How to Reconstruct Missing Crypto Cost Basis Without Guessing

Define the gap before calculating a result

A blank broker or software field does not by itself establish zero basis. Special rules may produce a zero-basis reporting result when required substantiation is unavailable, so classify the gap before drawing a conclusion.

• List each affected asset, wallet or account, acquisition period, quantity, and later disposition or transfer that depends on the missing record.
• Separate a broker form with a blank basis field from an amount that is known and supported to be zero.
• Preserve the original tax report, broker statement, software output, or workpaper that exposed the gap rather than overwriting it.
• Identify the filing, accounting, diligence, or reporting deadline that the reconstruction must support.

Build the evidence inventory

• Collect original exchange and broker exports, trade confirmations, account statements, fiat deposit records, and bank or payment records.
• Inventory self-custodied wallet addresses, transaction hashes, archived wallet labels, and any records tying an address to the taxpayer or entity.
• Retain prior-year returns, Form 8949 schedules, tax-lot reports, elections, spreadsheets, and unresolved opening-balance workpapers.
• Ask whether an old provider, administrator, accountant, counterparty, or email archive can supply records no longer visible in the current account.
• Treat blockchain data as evidence of on-chain movement, not by itself as proof of ownership, acquisition cost, off-chain consideration, or business purpose.

Rebuild the acquisition history by wallet or account

• Reconstruct acquisition date, time, quantity, cost, transaction costs, source, and destination wallet or account for each supported lot.
• Match transfers between accounts under common ownership so acquisition history follows the units instead of creating a duplicate acquisition or disposal.
• Reconcile reconstructed lots to opening and ending quantities and investigate negative balances, duplicate imports, and units that appear from an unknown source.
• Document any historical transition under Revenue Procedure 2024-28 as a conditional, irrevocable safe harbor for allocating unused basis to remaining units held as of January 1, 2025—not as a general 2026 reconstruction election.
• For broker-custodied units sold, disposed of, or transferred during 2026, preserve an identification recorded no later than the transaction date and time, or a standing order recorded before the transaction, under Notice 2026-20. A later reconstruction cannot create a timely identification.
• Keep gifts, inheritances, compensation, rewards, contributions, migrations, and other fact-dependent origins in separate review categories.

Separate supported results from unresolved items

• Tie each reconstructed amount to an evidence reference, source file, transaction, or documented professional conclusion.
• Distinguish verified facts, management representations, professional assumptions, and unresolved gaps in the workpaper.
• Do not silently insert a market price, software default, or estimated acquisition date merely to make a report calculate.
• Quantify which later dispositions remain affected so the taxpayer and tax professional can evaluate the appropriate next step.
• Retain alternate calculations only when their purpose, assumptions, and limitations are clearly labeled.

Prepare a reviewable handoff

• Deliver a source inventory, wallet- and account-level lot schedule, transfer-matching schedule, and exceptions register for the agreed period.
• Identify which basis amounts are supported, which depend on additional facts, and which remain unresolved.
• Provide the issuing broker with a focused evidence package when a Form 1099-DA appears incorrect; only the issuer can correct its form.
• Coordinate the final reporting method and tax positions with the taxpayer and engaged tax professional.
• Archive the source versions, calculations, approvals, and final handoff so the reconstruction can be explained later.