Why it matters
Your gain or loss on a sale is what you received minus your basis, so a missing or wrong basis changes the tax you owe. For crypto, US brokers report basis to the IRS on Form 1099-DA only for covered assets, which broadly means coins bought in 2026 or later in a custodial account and kept there until sold. For anything else the basis box can be left blank.
A blank box isn’t the same as zero. The IRS instructions for Form 1099-DA tell brokers to enter -0- “only if the digital asset sold actually had a basis of zero.” If you treated a blank as zero, you would pay tax on the whole sale price.
Example
Here is an illustration with round numbers. You buy 0.5 ETH for $1,500 and pay a $15 fee, so your basis is $1,515. You later sell it for $2,000 and pay a $20 fee. The Form 1099-DA instructions say proceeds are reduced by transaction costs, so proceeds are $1,980 and the gain is $465.
What to watch for
Coins you moved in from another platform count as noncovered, so the receiving broker won’t report their basis for you. Keep the purchase records from the platform where you bought them, because the IRS expects you to work out basis yourself before you file.