What your result means
Your final corpus is what the plan is projected to be worth on the last day, assuming you never miss a contribution and the return you entered holds. Total invested is simply your monthly amount multiplied by the number of months, and the gap between the two is compound growth.
The number to act on is today's purchasing power. A SIP running fifteen or twenty years spans enough time for inflation to halve what a pound buys, so a maturity value quoted in future pounds tells you very little about the life it will fund. Both figures sit side by side here for exactly that reason.
Watch the annualised return row. It is deliberately not "total gains divided by total invested" — that calculation flatters a SIP badly, because your final month's contribution has been invested for one month, not fifteen years. This is the money-weighted rate, the honest comparison against a savings account or a lump sum.
A SIP is a bet on consistency rather than on timing
Investing a fixed amount every month means you automatically buy more units when prices are low and fewer when they are high. Nobody has to be clever about it and nobody has to guess the bottom — the arithmetic of a fixed payment does the work. That is the entire mechanism, and it is why a SIP is the default recommendation for people who will never watch a market.
The trade-off is that a SIP cannot beat a well-timed lump sum in a rising market, because most of your money arrives late. What it beats is the far more common alternative: waiting for a better moment, and never investing at all.
How it works
Each month your contribution is added to the balance, and the whole balance then grows by one twelfth of the compounded annual return. Contributions default to the start of the month, which is what fund houses assume — you can switch to end-of-month in the advanced options for a slightly more conservative figure.
Because every instalment starts compounding from the month it lands, the earliest contributions do a disproportionate amount of the work. A pound invested in year one of a twenty-year plan at 12% grows to about £9.65; the same pound invested in year nineteen grows to about £1.27.
Fund charges are taken from the balance monthly rather than from your contribution, which is how an ongoing charges figure actually works. That is why the damage compounds too.
How to use this calculator
- Enter the amount you can invest each month.
- Set the expected annual return with the slider.
- Set how many years the SIP will run.
- For a realistic figure, open Advanced options and set the fund's ongoing charge and an inflation rate.
- Read the maturity value, then check the same figure in today's money underneath it.
Formula
M = maturity value, P = monthly investment, i = monthly return, n = number of instalments. The trailing (1 + i) is what makes it a start-of-month calculation; drop it for end-of-month.
Worked example
£500 a month for 15 years at a 12% annual return, with 3% inflation:
Investment gains £147,966
Maturity value £237,966
In today's money £152,741
Real return 8.74% a year
Now add a 1% ongoing charge, which is unremarkable for an actively managed fund. The maturity value drops to £216,416 — you hand over £12,298 in fees and end up £21,550 worse off, because the fees you paid in year three would otherwise have compounded for twelve more years. Switching to a 0.1% index fund keeps almost all of that difference.
Frequently asked questions
Is a SIP better than investing a lump sum?＋
Mathematically, investing everything immediately wins more often than not, simply because the money spends longer in the market. A SIP wins on behaviour: it removes the decision, it removes the temptation to wait for a dip, and it matches how salaries actually arrive. Most people do not have a lump sum to deploy, which settles the question.
What happens if I miss a month?＋
One missed instalment costs you that contribution plus all the growth it would have earned, which over a long plan is several times the amount itself. It does not break the plan — but the calculator assumes an unbroken run, so treat its figure as the best case for your consistency.
Should I use 12% or something lower?＋
Twelve per cent is the number most Indian SIP calculators default to and it reflects long-run domestic equity history; UK and global equity investors have typically seen less in nominal terms. Whatever you choose, run the plan again three points lower and see whether you still like the answer.
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Assumptions & limitations
A SIP projection is only as good as its assumptions. These are the ones baked in:
- Returns are assumed constant every year. Real markets are not — they deliver the average through a sequence of good and bad years, and the order those arrive in changes the outcome.
- Growth is compounded monthly from the annual rate you set, so 10% a year becomes 0.797% a month rather than a flat 0.833%.
- Fund charges are deducted monthly from the balance. Platform fees, trading costs and bid-offer spreads are not modelled separately — fold them into the charge figure if you want the full picture.
- Inflation is applied at a single constant rate to produce the "today's money" figures. Your personal inflation rate depends on what you actually buy.
- Nothing here is financial advice, and no projection is a promise. Use it to compare scenarios against each other, not to predict a number.
- Every instalment is assumed to be paid in full and on time for the entire term.