Your qualified appraiser will determine fair market value (FMV) for you. You can do an internet search to find appraisers qualified to value cryptocurrencies like bitcoin (BTC).
Generally, if the deduction for an item or a group of similar items is more than $5000, you must get a qualified appraisal.
Similar items refers to property in the same generic category or type. Although a BTC can be broken into multiple pieces (eight digits to the right of the decimal point), these pieces are all similar. Your claimed deduction will still be over $5000 (depending on the value of BTC at the time of appraisal, as cryptocurrency values can fluctuate wildly) even if you were to break the donations into 15 separate pieces.
For more information on qualified appraisals and donations in general, please see page 9 of Publication 561.
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