A practical guide to SIP investing

A systematic investment plan is a standing instruction: invest a fixed amount, on a fixed date, every month. That is the whole mechanism. Everything else written about SIPs is commentary on what that mechanism does.

What a SIP is not

Worth clearing up first, because the misconceptions are common and they matter.

A SIP is not a product. It is a way of buying one. You are not investing "in a SIP" any more than you are investing in a bank transfer. The risk you take comes entirely from the fund you buy.

A SIP does not guarantee returns. It does not protect against loss. If the fund you chose falls 30%, your SIP falls with it.

A SIP is not automatically safer than a lump sum. It spreads entry timing risk. It does nothing about the risk of the underlying asset, which is the larger risk over long periods.

The arithmetic

Each instalment compounds for however long it remains invested. The first one gets the full tenure; the last gets one month. Summing that geometric series gives:

The trailing $(1+i)$ is there because a SIP is an annuity-due — the instalment is invested at the start of the month, so it earns that month's return.

A worked example

₹10,000 a month, 10 years, assuming 12% a year:

$$i = \frac{12}{1200} = 0.01 \qquad n = 120 \qquad (1.01)^{120} = 3.300387$$ $$10{,}000 \times \frac{3.300387 - 1}{0.01} \times 1.01 = 23{,}23{,}391$$

You pay in ₹12,00,000. The projection is ₹23,23,391. The difference — ₹11,23,391 — is what the assumed rate contributes.

Under that assumption, and only under it.

Rupee cost averaging, honestly

The standard claim is that investing a fixed amount buys more units when prices are low and fewer when high, lowering your average cost.

That is arithmetically true. It is also frequently oversold.

What it genuinely does: removes the decision of when to invest, which most people get wrong, and matches the rhythm of salaried income.

What it does not do: beat a lump sum in a rising market. If markets go up over your investing period, investing everything on day one wins, because that money spent longer invested — and nobody knows in advance which kind of period they are in. Rupee cost averaging is insurance against bad timing, and like all insurance it has a cost.

The honest summary: a SIP is a good default because it is achievable and removes a decision you are unlikely to make well, not because it is mathematically superior.

Choosing an amount

Work backwards from a goal rather than forwards from a spare-cash figure. The Investment Goal calculator does the inversion.

One critical detail people routinely miss: set the target in future rupees, not today's. If you want the equivalent of ₹50 lakh in today's purchasing power in 20 years, at 6% inflation you should be targeting about ₹1.6 crore. Planning against the un-inflated figure is the most common error in this whole exercise. The Inflation calculator does that conversion.

The step-up

Most people's income rises. A flat SIP therefore represents a shrinking share of income every year.

A step-up SIP raises the instalment on each anniversary. A 10% step-up on ₹10,000 means ₹11,000 in year two, ₹12,100 in year three. Because the increases also compound, the effect on the final figure is larger than people expect — and it is often more achievable than committing to a bigger flat amount today.

The Step-Up SIP calculator compares the two directly.

Choosing a rate to assume

There is no correct answer, and anyone who gives you one confidently is overreaching.

Many people use 10–12% as a long-run illustration for diversified Indian equity funds, and lower figures for debt or hybrid funds. Whatever you pick is an assumption about the future, not a fact about it.

A genuinely useful habit: run the calculation twice — once at the rate you hope for, once several percentage points lower — and check whether the plan still works in the second case. If it only works at the optimistic rate, it is not a plan.

What the projections leave out

Every figure on this site, and on every SIP calculator anywhere, omits:

Taxes. Capital gains tax on redemption reduces what you actually receive. The treatment depends on the fund type and holding period, and the rules change; check the current position rather than assuming.

Costs. The expense ratio is deducted from returns before you see them, and exit loads may apply on early redemption. A 1% expense ratio over 30 years reduces the final figure by roughly a quarter, because every rupee taken early is also a rupee that never compounds.

Sequence. The projection assumes a steady rate. Real returns arrive in an unpredictable order, and order matters — particularly once you start withdrawing.

Behaviour. The largest gap between projected and actual outcomes is usually not the market. It is stopping the SIP during a fall, which converts a temporary decline into a permanent loss.

Setting one up

1. Decide the goal and its future-rupee amount.
2. Work out the required instalment; check it against what you can sustain in a bad month, not a good one.
3. Choose a fund. This guide takes no view on which — that is a decision about your own risk tolerance and horizon, and it is where a SEBI-registered investment adviser genuinely adds value.
4. Set the debit date shortly after your salary date, so the money leaves before it can be spent.
5. Set a step-up if your income is likely to grow.
6. Then leave it alone. Checking it monthly does nothing except make it harder to hold during a fall.

A closing caveat

This guide explains a mechanism and its arithmetic. It is not advice, it does not know your circumstances, and it takes no view on whether a SIP is right for you or which fund you should buy. For that, speak to a SEBI-registered investment adviser.