What Happens to Your SIP When the Market Falls
A clear explanation of how falling markets change the number of mutual fund units you acquire and how the arithmetic of rupee-cost averaging actually works.
A bank alert arrives on the fifth of the month: ₹10,000 has been debited for a Systematic Investment Plan (SIP). On the same morning, the news reports that the stock market has fallen sharply for three weeks straight. The value shown in the investment account looks lower than what was put in.
In that moment, continuing to send money into the account feels unnatural. The natural instinct is to protect cash by pausing the deduction until things look calm again. Understanding how the mechanics of a SIP behave during a market downturn helps separate the emotional discomfort from the actual arithmetic.
How a mutual fund purchase actually works
A mutual fund pool is divided into small, equal portions called units. The price of one unit on any given day is called the Net Asset Value, or NAV.
When a fixed sum of money is invested every month, the investor is not buying a fixed number of shares. Instead, the fixed sum is divided by that day's NAV to determine how many units are credited to the account.
The formula is simple:
$$\text{Units Received} = \frac{\text{Rupee Amount Invested}}{\text{NAV on the Date of Purchase}}$$
When market prices rise, the NAV goes up. A fixed ₹10,000 buys fewer units. When market prices fall, the NAV goes down. That same ₹10,000 buys more units.
This simple division is the engine behind what is commonly called rupee-cost averaging. The process does not require anyone to guess whether today's price is high or low. The fixed rupee amount automatically forces the purchase of fewer units when prices are high and more units when prices are cheap.
Walking through the arithmetic of a drop
To see how this works over time, consider a simple hypothetical example.
Assume an investor runs a SIP of ₹10,000 per month for five months. Over these five months, the market undergoes a steep fall and then recovers back to where it started.
Note: The numbers below are chosen solely to keep the arithmetic clear. They are an illustrative assumption, not a forecast or a reflection of any real fund.
| Month | Amount Invested | Hypothetical NAV (₹) | Units Allotted |
|---|---|---|---|
| Month 1 | ₹10,000 | ₹100 | 100.00 |
| Month 2 | ₹10,000 | ₹80 | 125.00 |
| Month 3 | ₹10,000 | ₹50 | 200.00 |
| Month 4 | ₹10,000 | ₹80 | 125.00 |
| Month 5 | ₹10,000 | ₹100 | 100.00 |
| Total | ₹50,000 | — | 650.00 |
In Month 1, the NAV was ₹100. By Month 3, the NAV had fallen by half, down to ₹50. In that third month, the account balance looked deeply negative. Yet because the NAV was ₹50, the ₹10,000 installment purchased 200 units—twice as many units as it did in Month 1.
By Month 5, the NAV returned to ₹100. The market did not make a new high; it merely returned to its starting point.
Now look at the outcome:
- Total money invested: ₹50,000
- Total units accumulated: 650
- Value of those units in Month 5 (650 units × ₹100): ₹65,000
- Average cost per unit (₹50,000 ÷ 650): ₹76.92
Even though the price ended exactly where it began, the total value of the investment grew from ₹50,000 to ₹65,000. This happened because the heavier accumulation of units took place when the price was depressed. The average purchase price (₹76.92) ended up significantly lower than the starting and ending price (₹100).
Why the urge to pause works against the method
When account values decline, the emotional desire to stop the loss is immediate. Human beings feel the pain of a financial loss roughly twice as intensely as they feel the pleasure of an equivalent gain.
If an investor pauses the SIP in Month 2 and Month 3 because the market is falling, the arithmetic changes entirely:
- The investor buys at ₹100 in Month 1.
- No units are purchased at ₹80 or ₹50.
- The investor resumes in Month 5 when prices feel "safe" again at ₹100.
In that scenario, the investor only ever buys units at the higher prices. The entire mathematical benefit of the SIP is eliminated.
A SIP is not an investment product in itself. It is simply an automated instruction given to a bank and a mutual fund. Its purpose is to remove human emotion from timing the purchase. Pausing an installment during a downturn re-introduces the very market timing that the automated method was created to avoid.
What rupee-cost averaging cannot do
While the arithmetic of a SIP works well in falling markets that eventually recover, it is essential to understand what the method cannot do.
First, a SIP does not protect against capital loss while the market is down. Equity mutual funds invest in businesses, and market values can drop substantially and remain down for prolonged periods—sometimes for years. If money is needed on short notice during a downturn, redeeming units will lock in a real loss.
Second, rupee-cost averaging cannot turn a poor investment into a good one. If an underlying portfolio holds assets that permanently lose their earning power, accumulating more units at lower prices merely increases the total loss. The method relies on the premise that the broad market or the underlying portfolio will eventually recover and grow over a multi-year horizon.
Finally, market recoveries are rarely symmetrical. Markets do not drop and rebound in neat, predictable five-month curves. They can drop sharply, rise slightly, drop further, and remain stagnant for long stretches. The arithmetic remains valid over time, but the path is often uncomfortable to watch.
The core takeaway
A falling market often feels like an operational failure of an investment. In reality, for an ongoing SIP, a downturn is simply the period where the cost of acquiring units drops.
The investment platform is indifferent to headlines, market sentiment, and account anxiety. It executes a mathematical order: divide a fixed rupee sum by the day's NAV. Understanding that single calculation makes it clear that the accumulation of units during difficult periods is what creates the lower average cost for the future.