What a plan is worth at the end of it
Future value is the mirror of discounting. Instead of asking what later money is worth today, it asks what today's money, and everything you add along the way, comes to by a date you choose. It is the arithmetic behind every projection of a savings plan, a pension pot or a deposit left alone for a decade.
The answer splits into two parts on this page: what the opening sum grows into by itself, and what the regular payments add. The split is worth looking at, because which part dominates tells you what to change. Early in a plan the payments carry almost all of it; late in a long one, growth on the money already there can dwarf anything you can afford to add.
The rate is the assumption, not the answer
Everything here is a straight line from one rate you typed. A single rate every period, forever, is not how any real investment behaves; it is a way of saying "on average, something like this". The honest way to use a future value is to run it at more than one rate and treat the spread as the answer, rather than the middle number as a prediction.
Nothing is taken off for tax, charges or inflation. Platform fees and fund charges come off the rate, so an investment expected to return a certain amount before charges should be entered at less than that. Inflation is the larger effect over long periods and the easiest to forget: a sum decades away buys much less than the same sum today, even when every figure here is correct.
Paying in at the start or at the end
A payment at the start of each period earns for that period; one at the end does not. Over a long plan that single period of difference compounds into a visible amount, and it is the difference between a standing order on pay day and one at the end of the month. The field is there because the two conventions produce different figures and most calculators quietly pick one.
Common questions
Why does this differ from the compound interest page?
The savings pages model an account month by month: interest accrues monthly and joins the balance on the compounding date you choose, which can be yearly, twice a year, quarterly or monthly. This page treats the plan as a stream of payments on one grid of equal periods, with the rate applied once per period. Choose a monthly period here and a monthly compounding there, with payments on the same rhythm, and the two agree closely; make the rhythms differ and they part slightly, because they are answering different questions.
Can I use this for a loan?
Not directly. A loan needs the payment that clears a balance to exactly zero over a term, which is found by searching rather than by compounding forward, and the loan calculator does that properly including the last instalment being a little different from the rest.
What if my payments change over time?
A single payment amount cannot express that, and this page will not pretend otherwise. A plan with a payment that rises each year is a stream of different amounts, which the IRR and present value pages can take as a list.
Why is there a limit on how large the answer can be?
Because every figure is held as a whole number of cents and multiplied in a way that stays exact. Beyond the limit the arithmetic would start losing precision quietly, so the page stops and says so rather than showing a number that has begun to drift. A plan that hits the ceiling part way through is refused for the same reason.
Is anything I type stored?
No. The calculation happens on the server as part of rendering the page, nothing is written down, and the access log for this site drops the query string precisely so that the numbers you enter are never recorded.
Sources
No figure on this page is read from a table, so there is nothing to source. What these documents define is the convention behind the arithmetic, and the regulated measures this calculator deliberately does not compute:
How this is worked out
The figures are arithmetic on the numbers you type, and nothing here is a forecast of what an investment will actually do.
Money is held in whole cents and every step is integer arithmetic, so the same inputs always give the same answer. A period is discounted in one division, with the annual rate divided by the number of periods in a year inside the same expression, and the result rounded to the nearest cent. That means the answer is a cent or two away from the textbook formula, which lets interest run to fractions of a cent forever. The table on the page is the calculation itself rather than a second pass over it, so the rows and the headline can never disagree.
This page treats money as a stream of payments on a grid of equal periods. The compound interest and savings pages model a bank account instead, where interest is earned every month and added to the balance on the compounding date you choose. The two answers differ by a little when the paying-in rhythm is not the compounding rhythm, and neither is wrong: they are answers to two different questions.