Cost basis is the original purchase price of a security — including commissions and fees — that the IRS uses to determine your taxable capital gain or loss when you sell. For active traders holding multiple lots of the same ticker at different prices, the cost basis method you elect before a sale can legally swing your tax bill by hundreds or thousands of dollars on identical proceeds.
Key Takeaways
- Cost basis equals purchase price plus commissions and fees, and is adjusted by stock splits, return-of-capital dividends, and wash sale rule disallowances.
- The IRS defaults to FIFO for equities — electing Specific ID (HIFO) before the sale is the most direct way to legally reduce realized gains.
- Short-term gains (held under 12 months) are taxed as ordinary income up to 37%; long-term gains qualify for 0%, 15%, or 20% rates — holding past 12 months can effectively halve your federal tax rate.
How Cost Basis Works
Cost basis starts with the purchase price per share multiplied by the number of shares, plus any commissions or fees charged at entry. From there, several corporate actions can adjust it:
- Stock splits: A 2-for-1 split halves the per-share basis while doubling share count — total basis stays constant.
- Return-of-capital dividends: Reduce your basis dollar-for-dollar; once basis reaches zero, further distributions are taxable as capital gains.
- Wash sales: A loss disallowed under IRC Section 1091 is added to the new lot’s basis, deferring — not eliminating — the tax benefit.
When you hold multiple lots of the same security at different prices, you must choose a cost basis accounting method. The IRS default for individual stocks is FIFO (First In, First Out). Active traders can instead elect Specific Identification, which allows designating any lot — including HIFO (Highest In, First Out) — to minimize the gain on a given sale. Average Cost is only permitted for mutual fund shares and ETFs, not individual stocks (IRS Publication 550).
The election must be made before the sale is executed. A broker confirmation — via screenshot or written acknowledgment — documenting which specific lot was designated serves as the required record.
Practical Example
A trader buys NVDA in two lots: 50 shares at $80/share ($4,000 total) in March, and 50 shares at $120/share ($6,000 total) in August. In November, they sell 50 shares at $140.
Under FIFO (IRS default): The March lot is sold first.
- Gain = ($140 - $80) x 50 = $3,000
- Held ~8 months → short-term, taxed at 22% = $660 tax
Under HIFO / Specific ID: The August lot is designated for sale.
- Gain = ($140 - $120) x 50 = $1,000
- Also short-term, taxed at 22% = $220 tax
Same ticker, same sale price, same holding period — $440 in tax savings purely from cost basis method selection. If the trader had instead held the March lot past 12 months before selling, the long-term rate (15% for most traders) would further reduce the FIFO tax to $450 — illustrating how both lot selection and holding period interact.
Cost basis is the original price you paid for a security, including fees. The IRS uses it to calculate your taxable gain when you sell. Choosing which lot to sell before executing the trade can legally reduce your tax bill by hundreds of dollars on the same position.
Common Mistakes
- Defaulting to FIFO without reviewing lots. On assets purchased at multiple prices in an uptrend, FIFO consistently sells the lowest-basis shares first, maximizing your taxable gain. Review open lots before selling.
- Trying to elect Specific ID after the trade. The IRS requires lot designation before or at the time of sale. Post-trade reassignment is not permitted — the window closes when the order executes.
- Ignoring wash sale adjustments. Traders who harvest losses and rebuy quickly often overlook that the disallowed loss shifts into the new lot’s basis. Failing to track this leads to errors on Form 8949 and potential underpayment.
- Assuming brokers handle crypto basis. Unlike equities — where brokers have reported covered-security basis on Form 1099-B since 2011 — crypto cost basis remains largely the trader’s responsibility. Gaps in records create audit exposure.
How JournalPlus Tracks Cost Basis
JournalPlus logs each trade entry with price, shares, and fees, giving you a per-lot cost basis record that persists alongside your trade notes and performance data. When you review a position before closing, you can see each open lot’s basis directly in your journal, making it straightforward to identify which lot to designate for Specific ID before executing the sale.
