You buy Bitcoin more than once. Different days, different prices, maybe different wallets or exchanges. Later you sell only some of what you hold. The tax question is not just “how much did I sell?” — it is “which Bitcoin did I sell?”

For U.S. federal tax purposes, Bitcoin is generally treated as property. When you dispose of property you acquired in separate lots, the gain or loss on each lot depends on that lot’s cost basis and holding period. Selling “some BTC” without identifying which lots you sold leaves Form 8949-style reporting incomplete. This article is a plain-language overview for U.S. readers, not advice for your accounts, software, or tax year.

Lots matter more than a single balance

A wallet or exchange balance is often one number on the screen. For tax records, that balance may be many lots: each purchase, gift received, mining or reward credit, or other acquisition that created a separate unit of property with its own cost and acquisition date.

When you sell part of a holding, you are usually disposing of specific units — whether you think of them that way or not. The dollar gain or loss is proceeds minus the basis of the units treated as sold. Which units you treat as sold can change both the amount of gain or loss and whether it is short-term or long-term.

Specific identification in plain terms

Specific identification means you designate which lots are disposed of in a given sale or trade, and you keep records that support that designation. In practice, that usually means being able to point to acquisition dates, quantities, and cost basis for the coins you claim you sold — not just a pooled average of everything you ever bought.

Brokers and tax software sometimes offer lot selection (for example, highest basis first, oldest first, or manually chosen lots). Those tools only help if your underlying lot history is complete and consistent. If the platform cannot see your self-custody purchases, gifts, or transfers, its default pick may not match your full tax picture.

Defaults and averages people encounter

Many people never choose lots. The exchange sells “Bitcoin,” the CSV shows a disposal, and tax software applies a default ordering or an average cost across a pool. Some platforms historically used approaches that look like average cost for convenience. Others default to first-in, first-out (FIFO) or similar ordering.

Those shortcuts can produce a number that files. They may not match what you intended, or what your records can defend. Averaging across lots with different holding periods can also blur short-term vs long-term character. Do not assume the exchange’s year-end export is the only acceptable method for every wallet and every year.

A simple example (illustrative only)

Suppose you bought 0.1 BTC for $4,000 in Year 1 and another 0.1 BTC for $8,000 in Year 2. In Year 3 you sell 0.1 BTC for $7,000. If you specifically identify the Year 2 lot as sold, your capital gain or loss is measured from the $8,000 basis. If instead the Year 1 lot is treated as sold, the same $7,000 proceeds are measured from the $4,000 basis — a very different result. Holding period character can differ too, depending on how long each lot was held before the sale.

Real facts include fees, partial lots, transfers between wallets, and incomplete exchange history. The example only shows why “I sold 0.1 BTC” is not enough detail for Form 8949-style lines.

What Form 8949-style reporting expects

Capital asset sales are typically reported with dates acquired, date sold, proceeds, and cost or other basis — often on Form 8949 and related schedules. Each disposal line implies a particular lot (or an aggregated group your records support). A single balance decrease on an exchange does not, by itself, fill those fields.

If you cannot tie a sale to acquisition lots, you may struggle to support basis, acquisition date, and holding period. That is a records problem first, then a reporting problem.

Records that make identification possible

Specific identification only works if you can show what you owned and what you sold. Capture and keep:

• Each acquisition: date, quantity, U.S. dollar cost basis (or fair market value when basis is determined that way), and source
• Transfers between your own wallets or accounts, so lots are not “lost” when coins move
• Each disposal: date, quantity, proceeds in U.S. dollars, fees, and which lot(s) you treated as sold
• Exchange and wallet exports, transaction IDs, and any lot-selection confirmations from a broker or platform

If you leave a platform, export history before access disappears. Reconstructing multi-year lots from memory in April is how small sales become expensive filings.

Practical tip

Before you sell, decide which lots you mean to dispose of and save a contemporaneous note — quantity, acquisition dates, basis, and where the coins sat. After the sale, match the export to that note. Waiting until tax software asks for cost basis with a blank field is usually too late to invent a clean trail. Confirm the approach that fits your accounts with a qualified professional.

This article is general information about common U.S. federal tax concepts related to identifying Bitcoin lots on a sale or other disposition. It is not tax, legal, or accounting advice for your situation. Rules depend on facts, account types, and how your records are kept. Consult a qualified professional before filing or making decisions.