Rupee-Cost Averaging
Why fixed-rupee investing quietly buys more units when prices fall and fewer when they rise — the maths that makes SIP work.
Why you care
Rupee-cost averaging is the quiet maths under a SIP. Because you invest a fixed rupee amount, not a fixed number of units, your money automatically buys more units in the months prices are low and fewer when they're high. You end up buying more of the cheap and less of the dear, without deciding anything.
Run the numbers
Invest ₹6,000 a month while the NAV moves 100, 80, 120, 100. You buy 60, 75, 50, 60 units — 245 in all for ₹24,000, an average cost of ₹97.96 per unit (illustrative). The average NAV over those months was ₹100. You paid less than average, purely because fixed rupees bought more units when it dipped.
Where this goes
Why you care
Rupee-cost averaging is what happens when you invest a fixed rupee amount at regular intervals: the fixed sum buys a variable number of units depending on the price that month. When the NAV is low, your ₹6,000 buys more units; when it's high, the same ₹6,000 buys fewer. Over time this tilts your purchases toward the cheaper months automatically, so your average cost per unit ends up below the simple average of the prices you paid across.
The reason it matters is that it takes the hardest, most emotional part of investing — deciding when to buy — off the table. You are structurally buying more when the market is down and scared, and less when it's up and euphoric, which is the exact opposite of what most people do on instinct. It's important to be honest about the limits, though. Rupee-cost averaging is not a guaranteed win and it does not beat simply being fully invested in a market that only rises. What it reliably does is lower your average entry cost relative to a lumpsum made at a bad moment, and remove timing risk and regret. Its real value is behavioural insurance, not alpha.
Run the numbers
₹6,000 invested every month across four months as the NAV swings (illustrative):
| Month | NAV | Units bought (₹6,000 ÷ NAV) |
|---|---|---|
| 1 | ₹100 | 60.0 |
| 2 | ₹80 | 75.0 |
| 3 | ₹120 | 50.0 |
| 4 | ₹100 | 60.0 |
| Total | avg NAV ₹100 | 245 units for ₹24,000 |
Your average cost per unit is ₹24,000 ÷ 245 = ₹97.96, even though the average NAV over the four months was ₹100. The gap is small here, but the direction is always the same: fixed rupees overweight the cheap months. The wilder the ride, the more pronounced the effect — which is the neat twist. For a disciplined SIP investor, volatility along the way is not something to fear; it's what lowers the average cost.
Where this goes
Rupee-cost averaging is inseparable from the SIP, which is simply the automated instruction that delivers it every month. Mechanically it all comes back to the NAV: the same rupee buys a different unit count as the price moves. Understanding it also inoculates you against two myths, that a SIP "can't lose" and that a SIP "beats" a lumpsum in a rising market. It does neither. It removes timing and regret, which for most investors is worth more than either myth promised.