Stock Average Calculator

How to use the stock average calculator

What this calculator does

This calculator finds the weighted average price paid for shares purchased in separate transactions. It combines each purchase price with the number of shares bought, so larger purchases have more influence than smaller ones. The result is an estimated per-share cost basis before brokerage fees, taxes, wash-sale adjustments, reinvested dividends, splits, transfers, or partial sales. It is useful for position tracking and scenario planning, but it is not a substitute for the tax-lot records maintained by your broker.

When to use it

Use it when you average down or average up, compare a planned purchase with your existing position, reconcile several fills from an order, or estimate the unrealized gain or loss at a current market price. The underlying calculation is a weighted mean; the NIST explanation of weighted means provides useful mathematical background.

How to calculate

1. The calculator opens with three demonstration buys and a current price. Review the live results and the immediately available example workbook.
2. Replace each Shares and Price value with your own transaction data. Use a period as the decimal separator and do not enter currency symbols.
3. Select Add another buy for additional purchases, or remove a row you do not need. At least two complete buys are required.
4. Optionally enter Current stock price to calculate current value, unrealized gain or loss, and return on cost.
5. Read the average purchase price and supporting metrics, then use Download Excel to save the current validated model. Reset clears the demonstration and all entered rows; Excel export remains unavailable until at least two valid buys are entered again.

Input guide

Current stock price is optional, accepts a nonnegative decimal in USD, and may be left blank. For example, 55.00 values the whole position at $55 per share. A higher current price increases current value, gain, and return; a lower price can produce a loss. Do not paste a dollar sign or scientific notation. Shares is required for every retained buy row and must be greater than zero; fractional shares such as 2.5 are accepted. Increasing shares gives that row more weight in the average. Price is also required and must be zero or greater; 45.00 means $45 per share. A higher purchase price raises the average in proportion to that row's share count. Blank half-completed rows, commas used as decimal separators, negative values, and mixed text are rejected.

Output guide

Average purchase price and Break-even price are the weighted price per share before omitted costs. Total shares is the sum of share quantities, and Total invested is the sum of shares multiplied by price for every buy. Current value equals total shares times current price. Unrealized gain/loss is current value minus total invested; a negative value indicates an unrealized loss. Return on cost expresses that gain or loss as a percentage of total invested. The summary pills show the number of buys, total shares, and position status. In the table, Purchase cost is each row's dollar contribution, Share of position is its percentage of total shares, and Running average shows the weighted average after that transaction.

Worked example

The startup example buys 10 shares at $50, 8 at $45, and 12 at $40. Total invested is (10 × 50) + (8 × 45) + (12 × 40) = $1,340, and total shares are 30. Dividing $1,340 by 30 gives an average purchase price of $44.67. At a current price of $55, the position is worth $1,650 and has an unrealized gain of $310, equal to 23.13% of the amount invested.