Investment Calculator
Project any investment: a starting amount, regular contributions (monthly or annual, at the beginning or end of each period), a return rate, and your choice of compounding. Flip the solve mode to work backwards — how much you must contribute to hit a target — and watch the year-by-year accumulation schedule update live.
Where the end balance comes from
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Compare platformsHow investment growth works
Three levers set your end balance: how much goes in, how long it stays, and the rate it earns. Compounding means each period’s growth is calculated on the running balance — contributions made early earn growth on their growth, which is why the interest line eventually overtakes the contribution line in the schedule below. The compounding frequency fine-tunes the result; the years invested and the contribution amount move it by orders of magnitude.
How it’s calculated
The per-contribution-period rate is i = (1 + r⁄m)m⁄k − 1, where r is the annual rate, m compounding periods per year, and k contributions per year. End balance = P(1 + i)n + C × [((1 + i)n − 1) ÷ i] with n = k × years, multiplied by (1 + i) for beginning-of-period contributions. Solve mode inverts the same equation: C = (target − P(1 + i)n) ÷ annuity factor. The schedule simulates period by period, so schedule totals match the formulas to the cent.
Projections assume a constant return with no taxes, fees, or volatility — real markets deliver none of those guarantees. Educational estimates only.
Accumulation schedule
Yearly deposits, growth, and ending balance. In solve mode the schedule uses the required contribution.
Worked example
Start with $10,000 and add $500 at the end of each month at a 7% return compounded monthly. After 20 years you’d hold $300,850.72 — $10,000 starting money, $120,000 of contributions, and $170,851 of growth. The same plan compounded annually lands at $292,465, and shifting deposits to the beginning of each month nudges it to $302,370. Working backwards: hitting $500,000 in those 20 years takes about $882.30 a month.
Common mistakes
- Planning around a double-digit return because a recent year delivered one — long-run averages include brutal years.
- Ignoring fees: a 1% expense ratio quietly turns a 7% return into 6% and costs tens of thousands over decades.
- Waiting for a “better time” — the years invested matter more than the entry point for regular contributors.
- Forgetting inflation: $300,000 in 20 years buys far less than it does today; check the real (after-inflation) value too.
Where it is used
- Projecting a brokerage, IRA, or 529 balance from a monthly auto-invest plan.
- Back-solving the monthly savings needed for a target like a down payment or retirement number.
- Comparing compounding or timing options a bank or broker advertises.
Frequently asked questions
What return rate should I assume?
The S&P 500’s long-run average total return is roughly 10% a year nominal — about 6–7% after inflation — per S&P Dow Jones Indices data back to 1926. Diversified planners often model 5–8% to stay conservative; assuming much more than 10% builds a plan on an above-market bet.
Does contributing at the beginning of the month really matter?
A little. Beginning-of-period deposits earn one extra period of growth each cycle. On $500 a month at 7% for 20 years it is roughly a $1,500 difference — nice, but far less important than the contribution amount and the years invested.
How much does compounding frequency change the result?
Less than people expect. $10,000 plus $500/month for 20 years at 7% grows to about $300,851 with monthly compounding versus about $292,465 with annual compounding — around 3% apart. Rate, time, and contributions dominate; frequency is a rounding-level effect.
How do I use the solve-for-contribution mode?
Switch the first dropdown to “Contribution needed for target”, enter your target amount, and the calculator back-solves the periodic deposit: C = (target − growth of your starting amount) ÷ annuity factor. With $10,000 down, 7% monthly-compounded, 20 years, hitting $500,000 takes about $882.30 a month.
Is this before or after taxes and fees?
Before both. In taxable accounts, dividends and realized gains are taxed along the way, and fund fees compound against you — a 1% annual fee at a 7% gross return behaves like a 6% return. Use a net-of-fee rate for realism.