How SIPs Work: Units, NAV, and Rupee-Cost Averaging

A systematic investment plan, or SIP, is one of the most widely used ways Indians invest in mutual funds. The idea is simple: a fixed amount is invested at a regular interval, usually monthly, without you having to decide each time. Yet many investors are unclear about what they are actually buying, why the number of units changes each month, and what averaging does and does not achieve. This article explains the mechanics from the ground up.

What a SIP is, and what it is not

A SIP is not a product. It is a method of buying a mutual fund scheme. You choose a scheme, an amount, and a date, and authorise automatic payments from your bank. On each SIP date, the amount is debited and used to buy units of that scheme. Stopping a SIP simply stops future purchases; the units you already own remain invested until you redeem them.

Units and NAV: the basic arithmetic

A mutual fund scheme is divided into units. The net asset value, or NAV, is the value of one unit: the market value of everything the scheme holds, minus its liabilities and expenses, divided by the number of units outstanding. Schemes publish NAVs on each business day, and AMFI's website is a common place to look them up.

When your SIP instalment goes in, the number of units you receive is simply the amount invested divided by the applicable NAV. If you invest ₹5,000 when the NAV is ₹50, you get 100 units. Which day's NAV applies depends on when the money reaches the fund and on cut-off time rules, which your fund house explains in its scheme documents.

Rupee-cost averaging, worked through

Because the amount is fixed and the NAV changes, you buy more units when the NAV is low and fewer when it is high. Here is a three-month example with ₹5,000 invested each month:

• Month 1: NAV ₹50, units bought = 5,000 ÷ 50 = 100
• Month 2: NAV ₹40, units bought = 5,000 ÷ 40 = 125
• Month 3: NAV ₹62.50, units bought = 5,000 ÷ 62.50 = 80
• Total invested: ₹15,000. Total units: 305. Your average cost per unit: 15,000 ÷ 305 ≈ ₹49.18
• The simple average of the three NAVs: (50 + 40 + 62.50) ÷ 3 ≈ ₹50.83

Your average cost per unit is lower than the simple average NAV over the period. This is the mathematical effect of buying a fixed rupee amount: the cheaper months contribute more units. It is called rupee-cost averaging.

What averaging does not do

Rupee-cost averaging reduces the impact of timing on your average purchase price. It does not guarantee a profit, and it does not protect you if the scheme's value falls and stays down. If the NAV in the example had ended at ₹45, your 305 units would be worth about ₹13,725, a loss on ₹15,000 invested, despite the favourable averaging. The outcome still depends on what the underlying investments do over time.

It is also worth knowing that investing a lump sum all at once has historically tended to beat spreading it out in rising markets, simply because more money is invested for longer. A SIP's practical advantage for most people is that it matches how income actually arrives — monthly — and removes the temptation to wait for a perfect moment.

The behavioural benefit is the real one

For many investors, the biggest value of a SIP is behavioural. An automated instruction keeps you investing through months when headlines are frightening, which is exactly when many people are tempted to stop. Market falls mean your fixed instalment buys more units; stopping during a fall gives up that effect. The SIP works best when it is boring and uninterrupted.

Practical details worth knowing

• Mandate: SIPs are usually paid through an auto-debit mandate set up with your bank. Mandates often have a maximum amount, so set it slightly above your SIP if you plan to increase it later.
• Direct and regular plans: most schemes offer a direct plan, bought without a distributor, and a regular plan, bought through one. The regular plan carries a higher expense ratio because it includes distributor commission. The difference compounds over long periods.
• Expense ratio: the annual cost of running the scheme, deducted from the scheme's assets and reflected in the NAV. It is disclosed in scheme documents and fund fact sheets.
• Exit load: some schemes charge a fee if units are redeemed within a set period. Each SIP instalment is treated separately for this purpose, so check the scheme's terms.
• Step-up: many fund houses let you increase the SIP amount automatically each year, which can help investments keep pace with rising income.
• Missed instalments: if your bank account lacks funds on the SIP date, the instalment may fail and some banks levy a charge. Repeated failures can lead to the SIP being cancelled.

Choosing the date and the amount

The SIP date matters less than people think. Over long periods, which day of the month you invest has little effect compared with how much and how long you invest. A practical choice is a date shortly after your salary arrives, so the money is there when the debit happens.

For the amount, a sensible starting point is what you can sustain comfortably without needing to stop in a tight month. A smaller SIP you keep for ten years usually does more for you than a large one abandoned after a year. You can model how different amounts and durations add up using a SIP calculator, remembering that any assumed return is only an assumption.

Checking that everything is working

Each instalment should appear in your account statement with the date, NAV, and units allotted. The consolidated account statement you receive from the registrars or depositories brings all your mutual fund holdings together. Checking it a few times a year confirms that instalments are going through and that units are being allotted to the right folio.

The short version

A SIP buys units of a mutual fund scheme with a fixed amount at regular intervals. Because the amount is fixed, you automatically buy more units when prices are lower, which reduces your average cost compared with the average price. That averaging does not guarantee returns; what the scheme invests in still decides the outcome. The method's lasting strength is that it turns investing into a habit that does not depend on predicting the market.