Understanding your annuity future value
The table below illustrates how the two components of the future value — contributions and growth — typically shift in relative size the longer contributions continue.
| Time horizon | Typical composition of future value |
|---|---|
| Early years | Total contributions make up most of the balance; growth has had little time to compound |
| Later years (e.g., 20+ years at a positive rate) | Growth can become a substantial or even majority share of the balance, since compounding accelerates over longer periods |
- This calculator assumes a constant monthly contribution and a constant annual rate of return for the entire period; in practice, contribution amounts often change over time (raises, life events) and investment returns fluctuate year to year rather than compounding smoothly.
- This calculator models an ordinary annuity (payments at the end of each period). An annuity due, with payments at the start of each period, would produce a slightly higher future value for the same inputs, since each payment would compound for one additional period.
- This is a projection based on the assumptions entered, not a guarantee; the SEC's Investor.gov resources emphasize that past or assumed average returns do not guarantee future results for market-based investments.
What is an annuity (accumulation phase)?
In finance, an annuity refers generally to a series of equal payments made or received at regular intervals; the accumulation phase — modeled by this calculator — is the period during which fixed contributions are made into an account and grow at a rate of return until a target future date, such as retirement. This is distinct from the payout phase, in which a lump sum is instead converted into a stream of regular withdrawals (see the related annuity payout calculator).
This calculator specifically computes an ordinary annuity, meaning each payment is assumed to occur at the end of each period (each month), which is the standard convention for most retirement contribution schedules such as 401(k) or IRA payroll deductions. A small mathematical difference exists between an ordinary annuity and an annuity due (payments at the start of each period), though this calculator uses the ordinary annuity convention throughout.
The result depends heavily on the assumed rate of return, which the U.S. Securities and Exchange Commission's Investor.gov resources note is never guaranteed for market-based investments — actual returns vary year to year and can be negative in some years, even if a long-term average return assumption is reasonable for projection purposes.
How to use this annuity calculator
- Enter the fixed monthly contribution amount.
- Enter the expected annual rate of return.
- Enter the number of years contributions will continue.
- Read the projected future value, total contributions made over the period, and the growth generated on top of those contributions.
The formula behind annuity future value
The future value of an ordinary annuity formula sums the future value of every individual monthly payment, each compounding at the monthly rate for the number of months remaining until the end of the period. This produces a closed-form calculation rather than requiring a month-by-month simulation.
On the calculator's default example — $500 monthly at a 7% annual rate for 20 years (240 months) — the future value is $260,463.33. Total contributions over that period are $120,000.00 (240 payments of $500), meaning growth from the rate of return alone accounts for $140,463.33 of the final balance.
Common mistakes
- Assuming the projected future value is guaranteed rather than a projection based on a fixed assumed rate of return, which market-based investments do not guarantee year to year.
- Forgetting to account for the difference between an ordinary annuity (payments at period end, used here) and an annuity due (payments at period start), which produces a slightly different future value for the same inputs.
- Not adjusting the projection when contribution amounts or the assumed rate of return change significantly, since this calculator holds both constant across the entire period.
- Overlooking taxes and fees on investment growth, which this calculator does not model and which reduce the actual after-tax, after-fee accumulated value in a taxable account.
- Confusing this accumulation-phase calculator with a payout calculator — this tool projects how contributions grow into a future lump sum, not how an existing lump sum converts into withdrawals (see the annuity payout calculator for that calculation).
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How much will $500 a month grow to at 7% over 20 years?
Contributing $500 monthly at a 7% annual rate of return for 20 years (240 months), using the ordinary annuity future value formula, produces a future value of $260,463.33. Of that total, $120,000 comes from contributions and $140,463.33 comes from investment growth.
What is the difference between an ordinary annuity and an annuity due?
An ordinary annuity assumes each payment occurs at the end of its period (the convention this calculator uses), while an annuity due assumes payments occur at the start of each period. Because annuity-due payments compound for one extra period each, an annuity due produces a slightly higher future value than an ordinary annuity for otherwise identical inputs.
Is the future value shown by this calculator guaranteed?
No. The future value is a mathematical projection based on the fixed monthly contribution and assumed annual rate of return entered. Actual investment returns for market-based accounts vary year to year and are never guaranteed, so the true accumulated value can differ, sometimes substantially, from this projection.
Does this calculator account for taxes on investment growth?
No. This calculator projects growth using the entered rate of return without adjusting for taxes on interest, dividends or capital gains, or for account fees. Actual after-tax, after-fee accumulation in a taxable account will generally be lower than this calculator's gross projection.
What's the difference between this calculator and the annuity payout calculator?
This calculator projects how a series of regular contributions accumulates into a future lump sum (the accumulation phase). The annuity payout calculator instead starts with an existing lump sum and calculates the level payment that lump sum can support over a set payout period (the distribution phase) — the two calculators model opposite directions of the same underlying annuity mathematics.
Kaynaklar
- U.S. Securities and Exchange Commission (SEC), Investor.gov. Compound interest and annuity calculators, and investing basics. investor.gov.
- Brealey RA, Myers SC, Allen F. Principles of Corporate Finance (13th ed.). McGraw-Hill, 2020. Chapter 2: How to Calculate Present Values (annuity formulas).
- Financial Industry Regulatory Authority (FINRA). Saving for retirement and understanding annuities. finra.org.