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.
Example: with Solve for End amount · Target end amount (solve mode) $500,000 · Starting amount $10,000 · Years invested 20 yrs → End balance: $300,850.72.
- Starting amount$10,000
- Total contributions$120,000
- Total interest / growth$170,851
Computed by the calculator below using its default values. Change any input to see your own numbers.
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.
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.
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.