What investing a fixed amount every month actually does
Every deposit you make starts its own clock. The $500 you invest this month compounds for the entire remaining term; the $500 you invest in the final month compounds for essentially no time at all. A monthly investing projection is really just the sum of a few hundred separate compound-growth calculations, each one running for a different length of time.
That structure explains the shape of the result. At $500 a month and an 8% return, twenty years produces roughly $296,000 from $120,000 of deposits. Thirty years — half again as long — produces about $750,000 from $180,000. The extra decade adds 50% more deposits and more than doubles the outcome, because it is the early deposits, the ones with the longest runway, that do the heavy lifting.
The practical consequence is that time in the market matters more than the size of the deposit, at least across the ranges most people are choosing between. Someone investing $300 a month starting at 25 finishes ahead of someone investing $500 a month starting at 35, on the same assumptions, despite depositing less in total.
The formula, and why the timing of each deposit matters
The standard closed form is:
Future value = P × [((1 + i)n − 1) ÷ i] × (1 + i)
where P is the monthly amount, n the number of months, and i the monthly rate. The bracketed part sums the growth of every deposit. The trailing × (1 + i) is the part worth understanding: it assumes each deposit lands at the start of the month rather than the end, so every one earns an extra month of growth.
That is called an annuity due, and it is what essentially every monthly investing calculator assumes, because automatic transfers usually go out on the 1st. It is a small difference per month and a visible one over decades — dropping it would reduce a twenty-year projection by roughly the value of one month's growth on the whole balance.
Why two calculators give two different answers
Enter identical numbers into two reputable calculators and you can get answers thousands of dollars apart. It is almost never a bug. It is one specific, rarely disclosed assumption: how the annual rate is converted into a monthly rate.
- Divide by twelve. A 12% annual return becomes 1.0% a month. Simple, and by far the more common choice.
- Take the twelfth root. A 12% annual return becomes 0.949% a month, because 1% compounded twelve times produces 12.68% a year, not 12%.
The second is arithmetically tidier: it makes the annual figure you typed the figure you actually get. The first is what most published calculators use, and what a bank quoting a "12% rate compounded monthly" means. Neither is wrong so long as you know which you are looking at.
The gap is not academic. On $10,000 a month at 12%:
- 5 years — about 1.7% apart
- 10 years — about 3.7% apart
- 20 years — about 8.6% apart
- 30 years — about 14.6% apart
This calculator defaults to dividing by twelve, matching the majority of published tools, and lets you switch under More Options. If your number here disagrees with another site, that switch is the first thing to check.
What to put in each field
Invested each month. What actually leaves your account, not what you hope to manage. A projection built on an aspirational figure tells you about a person who does not exist.
Expected return. An annual rate, before fees and taxes. See the FAQ below on choosing a defensible one — the short version is that this field, not the deposit, is where optimism does the most damage.
For how long. Years of contributions. The calculator assumes you stop depositing and cash out at the end; leaving the balance invested longer would grow it further.
Annual step-up. The percentage you raise the monthly amount by each year. Zero gives a level plan.
Inflation. Optional, under More Options. Above zero, the result gains a today's-money line — often the more useful figure, since $750,000 in thirty years does not buy what $750,000 buys now.
Stepping up your contribution as your income grows
A level monthly amount quietly shrinks in real terms. $500 a month invested for thirty years is $500 in year one and $500 in year thirty, by which point it represents a much smaller share of your income and buys considerably less.
Stepping up means raising the deposit by a set percentage each year, usually timed to a pay rise so it never feels like a cut. The increase compounds: a 10% step-up on $500 gives $550 in year two, $605 in year three, $665 in year four. That is the convention this calculator uses and the one the industry quotes, though it is worth checking, because at least one major fund house describes a step-up as a flat annual increase in its own worked example while publishing a corpus figure that follows neither reading.
The effect is large, and larger than most people expect, because the increases apply to the early years too — the deposits with the most time to compound. Turning on even a modest step-up typically changes the ending figure more than adding a percentage point to the assumed return, and unlike the return, it is a decision you actually control.
Monthly investing versus a lump sum
If you already have the money, investing it all at once usually finishes ahead. The reason is mechanical rather than clever: every dollar is exposed for the full period instead of trickling in. Studies of long US market history put lump sum ahead roughly two-thirds of the time, which is another way of saying markets rise more often than they fall.
Monthly investing wins in the other third — when the market falls after you start, so your later deposits buy in cheaper. It also has an advantage no spreadsheet captures: it is the only option most people have, because income arrives monthly rather than in one piece, and it removes the decision of when to buy, which is where a lot of self-inflicted damage happens.
The comparison tab runs both on identical assumptions so you can see the size of the gap rather than the direction of it. If the gap is small relative to how much a badly timed lump sum would bother you, that is a real answer.
SIP, dollar-cost averaging, and what the names mean
Three terms describe overlapping things, and the differences are mostly geographic.
A Systematic Investment Plan (SIP) is the standard Indian term for investing a fixed amount into a mutual fund at regular intervals by automatic debit. It is an enormous market — monthly SIP inflows run into the tens of thousands of crores — which is why most SIP calculators quote rupees.
Dollar-cost averaging is the American name for the same behaviour, though it is often used more narrowly to mean deliberately spreading out a lump sum you already hold, rather than investing income as it arrives.
An automatic monthly transfer into an index fund, or payroll deferrals into a 401(k), is the same arithmetic again under no particular name.
The mechanics on this page apply to all three. What differs is the wrapper — a 401(k), an IRA, an ELSS, a taxable brokerage account — and those differences affect your tax, not the compound growth.
What this calculator does not cover
- Fees. No expense ratio, no platform charge. A 0.5% annual fee costs more over thirty years than most people guess; check your fund's published figure.
- Taxes. Nothing here knows whether you are in a retirement account or a taxable one, and that difference can be larger than several years of contributions.
- Volatility and sequence. The projection is a smooth curve. Real returns arrive unevenly, and the order matters a great deal if you need the money at a fixed date.
- Missed or paused contributions. It assumes you never skip a month.
- Whether the return is realistic. It calculates what you typed. It has no view on whether the market will deliver it.
Frequently asked questions
How much will I have if I invest $500 a month?
At an 8% annual return, $500 a month becomes roughly $296,000 after 20 years, of which $120,000 is your own deposits and the rest is growth. Stretch it to 30 years and the same $500 reaches about $750,000, because the deposits you made in the early years have had decades to compound. The single biggest lever in that arithmetic is time, not the amount, which is why starting smaller and earlier usually beats waiting until you can afford more.
Why does another calculator give me a different number?
Almost always because of how it converts your annual rate into a monthly one. Dividing by 12 treats 12% as 1% a month; taking the twelfth root treats it as about 0.949% a month, since compounding 1% twelve times produces more than 12% a year. Both are in wide use and neither is a mistake, but they diverge as the horizon lengthens, reaching roughly 8.6% apart over 20 years. This calculator lets you switch between them so you can match whichever tool you are comparing against.
What is a SIP, and is it different from what I do in the US?
A Systematic Investment Plan is the standard Indian term for investing a fixed amount into a mutual fund at regular intervals, usually monthly by automatic debit. Mechanically it is the same thing an American does with an automatic monthly transfer into an index fund, or with payroll deferrals into a 401(k). The name differs and the tax wrappers differ, but the arithmetic on this page applies either way.
Should I invest monthly or put in a lump sum?
If you already hold the money and returns turn out positive, investing it all at once finishes ahead, because every dollar is exposed for longer. Studies of historical US data find lump sum wins roughly two-thirds of the time for that reason. Monthly investing wins when markets fall after you start, and it is the only option available to most people anyway, since the money arrives with each paycheck rather than in one piece. The comparison tab shows both paths on the same assumptions.
What return rate should I assume?
Something you can defend rather than something that makes the total look good. US large-cap stocks have returned roughly 10% a year nominally over very long periods, which is closer to 7% after inflation, and any single decade can land far from that. A projection built on 12% is not a plan, it is a hope with a spreadsheet attached. Running the same numbers at a rate two or three points lower is a quick way to see how much your conclusion depends on the assumption.
Does this account for fees, taxes or inflation?
Inflation only, and only if you ask. Open More Options and enter an inflation rate to see what the final figure is worth in today's money, which is usually the number that actually matters. Fund expense ratios and taxes are not modelled: a 0.5% annual fee reduces a long projection by more than most people expect, and the tax treatment depends entirely on whether you are investing inside a retirement account or a taxable brokerage account.
These results are estimates for general information only and are not investment advice. Projections assume a constant rate of return, uninterrupted contributions and no fees or taxes, none of which holds in practice. Past market returns do not predict future performance, and the value of investments can fall as well as rise. Confirm any figure against your own account statements and fund documents, and speak to a qualified financial adviser before making investment decisions.