Old exchanges shut down, CSV files lost, or bitcoin moved to a hardware wallet years ago: among long-standing crypto investors in particular, the original acquisition costs are often missing by the time a sale comes around.
In Austria that does not mean the gain is automatically tax free. Quite the opposite. Where an Austrian crypto service provider obliged to withhold tax cannot use the actual tax data, the law provides for a flat-rate assumption of the acquisition costs for the purposes of that deduction. At the same time, this does not settle the tax question once and for all.
To withhold capital gains tax correctly, the Austrian provider has to know the acquisition data that is relevant for tax.
That mainly covers:
The details can be passed on as early as the moment the bitcoin arrives with the provider. At the latest, they have to be available and shown to be plausible immediately before a taxable realisation.
This matters above all where bitcoin was originally bought on a foreign exchange, or moved between different wallets over a period of years.
If the investor cannot supply usable acquisition data, or if the figures provided are obviously implausible, the Austrian party obliged to withhold has to apply a statutory flat-rate rule for the tax deduction.
Under that rule the acquisition costs are generally set at 50 percent of the sale proceeds. At the same time it is assumed that the holding is taxable new assets.
Put simply, that means:
The amount withheld at first therefore comes to 13.75 percent of the entire sale proceeds. The flat rate can work out favourably or unfavourably, depending on how high the actual acquisition costs were.
This point matters especially. Where the acquisition costs for the tax deduction are only determined on a flat-rate basis, the deduction generally has no final settlement effect. It therefore does not automatically dispose of the income tax for good.
The actual income may have to be determined in the course of the income tax assessment. That can work in the investor's favour as much as against it.
An investor sells bitcoin for 40,000 euros. The exchange does not know the original purchase price and applies a flat rate of 20,000 euros in acquisition costs. As a result, 5,500 euros in capital gains tax are withheld. Later, however, the investor is able to show that the bitcoin was in fact bought for 30,000 euros.
The actual gain therefore comes to only:
The actual tax at 27.5 percent comes to 2,750 euros. The excessive deduction may be corrected accordingly.
Anyone who finds the documents only after the sale does not necessarily have to wait until the next tax return. In 2025 the Austrian finance ministry clarified that an Austrian crypto service provider can still correct a capital gains tax deduction initially made on a flat-rate basis, provided the actual tax data is submitted and made plausible at a later point. Such a correction is generally possible until the end of the calendar year in question. For investors that means missing documents are worth hunting for even after a sale, and worth submitting to the provider as quickly as possible.
All figures come from the worked example above. The length of each bar shows the euro amount in relation to sale proceeds of 40,000 euros (100 percent).
Source: worked example in this article based on the Austrian flat-rate rule for capital gains tax (50 percent of the sale proceeds as acquisition costs, 27.5 percent tax). As of August 19, 2026.
A wallet shows how many bitcoin are held and when particular blockchain transfers took place. It does not automatically prove how much the investor originally paid for the coins. Reconstructing that may therefore call for several sources:
Reconstruction becomes particularly awkward where bitcoin was not bought directly for euros but came, for instance, from an earlier swap of ether into bitcoin. In tax-neutral crypto-to-crypto swaps the original acquisition costs carry over to the coins received.
Anyone who has bought bitcoin in several tranches over the years does not necessarily need the price of every individual satoshi later sold. For bitcoin of the same kind acquired one after another at the same crypto address or wallet, the moving average price generally applies.
Where individual older purchase records are missing, however, the entire average calculation can be affected. Acquisition costs applied on a flat-rate basis are not taken into the ordinary moving average price. They stay separate for tax purposes.
Historical records become more important still where the bitcoin may have been acquired before March 1, 2021.
Coins of that kind may be Austrian legacy holdings and fall under entirely different tax rules. Where the corresponding evidence is missing at the provider obliged to withhold, the statutory flat-rate calculation assumes taxable new assets instead.
An old bank statement or exchange settlement can make a considerable difference to the tax in such a case. Anyone claiming to have bought bitcoin as early as 2019 or 2020 should therefore be able to document it with robust records.
Missing acquisition costs for bitcoin do not mean that the Austrian tax office automatically treats the entire sale proceeds as a gain.
For the tax deduction there is a statutory flat-rate rule instead. Where the Austrian crypto service provider cannot use the actual tax data, 50 percent of the sale proceeds are generally applied as acquisition costs. Capital gains tax of 27.5 percent is then withheld on the remaining flat-rate gain.
That calculation is not automatically final. A flat-rate deduction generally carries no final settlement effect. Where the actual acquisition costs are documented later, a correction may be required or possible.
With larger bitcoin holdings in particular, it is therefore worth reconstructing old exchange accounts, bank statements and blockchain data before the coins are sold.
