Average Cost Basis Visualizer

How the moving-average ledger works

This is a single-position, long-only accounting illustration. It pools purchase cost, including buy fees, and removes average basis at each sale. It is not a broker statement, a tax-lot engine or a recommendation about when to trade.

Enter a consistent history

1. Use one instrument, account and currency. The symbol is a label; changing it does not clear or switch holdings. The dollar sign assumes USD and performs no currency conversion.
2. Clear the constructed sample and enter complete dated buys and sells, including fees. Same-date order matters: a sale cannot use shares from a later row on that date.
3. Enter a manual valuation price. Zero is supported; blank, negative or nonfinite values do not become a zero-price loss.
4. For imports, use the exact headers below or a JSON array with those keys. Fees can be omitted and then default to zero; present blank numeric cells are errors. Import replaces the entire position only after all rows pass validation.
5. Export before leaving. The page does not save a portfolio or fetch transactions from a brokerage.

date,type,shares,pricePerShare,fees
2024-01-15,buy,100,150,10
2024-03-20,buy,50,165,10
2024-05-10,sell,30,180,10

A constructed calculation

The two example buys cost $15,010 and $8,260, for 150 shares with total basis $23,270 and an unrounded average of $155.133333… . A sale of 30 shares at $180 with $10 selling fees receives $5,390, removes $4,654 of basis and produces $736 of modeled realized P&L. The remaining 120 shares retain $18,616 of basis and the same average cost.

Buying changes the average; a partial sale at that average removes cost proportionally. A complete sale resets remaining shares and cost to zero. The display may round while subsequent calculations and CSV exports preserve numeric precision.

Use each output for its stated purpose

Tax methods and adjustments

IRS Publication 550 distinguishes ordinary stock identification from average basis allowed for certain mutual fund or eligible dividend-reinvestment-plan shares. Do not assume that a pooled average shown here is an acceptable tax basis. Specific identification requires the applicable records; when shares cannot be adequately identified, the IRS describes first-acquired shares. This calculator implements neither tax-lot identification nor FIFO.

Reinvested dividends can create additional purchases and tax consequences; cash distributions and basis adjustments require separate records. Splits and other corporate actions are unsupported. Use broker records and the applicable tax rules for reporting.

Frequently Asked Questions