How to prove your crypto cost basis when you have lost exchange records

18 October 2026

Proving your crypto cost basis without exchange records is possible, but it requires assembling alternative evidence from whatever sources remain available. Tax authorities generally accept reasonable reconstruction methods, provided you can demonstrate a good-faith effort and support your figures with verifiable data.

What the tax authority expects

Most tax agencies require you to report capital gains or losses on each crypto transaction. The cost basis - what you originally paid for the coin - directly determines whether you owe tax. Without records from the exchange where you bought, you must reconstruct that number using other information.

The key principle: you need to show the purchase price and purchase date for each coin you later sold or traded. The tax authority does not require a single specific document type, only credible evidence.

Step 1: gather whatever records still exist

Start by checking every source you might have used:

• Email confirmations - Search your email inbox for order receipts, withdrawal confirmations, or deposit notifications from the exchange. Even partial records help.
• Bank or credit card statements - Look for deposits to the exchange. The date and amount of a fiat deposit often match a purchase, though not always if you deposited cash before buying.
• Blockchain explorers - Your public wallet address shows every incoming transaction. For each deposit, note the date, the amount of crypto received, and the sending address (which may be the exchange’s hot wallet). You will not see the price paid, but you have the quantity and date.
• Screenshots or notes - Any personal record you kept, even a handwritten log, counts as evidence.
• Old tax returns - If you filed previous years, those returns may contain cost basis figures you reported.

Step 2: Use Blockchain Data to Estimate Dates and Amounts

If you have your public wallet address, you can reconstruct the quantity and date of each purchase. You still lack the price, but the date gives you a range.

Open a blockchain explorer for the network you used (e.g., Etherscan for Ethereum, Blockchair for Bitcoin). Enter your wallet address and export the transaction history. For each incoming transaction:

• Record the date and time.
• Record the amount of crypto received.
• Identify the sending address. If it belongs to a known exchange hot wallet (many are publicly listed), you can reasonably tie that transaction to an exchange purchase.

You now know when you acquired each batch and how much you got. The missing piece is the price you paid.

Step 3: Reconstruct the Price from Historical Data

Once you have the acquisition date, use a reputable historical price source. Acceptable options include:

• CoinMarketCap historical data - Free and widely cited.
• CoinGecko historical data - Similar coverage.
• CryptoCompare API - Used by some tax software.
• Exchange-specific archives - Some exchanges post historical order books or price snapshots.

For each acquisition date, look up the daily average price or the closing price of that crypto on that date. If you know the approximate time of day, use the hourly price. If not, the daily average is usually acceptable.

Document each step: the source URL, the date queried, and the price you extracted. This creates an audit trail.

Step 4: Aggregate into a Reconstructed Cost Basis

Combine the evidence from steps 1 - 3 into a single list. For each batch of crypto you acquired, show:

• Date of acquisition (from blockchain or email)
• Amount acquired (from blockchain)
• Price per unit on that date (from historical data source)
• Total cost basis (amount × price)

If you have partial records from email or bank statements, use those in preference to historical averages. Where you have no direct record at all, the historical average is a fallback.

Step 5: explain your method when filing

When you file your tax return, attach a note explaining that exchange records were lost and describing your reconstruction method. Include:

• Why original records are unavailable (e.g., exchange went bankrupt, account was hacked, platform deleted your history after closure).
• Which sources you used (blockchain explorer, historical price API, bank statements).
• Any assumptions you made (e.g., “used daily average closing price from CoinMarketCap for each acquisition date”).

This note does not guarantee acceptance, but it shows good faith and often prevents penalties for negligence.

What if you have no records at all?

If you cannot identify even the wallet addresses you used, reconstruction becomes nearly impossible. In that case:

• Report zero cost basis - This means you treat the entire proceeds as gain, which overstates your tax. You can later amend if you find records, but filing with zero basis avoids a failure-to-file penalty.
• Contact the exchange - Even a bankrupt exchange may have backup tapes or a court-appointed administrator who can provide transaction history. This is slow but sometimes works.
• Hire a crypto tax specialist - A professional may have access to forensic tools that can trace blockchain activity you cannot see.

Common mistakes to avoid

• Using the wrong historical price - Do not use the price on the day you sold the coin as the cost basis. That would give you zero gain, which is incorrect and likely to trigger an audit.
• Ignoring fees - If the exchange charged a fee when you bought, that fee is part of your cost basis. Include it if you have the fee amount. If not, state that fees are not included due to lost records.
• Mixing wallets - If you transferred coins between wallets, trace each batch separately. Do not average all coins into one lump sum unless you used the average cost method consistently.

When you should seek professional help

If the amounts involved are large (over $10,000 in total cost basis), or if you are facing an audit, hire a tax professional who handles crypto. The reconstruction method described here is a reasonable approach for routine filing, but an auditor may demand more detail. A professional can negotiate on your behalf and may have access to data sources you do not.

The bottom line: lost exchange records do not excuse you from reporting. Reconstruct your cost basis using blockchain data and historical prices, document every step, and file with a clear explanation. That is your best chance of avoiding penalties.

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