Future Value Calculator

This future value calculator answers a single question precisely: given an amount today, a contribution plan, a rate of return and a horizon, what is the balance at the end? It handles the cases that trip up simpler tools — a zero rate, no contributions at all, contributions at the beginning rather than the end of each period, and a compounding frequency that differs from the contribution frequency.

It also reports what most calculators leave out. Every result comes with the split between money you paid in and money the market added, the same figure restated in today's purchasing power, and a scenario band showing how much of the projection depends on the return assumption rather than on your saving.

Nothing is hidden behind an email form, and every result is reproducible: the formulas are written out below the tool, and the schedule shows the arithmetic year by year so you can check any row by hand.

The headline figure is a nominal future value — the number of dollars in the account at the end of the horizon. On its own it overstates what the money will buy, which is why the inflation-adjusted figure sits directly beside it. At 2.5% inflation a balance thirty years out has roughly half the purchasing power its nominal value suggests.

The growth share is the most diagnostic number on the page. A projection that is mostly contributions is under your control: it will land close to plan whatever markets do. A projection that is mostly growth is a bet on the return assumption holding for decades, and the lower scenario tells you what happens if it does not.

The schedule is where the compounding becomes visible. Early rows are dominated by contributions and the growth column is small; late rows invert that relationship entirely. If the late-year growth figures look implausibly large next to your contributions, that is the exponent doing its work — and a good moment to re-run the projection at a lower rate.

The schedule has two levels of detail. The yearly view is the summary; the per-period view shows one row per contribution, which is where you can see exactly when each deposit lands relative to the growth it earns. The two always reconcile, because the yearly rows are aggregates of the periodic ones.

The calculation has two independent terms. The starting amount compounds for the whole horizon. Each contribution compounds only from the moment it is made, so the contribution stream is valued with the annuity factor rather than the simple growth factor.

Because the two terms do not interact, the complete future value is their sum. A zero starting amount reduces the result to the annuity term alone; a zero contribution reduces it to the lump sum term.

Zero-rate inputs are handled explicitly rather than by dividing by a near-zero denominator: at a rate of zero the annuity term becomes the contribution multiplied by the number of periods, which is the correct limit. Money is carried at full precision through the schedule and rounded to cents only for display.

Take $10,000 today, $300 a month for 20 years, a 7% nominal return compounded monthly and contributions made at the end of each month. The monthly rate is 0.0058333 and there are 240 contributions.

The starting amount grows to $10,000 × 1.0058333^240 = $40,387.39. The annuity factor is (1.0058333^240 − 1) / 0.0058333 = 520.9267, so the contributions are worth $156,278.00. The total is $196,665.39 against $82,000 paid in, meaning $114,665.39 — 58% of the ending balance — is growth.

Restated at 2.5% inflation, that $196,665 has the purchasing power of about $120,019 today. Both figures describe the same account; only the second one can be compared against what you spend now.