A SIP calculator answers one question: if I invest a fixed amount every month and it grows at some assumed rate, how much could I end up with? The answer comes from a single piece of textbook mathematics, the future value of an annuity, and everything else the tool shows — the invested total, the estimated gains, the year-by-year schedule — is arithmetic built on top of it. Take $500 a month at an assumed 12% a year for 10 years: the calculator returns $116,169.54, made of $60,000 you put in and $56,169.54 in estimated gains. This guide derives that number by hand, explains each input, shows what the calculator deliberately leaves out, and flags the mistakes that make people trust a projection more than they should. All return rates here are illustrative assumptions used to demonstrate the maths, never predictions or promises about any real fund.

What a SIP calculator is

A Systematic Investment Plan, or SIP, is an instruction to invest a fixed amount at regular intervals — almost always monthly — into a mutual fund. Each instalment buys units at that day’s price, so a falling market buys you more units and a rising market buys fewer. A SIP calculator models that habit in money terms: it does not track units or prices, it projects the value of the growing pot under one simplifying assumption, that the pot grows at a steady rate every month. That assumption is the calculator’s greatest strength and its greatest weakness. It makes the maths clean enough to compute instantly, and it guarantees the answer will differ from reality, because no fund grows steadily. The honest way to use the tool is to treat its output as a scenario — “if the return were 12% every year, this is where the plan lands” — and then to run the same plan at 8% and at 10% and compare. The range between those scenarios is far more informative than any single headline number, and it is the reason this site’s calculator puts the return rate front and centre instead of hiding it behind a default.

The tool on the SIP Calculator page has two modes. In growth mode you enter a monthly amount and it projects the final value. In goal mode you enter a target value and it solves for the monthly amount that target needs, using the same formula run backwards. Both modes share the same engine, and both accept three optional refinements under Advanced options: an initial lump sum invested on day one, an annual step-up percentage that raises the monthly amount each year, and an expense ratio that is deducted from the assumed return before compounding. Those refinements are handled month by month in the engine rather than by the closed-form formula, because a step-up breaks the “same amount every month” premise the formula needs. The closed form and the month-by-month engine agree exactly for a flat plan with no lump sum and no expenses, which is a useful cross-check: if you ever doubt a result, compute the formula by hand for the flat case and compare.

The three inputs, and what each one does

The monthly amount is the only input you control directly, and it scales the answer linearly: double the monthly amount and the projected value doubles, exactly, as long as the return and the duration stay the same. That linearity is why the tables in this guide can be read across rows — $100, $300 and $500 a month at the same rate and duration are in exact proportion. The assumed annual return is the most powerful input and the most dangerous one, because small changes compound into large differences over long periods. Moving the assumption from 10% to 12% on a $500 monthly plan over 20 years moves the result from $382,848.45 to $499,573.96, a gap of $116,725.51 created by two percentage points. Duration is the quiet superpower: time does more work than either of the other inputs, because compounding is exponential in time. The same $500 at 12% reaches $116,169.54 in 10 years, $499,573.96 in 20 years, and $1,764,956.89 in 30 years — the third decade alone adds more than the first two decades combined. If you remember one sentence from this section, make it this: start early, because no realistic increase in the monthly amount can buy back a lost decade.

The third input, duration, deserves one practical note. The calculator accepts years plus up to 11 extra months, because real plans rarely start and end neatly on anniversaries. A plan of 10 years and 6 months is 126 monthly instalments, not 120, and those six extra instalments plus their compounding are worth real money. When comparing two plans, always compare identical durations; a plan that looks better only because it runs six months longer is not better, it is longer.

The formula, derived in plain language

Start from the simplest case: you invest M dollars at the end of every month, and the pot grows by a monthly rate i. After one month you hold M. After two months you hold M(1+i) + M, because the first instalment has grown for one month and the second has just arrived. After three months you hold M(1+i)² + M(1+i) + M. The pattern is a geometric series, and the standard formula for its sum gives FV = M × (((1+i)ⁿ − 1) ÷ i) for n monthly instalments. That is the future value of an ordinary annuity, and it is the formula most textbooks print first.

This calculator, however, assumes contributions at the start of each month — the way most real SIPs work, with the instalment invested and then left to grow for the full month. Every instalment therefore gets one extra month of growth compared with the end-of-month version, and the whole sum is multiplied by one extra (1+i). That is the origin of the final × (1+i) in FV = M × (((1+i)ⁿ − 1) ÷ i) × (1+i), the future value of an annuity due. The difference is small on one instalment and material over a decade: for $500 a month at 12% over 10 years, the end-of-month version gives $115,019.34 and the start-of-month version gives $116,169.54, a difference of $1,150.20 that belongs entirely to that extra month of growth on every instalment. If you reproduce this calculator in a spreadsheet and get the smaller number, you have almost certainly used the wrong payment-timing setting — the companion guide on building a SIP calculator in Excel explains the fix, which is a single argument in the FV function.

Two conversions hide inside the symbols and cause most hand-calculation errors. First, i is the monthly rate, not the annual rate: at 12% a year, i = 0.12 ÷ 12 = 0.01, or 1% a month. Second, n is the number of months, not years: 10 years means n = 120. Plugging 0.12 and 10 into the formula instead of 0.01 and 120 is the single most common arithmetic mistake in SIP calculations, and it produces nonsense large enough that you should notice — but only if you have a rough sense of the right answer first, which is exactly what the worked example below is for.

Worked example: $500 a month at 12% for 10 years

Take M = $500, an annual return of 12%, and 10 years. Step one: the monthly rate i = 0.12 ÷ 12 = 0.01. Step two: the number of months n = 10 × 12 = 120. Step three: compute (1+i)ⁿ = 1.01¹²⁰ = 3.300386. Step four: subtract 1 to get 2.300386, and divide by i = 0.01 to get 230.0386. Step five: multiply by the monthly amount, 500 × 230.0386 = $115,019.34 — that is the end-of-month figure. Step six: multiply by (1+i) = 1.01 for start-of-month timing, giving $116,169.54. The invested total is simply 500 × 120 = $60,000, and the estimated gains are the difference, $116,169.54 − $60,000 = $56,169.54. Every other figure in the table below follows the same six steps with different inputs, and you can verify any cell with a pocket calculator in under a minute.

Read the table down the duration column, not just across it, because that is where the story lives. The $500 plan roughly quadruples from decade one to decade two, and more than triples again in decade three, while the amount invested only doubles and then triples. The gains column tells the same story from the other side: after 10 years gains are slightly less than the amount invested ($56,169.54 vs $60,000); after 30 years gains are nearly ten times the amount invested ($1,584,956.89 vs $180,000). Compounding is slow to start and overwhelming late, which is why a projection that looks disappointing at year five can look extraordinary at year twenty-five — and why judging a SIP by its first few years is the worst possible test of it.

Reading the result: invested, gains, and the schedule

The year-by-year schedule on the calculator page is the same computation broken into annual snapshots: each row shows that year’s monthly amount (which rises if you set a step-up), the total invested so far, the gains so far, and the total value. Use it to answer the questions the headline cannot: when do gains overtake the amount invested? For the $500 plan at 12%, the crossover happens around year eleven — before that, most of what you see is your own money coming back to you. What does a single bad year do? The schedule cannot show that, because it assumes the smooth rate throughout; for bad years you need to understand the next section, on what the calculator leaves out.

What the calculator leaves out — on purpose

Taxes are the largest omission, and the page says so plainly. Capital gains tax on mutual fund redemptions depends on the fund type, the holding period of each instalment, and the tax rules in force when you sell — none of which a projection can know in advance — so the calculator shows a pre-tax figure. If your jurisdiction taxes long-term gains, the spendable result will be lower than the headline, and the gap grows with the gains column. Fund expenses are handled differently: the expense ratio field deducts the charge from the assumed return before compounding, so a 12% assumption with a 1% expense ratio compounds at 11% and the same $500 plan ends at $109,493.64 instead of $116,169.54. That $6,675.90 difference is the honest price of a 1% yearly charge over a decade, and it is worth entering a real fund’s published expense ratio rather than leaving the field at zero. Market volatility is the second omission and the more important one: real returns arrive as a sequence of good and bad years, and the order matters. Two plans with the same average return can end in different places if the bad years fall early or late, and a SIP’s averaging softens but does not remove that sequence risk. The calculator’s smooth return is a planning simplification, not a description of any market.

A SIP calculator also does not choose a fund, does not recommend an amount, and does not know your circumstances. “Best SIP” lists in search results are marketing, not maths, and this guide deliberately makes no fund recommendation: the right fund depends on your goal, horizon, and tolerance for falls, and a calculator cannot assess any of those. Treat any tool that names a specific fund alongside a projected return with suspicion — the projection is generic, and the recommendation is advertising.

Common mistakes when using a SIP calculator

The first mistake is typing a past return as if it were a future one. A fund that returned 15% over the last five years has not promised 15% over the next five, and projecting at the highest past figure is how plans get built on sand. Run 8% and 10% alongside 12% and plan around the lower end. The second mistake is forgetting inflation: $116,169.54 in ten years’ time buys less than $116,169.54 today, and the companion guide on SIP returns and inflation shows the adjusted figures. The third mistake is comparing plans of different durations or different payment timings without noticing — a step-up plan, an end-of-month plan, and a flat start-of-month plan are three different calculations, and only like-for-like comparisons mean anything. The fourth mistake is stopping and starting: every gap in contributions is a gap in compounding, and the schedule shows exactly what a missed year costs. The calculator is at its most useful when it stops you making these four mistakes, not when it prints its biggest number.

Sources: Standard future-value-of-annuity mathematics (annuity-due form); worked figures computed for this guide from the stated formula; CalcFolk SIP Calculator engine (monthly compounding, start-of-month contributions). All return rates are illustrative assumptions.