits, then refine optional assumptions.
Set timing and frequency. Open advanced assumptions for withdrawals, contribution increases, and inflation.
What your results mean
Projected future balance is the modeled ending balance after deposits, withdrawals, and growth. Total deposited keeps the initial balance separate from recurring contributions, while interest or growth is the residual after all cash flows.
The effective annual rate shows the one-year result of a nominal rate at the selected compounding frequency. Doubling time applies to the rate alone; regular contributions can make the account balance double sooner.
Inspectable math
Formula and cash-flow methodology
With no cash flows, the calculator uses A = P(1 + r/n)^(nt), where A is future value, P is principal, r is the nominal annual rate, n is the number of compounding periods per year, and t is years.
APY mode converts the effective annual rate directly into growth over each exact fraction of a year. Nominal mode first derives (1 + r/n)^n - 1 as the effective annual rate. Weekly uses 52 periods, biweekly 26, monthly 12, quarterly 4, and daily compounding 365.
Timing convention
The engine advances between exact scheduled event times, so mismatched frequencies are deterministic and daily compounding is never replaced with monthly compounding. At a shared period boundary it applies accrued growth, then end-of-period contributions, then withdrawals, records the balance, and finally applies a beginning-of-period contribution for the next period. A beginning contribution at the start of the scenario is invested immediately. If a withdrawal would exhaust the available balance, the scenario is rejected instead of drifting below zero.
Worked example
For $5,000 at a 5% nominal annual rate compounded monthly for five years with no additional cash flows, 5000 × (1 + 0.05/12)^(12×5) = $6,416.79. The difference, $1,416.79, is modeled interest.
Assumptions and limitations
- Returns are constant in the model; real investments and variable-rate accounts do not grow smoothly.
- Taxes, account fees, fund expenses, contribution limits, and penalties are not included.
- A modeled year is divided into exact frequency fractions; daily compounding uses 365 periods and does not model leap days.
- Inflation changes the displayed buying-power comparison only. It does not change the nominal cash-flow schedule.
- Contribution increases apply after each complete year of scheduled contributions; withdrawals remain level.
Common questions
Frequently asked questions
What is the difference between a nominal rate and APY?
The growth produced by a nominal annual rate depends on how often it compounds. APY already includes the effect of compounding over one year, so the same APY produces the same one-year growth regardless of the frequency selector.
Can I use this as a daily compound interest calculator?
Yes. Choose Daily under compounding frequency. The engine uses 365 compounding periods per modeled year and schedules cash flows at their own selected frequency.
How are contributions and withdrawals ordered?
Growth is applied to the event date first. End-of-period contributions are added before withdrawals. The period balance is then recorded, followed by any beginning contribution for the next period.
Why might my real value be lower than my future balance?
The inflation-adjusted figure converts future dollars into estimated today’s purchasing power. It can be lower even while the nominal balance grows.
Sources
- Consumer Financial Protection Bureau: How does compound interest work? (accessed July 26, 2026)
- Investor.gov: Compound Interest Calculator (accessed July 26, 2026)
Last reviewed July 26, 2026.