The stock average cost calculator combines as many as three purchase lots into total shares, total cost, and weighted average cost per share. If you supply a current price, it also estimates current value, unrealized gain or loss, and percentage return. Every price comes from you; the tool does not retrieve market quotes.
How to use this calculator
Enter shares and purchase price for the first lot. Add later lots only when their share quantity is above zero; a zero-quantity lot is ignored. Optionally enter an observed current share price for valuation. Keep all prices in the same currency and include purchase costs in the relevant per-share price if you want them reflected.
Formula & calculation method
Average cost = Σ (shares bought × purchase price) / total sharesEach lot cost equals quantity multiplied by purchase price. Total cost is the sum of lot costs, and average cost divides that total by all shares. Current value equals total shares times the optional current price. Unrealized result subtracts total cost from current value, and percentage return divides it by total cost.
Worked example
Buying 100 shares at 20 and 50 shares at 26 costs 2,000 plus 1,300, or 3,300 for 150 shares. Average cost is 22 per share. At an entered current price of 30, current value is 4,500 and unrealized gain is 1,200 before any unentered fees or taxes.
Prepare your inputs and compare scenarios
Before entering numbers, define the question you want the calculator to answer and identify the source of each value. Review the fields below and use the requested unit or period for each one. When a value comes from a bill, document, or measurement, copy it with its meaning and units rather than converting from memory.
- Lot 1 shares
- Lot 1 price per share
- Lot 2 shares (optional)
- Lot 2 price per share
- Lot 3 shares (optional)
- Lot 3 price per share
- Current price per share (optional)
Calculate your current or baseline situation first and keep the result. Then change only one input and recalculate to see its effect clearly. If a result looks surprising, check decimal separators, percentages, dates, and selected options before changing values. Comparing two realistic scenarios provides more useful context than relying on one isolated number.
Make financial comparisons on a like-for-like basis
A financial result becomes useful when every scenario uses the same definitions. Keep the currency, time horizon, payment frequency, and treatment of fees consistent. Distinguish a nominal rate from an effective rate, and money you contribute from growth produced by the calculation. A single total rarely tells the whole story: cash flow, total cost, and risk can point in different directions.
Use the calculator as a comparison model rather than a prediction. Run a cautious case, an expected case, and a stronger case where appropriate. Then compare the outputs with an official quote, account statement, payslip, or product disclosure. If an offer includes introductory rates, variable charges, taxes, or penalties, record those separately instead of forcing them into an input that has a different meaning.
- Write down the source and date of every rate or balance.
- Compare both the periodic amount and the total over the full period.
- Keep fees, taxes, and uncertain future returns visible in your review.
Understanding the result
Average cost is the break-even share price for the entered purchase costs before any unincluded fees, taxes, or dividends. Total shares and total cost provide the quantities behind that average. When a current price is added, unrealized gain or loss is only a valuation at that price, not cash already received. Percentage return uses total cost as its denominator, so review it beside the currency gain rather than treating a large percentage as a large absolute amount.
A practical comparison to try
Enter your first two lots and check the weighted average manually using their total costs. Then add a small third lot at a very different price; the average should move only in proportion to that lot's weight. Finally, change only the optional current price to separate market-value sensitivity from purchase history. Removing the current price returns the tool to cost-only results and leaves no impression of a live quote.
Common mistakes to avoid
- Averaging purchase prices without weighting them by share quantities.
- Entering a share quantity for an optional lot but leaving its price at zero.
- Treating the user-entered current price as a live market feed or realized sale proceeds.
Why this is useful
Weighted averaging avoids the common error of giving a small purchase the same influence as a large one. Optional valuation separates historical cost from a price-based estimate, and the three-lot structure makes repeated calculations transparent without needing a spreadsheet.
Limitations to keep in mind
Three lots may not cover a full transaction history. Commissions, spreads, taxes, corporate actions, splits, dividends, currency conversion, and realized sales are not automatically included. An unrealized result depends entirely on the entered price and is not a live quote or instruction to buy or sell.
Frequently asked questions
Is the current price updated automatically?
No. Enter a price from a source you trust; no live market data is fetched or implied.
Why is it a weighted average?
Each purchase price is multiplied by its share quantity, so larger lots correctly have greater influence.
Are dividends included in the return?
No. The percentage shown compares current share value with entered purchase cost only.