Track every purchase lot, average down with confidence, and know your true cost basis — in real time.
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Download a CSV of all your purchase lots including dates, share quantities, prices, and totals — ready for your accountant or tax return.
Practical articles for self-directed investors
Your brokerage platform almost certainly shows you the wrong average price. For most investors who buy in multiple lots, the figure displayed is a simple average — and that number can mislead you every time you make a decision.
A simple average adds up all your purchase prices and divides by the number of transactions. It treats a purchase of 10 shares exactly the same as a purchase of 10,000 shares. That's mathematically indefensible as a measure of your cost basis.
| Lot | Shares | Price | Total cost |
|---|---|---|---|
| 1 | 100 | $12.00 | $1,200 |
| 2 | 500 | $9.50 | $4,750 |
| 3 | 200 | $11.00 | $2,200 |
| Total | 800 | $8,150 |
The simple average of those three prices: ($12.00 + $9.50 + $11.00) ÷ 3 = $10.83
The weighted average: $8,150 ÷ 800 shares = $10.19
That $0.64 gap represents the difference between your perceived break-even and your actual break-even. If you sold at $10.50 thinking you were profitable, you would be — but by far less than the simple average implied.
The weighted average share price (WASP) answers a precise question: if I were to liquidate all my shares right now, what is the average price I paid per share? That's your true cost basis. It's the figure your accountant uses, it's what the ATO wants for capital gains calculations, and it's the only number that tells you whether you're actually ahead or behind.
The difference between simple and weighted averages grows when your lot sizes vary significantly — which is common for investors who dollar-cost average over time, or who make one large initial purchase and later smaller top-ups.
Always calculate your cost basis using the weighted average method. Use the calculator above to enter each purchase lot — the WASP updates in real time. Export the result to CSV for your records.
Buying more of a stock as it falls can be one of the best moves you ever make — or it can multiply a mistake. The strategy itself isn't the problem. Doing it without a framework is.
When you buy additional shares at a price below your current average cost, you lower your weighted average share price. This reduces the price at which you need to sell to break even. For example, if you bought 500 shares at $20.00 and add 500 more at $14.00, your WASP falls to $17.00 — you now only need a recovery to $17.00, not $20.00, to break even.
| Action | Shares | Price | New WASP |
|---|---|---|---|
| Initial buy | 500 | $20.00 | $20.00 |
| Average down | 500 | $14.00 | $17.00 |
| Average down again | 1,000 | $11.00 | $14.00 |
1. Has anything changed fundamentally? A falling price is only an opportunity if the reason you originally bought the stock still holds. If the business has deteriorated, a lower price may simply be accurate. Averaging down into a broken thesis compounds the original error.
2. Can you afford to be wrong again? Averaging down concentrates your position. Before adding, ask: if this falls another 30%, what is my total exposure, and am I comfortable with that?
Many disciplined investors use a predefined scale-in plan rather than improvising. For example: buy in thirds — an initial position, a second tranche if it falls 15%, and a third if it falls 30%. Use the WASP calculator to model the effect of different add sizes before committing.
Never average down simply because the price has fallen. Never average down into a position that is already oversized in your portfolio. And never use it as a substitute for cutting a loss when the investment case is genuinely broken.
Dollar-cost averaging (DCA) is one of the most widely recommended investing strategies. But the research on whether it outperforms lump-sum investing is more nuanced than most guides acknowledge.
Dollar-cost averaging means investing a fixed dollar amount at regular intervals — say, $500 every month — regardless of price. Because the amount is fixed rather than the number of shares, you automatically buy more shares when prices are low and fewer when prices are high.
| Month | Price | Shares bought | Running WASP |
|---|---|---|---|
| Jan | $25.00 | 20.0 | $25.00 |
| Feb | $20.00 | 25.0 | $22.22 |
| Mar | $18.00 | 27.8 | $20.55 |
| Apr | $22.00 | 22.7 | $21.12 |
The simple average of those four prices is $21.25. The actual WASP is $21.12 — slightly lower, because the fixed-dollar approach automatically bought more shares at the lower prices in February and March.
Academic research consistently shows that lump-sum investing outperforms DCA roughly two-thirds of the time in rising markets. The reason is simple: markets go up more often than they go down, so money sitting on the sidelines waiting to be deployed earns less than money already invested.
DCA removes the psychological pressure of timing the market. A systematic DCA plan bypasses the stress of trying to pick the bottom entirely. You invest on the schedule, not on your feelings about where the market is heading.
Regular DCA purchases create exactly the multi-lot scenario where your weighted average share price matters most. After six months of monthly purchases, your true break-even is the WASP across all purchases. Use the calculator above to keep a running total, and export to CSV periodically so you have a complete record for tax time.
Every time you sell shares in Australia, you need to calculate your capital gain or loss. That calculation starts with your cost base — and getting it right can make a significant difference to your tax bill.
Under Australian tax law, the cost base of a share is essentially what it cost you to acquire it. For most investors buying shares through a broker, the cost base is the purchase price plus any brokerage commission paid on the buy. When you sell, your capital gain is the sale proceeds minus the cost base.
If you've bought the same stock at different times and prices, the ATO generally uses a First In, First Out (FIFO) approach by default — meaning your earliest-purchased shares are treated as sold first. This matters because shares held for more than 12 months qualify for the 50% CGT discount for individuals.
The ATO requires you to keep records for every share transaction for at least five years after the relevant tax return is lodged. For each purchase, record: the date, number of shares, price paid per share, total consideration, and brokerage paid.
Your weighted average share price is an essential part of this record-keeping. Knowing your WASP tells you the average cost per share across all parcels, which helps you estimate the tax impact of a potential sale before you make it.
Enter each purchase lot into the calculator including the date, share count, and price. Use the Export CSV function to download a complete record. Save this file alongside your brokerage confirmations — it's exactly what your accountant needs at tax time.
Of all the cognitive biases that affect investors, the sunk cost fallacy may be the most costly. It causes rational people to make provably irrational decisions — and it's triggered specifically by knowing what you paid for something.
A sunk cost is money already spent that cannot be recovered. The fallacy is allowing that irretrievable cost to influence future decisions. In share investing, it sounds like this: "I paid $18 for this stock and it's now at $9. I can't sell at a loss — I'll wait until it gets back to $18." The $18 purchase price is a sunk cost. The market doesn't know or care what you paid.
Consider two investors. Investor A bought a stock at $18 and it's now $9. Investor B received the same stock as a gift when it was at $9. Both own an identical asset with identical future prospects — yet Investor A feels anchored to $18 and hesitates to sell. Both should make the same decision. The only difference is psychological.
Your weighted average share price is not irrelevant — it matters for tax calculations, for measuring performance, and for understanding your true cost basis. What it should not do is anchor your sell decisions. Know your WASP precisely; then set it aside when asking whether to hold or sell. The two questions are different: "what did I pay?" is accounting. "Should I keep holding?" is forward-looking analysis.
Behavioural economics research shows that losses feel roughly twice as painful as equivalent gains feel good. This asymmetry makes investors hold losing positions far too long and sell winning positions too early. Awareness of this bias doesn't eliminate it, but it creates a pause before acting — and that pause is often enough to make a more rational decision.
As a portfolio grows beyond a handful of stocks, cost basis tracking becomes genuinely difficult. Brokerage apps show you current value but rarely your true average purchase price per stock. Here's a practical system.
Most retail brokerage platforms calculate average price using a simple average of your transactions, not a weighted average. Some don't account for brokerage commission in the cost base at all. Others reset or distort the figure if you've ever partially sold a position.
For each stock you hold, track: the ASX or exchange code, each purchase date, the number of shares in each lot, the price paid per lot, and brokerage paid. Everything else — your WASP, total invested, unrealised gain/loss — can be derived from those five data points.
| Date | Shares | Price | Brokerage |
|---|---|---|---|
| 12 Mar 24 | 50 | $44.20 | $9.50 |
| 8 Jun 24 | 30 | $41.80 | $9.50 |
| 15 Oct 24 | 80 | $38.50 | $9.50 |
For each stock, use the WASPCalc calculator to enter your purchase lots and export a CSV. Name the file with the ticker and date: BHP_lots_2024.csv. Store these files in a single folder alongside your brokerage confirmation emails. Once a year — or whenever you buy more shares — update the relevant CSV and re-export.
When you sell all or part of a position, record: the sale date, number of shares sold, the price received, and brokerage. Note which lots were sold (usually FIFO). Calculate the capital gain or loss as: proceeds minus cost base.
A file-based approach works well for portfolios up to 15–20 positions with infrequent trading. Beyond that, or if you trade frequently or hold international stocks in multiple currencies, a dedicated portfolio tracking application may be worth the cost.