Stock Average Calculator (VND)
If you accumulate a position in a Vietnamese stock across several buys at different prices, the cost basis that actually decides whether you are up or down is not the simple mean of those prices — it is the share-weighted average cost. Each purchase price is weighted by how many shares you bought at it, so a larger lot pulls the average closer to its price. This is the number your break-even point sits on.
Take the illustrative inputs in the calculator above: you buy 1000 shares at 50.000 VND, the price falls, and you add 1500 shares at 42.000 VND. A quick "average the two prices" gives 46.000 VND — and it is wrong. Because the second lot is larger, the true weighted average is only 45.200 VND per share, pulled 4.800 VND below your original 50.000 VND. That is exactly what averaging down does: buying more at a lower price lowers your cost basis, and therefore your break-even price.
The tool lets you enter up to three buy lots (price and quantity each) plus a current market price, then instantly shows your average cost, total shares, total cost paid, and unrealized profit or loss. Every figure on this page comes from the example inputs — replace them with your own fills. One caveat: the example excludes brokerage fees and the sell tax (Vietnam levies a 0.1% personal income tax on the sale value of listed shares), so your real break-even price will sit slightly above the raw average cost.
How the calculator computes your average
The core formula
The weighted average cost basis of a position built from several buys is:
Average cost = Σ(price_i × qty_i) / Σ qty_i
| Symbol | Meaning |
|---|---|
| price_i | The fill price of buy lot i |
| qty_i | Shares bought in lot i |
| Σ(price_i × qty_i) | Total cash paid (before fees) |
| Σ qty_i | Total shares now held |
The numerator is the total amount you spent; the denominator is the total number of shares. Because each price is multiplied by its quantity, the lot with more shares drags the average toward its price — which is why it is a weighted mean.
Why not just average the prices?
The simple mean of 50.000 and 42.000 is 46.000 VND — correct only if you bought equal share counts in both lots. In the example the second lot is 1500 shares (more than the 1000 of the first), so the true cost basis is 45.200 VND, below the simple mean. Confusing the two makes you think you have broken even when you have not.
Unrealized P/L
Once the average cost is known, the tool computes:
- Market value = total shares × current price
- Unrealized P/L (VND) = market value − total cost paid
- Unrealized P/L (%) = P/L ÷ total cost × 100
It is unrealized because nothing is locked in until you actually sell. The 55.000 VND current price in the example is an illustrative figure, not a quote.
What the model leaves out
The calculator ignores trading commissions (typically ~0.1–0.35% of order value depending on broker) and the sell-side tax (0.1% of sale proceeds). Both push your real break-even price slightly above the raw average cost. It also assumes you only buy and never sell down mid-way; if you do trim, the weighted-average basis of the remaining shares is unchanged under this method.
Worked example: two buys, one weighted average
Illustrative inputs (fees and taxes excluded):
| Buy lot | Price (VND) | Quantity (shares) | Amount (VND) |
|---|---|---|---|
| Lot 1 | 50.000 | 1.000 | 50.000.000 |
| Lot 2 | 42.000 | 1.500 | 63.000.000 |
| Total | 2.500 | 113.000.000 |
Average cost = total paid ÷ total shares = 113.000.000 ÷ 2.500 = 45.200 VND per share.
Before adding the second lot your basis was 50.000 VND (lot 1 only). Buying 1.500 more shares at a lower price pulled the basis down to 45.200 VND — a drop of 4.800 VND per share. That is averaging down: the point is to lower your break-even so the price only has to recover part-way for you to be whole again.
Unrealized P/L at the current price of 55.000 VND
| Metric | Value |
|---|---|
| Total shares | 2.500 |
| Total cost paid | 113.000.000 VND |
| Average cost | 45.200 VND/share |
| Market value (at 55.000) | 137.500.000 VND |
| Unrealized P/L | 24.500.000 VND (21.7%) |
At a current price of 55.000 VND — above the 45.200 VND basis — the position shows an unrealized gain of 24.500.000 VND, about 21.7% of cost. If the price fell back to exactly 45.200 VND, P/L would be zero (break-even). Swap in your own fill prices, and remember to add commissions plus the sell tax to find your true break-even.
Frequently asked questions
How do I calculate my average cost across multiple buys?
Divide total cash paid by total shares: Average cost = Σ(price × quantity) ÷ Σ quantity. Example: 1000 shares at 50.000 VND plus 1500 at 42.000 VND gives 113.000.000 VND across 2.500 shares → 45.200 VND per share. This is a share-weighted mean, not the simple average of the two prices.
What does "averaging down" mean?
It means buying more shares as the price falls to lower your weighted average cost. In the example, after 1000 shares at 50.000 VND you add 1500 at 42.000 VND, pulling the basis from 50.000 down to 45.200 VND — a drop of 4.800 VND/share. A lower break-even means the price only has to recover part-way, but you are also increasing your bet on a falling stock.
Why is the weighted average not just the mean of the prices?
The simple mean is correct only when you buy equal share counts each time. The mean of 50.000 and 42.000 is 46.000 VND, but because the second lot is larger (1500 vs 1000 shares) the true cost basis is 45.200 VND. You must weight each price by its quantity to get the right number.
How is unrealized P/L calculated?
Unrealized P/L = (current price − average cost) × shares, equivalent to market value − total cost. For the example: 2.500 shares at a 45.200 VND basis, current price 55.000 VND → an unrealized gain of 24.500.000 VND (21.7%). It is unrealized because it only becomes real when you sell.
Is my break-even price the same as my average cost?
Close, but not exactly. The average cost (45.200 VND here) ignores brokerage commissions (roughly 0.1–0.35% per order) and the sell tax (0.1% of sale proceeds in Vietnam). Add those and your real break-even sits a little above the raw average. This tool shows the clean cost basis so you can verify it, then layer fees and tax on top yourself.
Should I average down on a losing position?
There is no universal answer. Averaging down lowers your basis (from 50.000 to 45.200 VND in the example) and shortens the path back to break-even, but it also concentrates more capital in a falling stock — if the business is genuinely deteriorating, your loss grows. A pragmatic rule: only add if your original thesis still holds, cap any single position's weight, and never borrow to average down.
If I sell part of the position, does my average cost change?
Under the weighted-average method, the average cost of the remaining shares stays the same after a partial sale — selling reduces your share count but not your basis per share. The P/L on the sold portion is realized at the sale price. This calculator covers buy-only positions; if you have trimmed, enter the remaining shares against their respective buy lots.