How to calculate your cost basis — stocks, crypto, and the FIFO vs Specific ID decision

Cost basis is the number that determines how much tax you owe every time you sell an investment. Get it wrong and you either overpay the IRS. Or you create an audit risk. The method you choose — FIFO, Specific Identification, or Average Cost — can change your tax bill by thousands of dollars on the exact same trade. And for crypto specifically, the rules changed meaningfully in 2025 and continue evolving into 2026. Here's how it actually works.

What cost basis is

Cost basis is what you paid for an investment — the purchase price plus any commissions or fees. When you sell, your taxable gain or loss is simply: proceeds minus cost basis. Sell for more than your basis, you owe tax on the gain. Sell for less, you have a deductible loss (subject to wash sale rules, covered below).

The complexity arises when you've purchased the same investment multiple times at different prices — creating multiple "lots," each with its own basis. When you sell only part of your position, which lot's basis applies? That's where cost basis methods come in, and the method you choose can meaningfully change your tax outcome on the exact same sale.

The three cost basis methods

Worked example — stocks

You bought Apple stock in three lots over two years, and now want to sell 150 shares at $210/share:

*Average cost is shown for illustration only — not available for individual stocks under current IRS rules.

HIFO produces $5,000 less taxable gain than FIFO on this exact same sale — but part of that gain (from Lot 3, purchased in March 2026) would be short-term, taxed at ordinary income rates rather than the lower long-term capital gains rate. Whether HIFO or FIFO produces the better after-tax outcome depends on your tax bracket and whether the short-term portion is offset by losses elsewhere. Method selection isn't just about minimizing the gain shown on paper — it's about controlling the character, timing, and amount of taxable income in the context of your full tax picture.

What changed for crypto in 2025–2026

Crypto cost basis rules underwent a significant structural change that every crypto investor needs to understand:

• Wallet-by-wallet tracking (effective January 1, 2025). Previously, many investors used a "universal wallet" approach — treating all holdings of the same crypto across every exchange and wallet as one combined pool, allowing cherry-picking of the most tax-efficient lots regardless of where assets actually sat. Current guidance effectively shifts this toward account-by-account tracking: each exchange account and each self-hosted wallet is treated as a separate ledger for cost basis purposes, with FIFO and Specific ID applied independently within each one.
• Form 1099-DA reporting (2026 transactions, reported in early 2027). Exchanges must now report both gross proceeds and adjusted cost basis for "covered" digital assets — those acquired and sold on the same platform. Assets transferred between platforms become "non-covered," since cost basis doesn't automatically transfer on the blockchain, shifting the burden of accurate basis reporting back onto the taxpayer for those transfers.
• Contemporaneous identification requirement (2025 forward). Specific Identification for crypto now requires identifying the lot before or at the time of the sale — not after the fact at tax filing. This closes the door on after-the-year-end optimization approaches that some investors previously used.
• Transitional relief through 2026. IRS guidance has extended relief allowing taxpayers to apply their own lot identification in their books and records, even if it differs from a broker's internal default method, provided documentation supports the choice and it's recorded before disposal. In practice, this means your own records can currently override an exchange's default method — for now. This relief is transitional and its future beyond 2026 is not yet settled — check current guidance before relying on it.

The wash sale rule

The wash sale rule (IRC §1091) disallows a capital loss deduction if you buy a "substantially identical" security within 30 days before or after the sale that generated the loss — a 61-day window in total. The disallowed loss isn't lost forever; it's added to the cost basis of the replacement shares, effectively deferring the loss rather than eliminating it. This rule has long applied to stocks and applies across all your accounts, including IRAs and a spouse's accounts. One costly nuance: if the replacement shares are purchased in an IRA, the disallowed loss is not added to the IRA's basis — it's effectively lost permanently, since IRAs don't track cost basis the way taxable accounts do. Avoid repurchasing in an IRA within the wash sale window.

Stock splits and DRIP reinvestment

Two common events change your cost basis calculation without you actively buying or selling:

Stock splits change the number of shares you own but not your total cost basis. A 4-for-1 split turns 100 shares worth $200/share ($20,000 total) into 400 shares — your total investment remains $20,000, but your per-share basis drops to $50. Your brokerage should automatically adjust this, but always verify the adjustment is correct on your 1099-B before filing.

DRIP (Dividend Reinvestment Plan) purchases create a new cost basis lot every time a dividend is automatically reinvested — even small quarterly reinvestments. Over years of participation, this can produce dozens of tiny lots, each with its own purchase date and basis. This is one of the most common sources of cost basis tracking errors, since investors often forget these automatic purchases happened at all. Average cost is permitted for mutual funds and certain broker-held DRIP shares specifically, which can simplify tracking considerably compared to tracking each reinvestment as a separate lot — confirm with your broker whether your specific DRIP setup qualifies.

Frequently asked questions

Which cost basis method should I use?

There's no universally correct answer — it depends on your goals. FIFO is simplest and requires no ongoing lot-level record-keeping, making it the practical default for most casual investors. Specific Identification (including HIFO) offers the most tax control but requires meticulous, contemporaneous documentation — if you claim it but can't substantiate adequate identification during an audit, the IRS may default the sale to FIFO, often resulting in a higher tax bill plus potential penalties. Active traders and those with larger positions where the tax difference is material generally benefit most from the added record-keeping burden of Specific ID.

Can I switch cost basis methods between tax years?

Generally yes, for future transactions — you can change your method from one year to the next without IRS approval. However, the method must be applied consistently to a given lot once it's been sold and reported; you cannot retroactively change how a prior sale was calculated after filing. Consult a tax professional before changing methods if you have a complex position.

What happens if I don't specify a cost basis method?

FIFO applies automatically as the default for both stocks and crypto. If you intended to use Specific Identification but didn't properly document your lot selection before the sale — for stocks, before settlement date; for crypto, before or at the time of disposal — the IRS treats the sale as FIFO, potentially producing a different (often larger) taxable gain than you expected.

Do I need crypto tax software, or can I track this myself?

For simple situations — a handful of purchases on a single exchange, held and sold on that same platform — manual tracking or basic broker reporting may be sufficient, especially now that 2026-forward "covered" transactions get 1099-DA reporting. For anyone with transfers between wallets, multiple exchanges, DeFi activity, staking rewards, or a long transaction history, dedicated crypto tax software that automates per-wallet lot tracking is strongly advisable. The wallet-by-wallet requirement effective 2025 makes manual reconstruction genuinely difficult for anyone with a nontrivial transaction history across platforms.

How does cost basis affect tax-loss harvesting?

Cost basis is the foundation of tax-loss harvesting — you need accurate, lot-level basis to identify which specific positions have unrealized losses worth harvesting. Specific Identification is particularly valuable here, since it lets you precisely select which losing lots to sell for the deduction while retaining winning lots. See our tax-loss harvesting guide for the full strategy, including how the wash sale rule interacts with harvesting decisions.