In this article
You bought 100 shares at ₹500. The price has dropped to ₹400. Buying 100 more would bring your average cost down, but by how much, and does it actually help? The arithmetic is simple, and it is worth seeing once, because it shows both the benefit and the trap of “averaging down”.
The formula
To find your average buying price, add up everything you paid and divide by everything you now hold. Do not average the prices: a purchase of 200 shares matters twice as much as one of 100.
One example, three purchases
You buy in three steps as a share keeps falling. Each purchase changes the average by a smaller amount, because the old holding grows larger.
| Step | Bought | Price | Spent so far | Shares held | Average price |
|---|---|---|---|---|---|
| 1 | 100 | ₹500 | ₹50,000 | 100 | ₹500 |
| 2 | 150 | ₹400 | ₹1,10,000 | 250 | ₹440 |
| 3 | 200 | ₹320 | ₹1,74,000 | 450 | ₹386.67 |
After the third purchase you hold 450 shares that cost ₹1,74,000 in total, an average of about ₹386.67 each.
The average always stays above the latest price while the price is falling, and it moves toward it more slowly than the price itself.
Without averaging you would need a 56% rise from ₹320 to get back to ₹500. With the third purchase, only about 21%.
That looks like an easy win, and it is the main attraction of averaging down. The catch is on the next line of the table.
- After buy 1
- After buy 2
- After buy 3
Your cost per share went down, but the total money at risk more than tripled. If the share keeps falling, you lose on a bigger base.
Why a fixed amount buys at a lower average
There is a second kind of averaging, which is investing the same amount at regular times (as in a monthly SIP) instead of buying the same number of shares. Because a fixed amount buys more shares when the price is low, it automatically weights the cheaper prices more heavily.
| Month | Price | Amount invested | Shares bought |
|---|---|---|---|
| 1 | ₹100 | ₹10,000 | 100 |
| 2 | ₹80 | ₹10,000 | 125 |
| 3 | ₹50 | ₹10,000 | 200 |
| 4 | ₹80 | ₹10,000 | 125 |
| 5 | ₹100 | ₹10,000 | 100 |
The price falls from 100 to 50 and recovers to 100. Total invested ₹50,000 for 650 shares.
- (100 + 80 + 50 + 80 + 100) ÷ 5
- Ignores how many shares you got
- Not what you actually paid
- ₹50,000 ÷ 650 shares
- Cheaper months bought more shares
- The harmonic mean of the prices
The gap is the benefit of buying a fixed amount: you paid about 6% less per share than the average price.
The price is flat from start to finish, but the investor gains 30%. Falling prices in the middle gave extra shares at a discount.
This is the idea behind rupee-cost averaging. It does not guarantee a profit, and it works only if prices recover. See the SIP calculator article for the long-run view.
Why losses are harder to recover than they look
A 50% fall needs a 100% rise to recover. This asymmetry is why a lower average cost feels so valuable. The same maths is explained in up 50%, down 50%.
A 10% loss needs 11%. A 50% loss needs 100%. A 75% loss needs 300%.
Quick practice
- You hold 40 shares at ₹250 and buy 60 more at ₹200. Average? (10,000 + 12,000) ÷ 100 = ₹220.
- The price is ₹180. What gain do you need to break even? 220 ÷ 180 − 1 = 22.2%.
- You invest ₹5,000 at ₹50, then ₹5,000 at ₹25. Average cost? 10,000 ÷ 300 shares = ₹33.33, not ₹37.50.
- Should you average down on every fall? No. Decide in advance how much you are willing to hold in one share.
This article explains arithmetic, not what to buy, and it is not investment advice. Past prices do not predict future ones, so make decisions based on your own situation or with a registered adviser.
Questions people ask
How do you calculate the average price of a stock?
Divide the total money you spent (quantity × price for each purchase, plus costs) by the total number of shares you hold.
What is averaging down?
Buying more of a share after its price has fallen, so your average cost per share drops and the price needs a smaller rise to reach your break-even.
Is averaging down a good idea?
It depends on why the price fell. It lowers your average cost, but it also puts more money into the same share. It can hurt if the share keeps falling.
What is rupee-cost averaging?
Investing a fixed amount at regular intervals, as in a SIP. You buy more units when prices are low and fewer when they are high.
Why is my average lower than the average of the prices?
Because a fixed amount buys more shares at the low prices. The result is the harmonic mean of the prices, which is always at or below the simple mean.
Should brokerage be included in the average price?
For a true break-even figure, yes. Add buying and selling costs and taxes to the total cost.