How to Accurately Report Taxes on Short Sales of Stocks

A short sale (blankning) is when you borrow shares, sell them, and later buy back equivalent shares to return to the lender. While the mechanics are familiar to active traders, the Swedish tax treatment has some important differences from how your broker reports these transactions.

The broker's control statement reports the disposal of shorted shares the same way as an ordinary share sale — it usually does not show that the sale was a short sale. This means a prefilled Swedish return can show the sale proceeds, but you may still need to reconstruct the correct K4 acquisition cost yourself.

For the short seller, Skatteverket says the gain calculation is based on the sale of the borrowed shares and the covering purchase. The borrowing fee is not part of the share acquisition cost on K4; it is deducted separately in capital income at point 8.1.

If you paid dividend compensation to the lender, that item also gets special treatment: if you did not receive the dividend yourself, the compensation should reduce the proceeds in the capital-gain calculation; if you did receive the dividend yourself, the compensation is instead deducted as another capital expense.

The most important special rule is this: if, when you borrowed the shares, you already owned identical shares, Skatteverket says those shares are deemed sold first and taxation takes place immediately on that quantity.

In that situation, the acquisition cost is the average cost basis of your previously owned shares, and the later return of the borrowed shares is not itself taxed. If, after the short sale, you buy more shares of the same kind than the number that must be returned, then the average cost method must be used when computing the short-sale result.

If the transaction was a true short

If you did not own identical shares at the moment of the borrow, the K4 line for the share short works like this:

• Ersättning (proceeds) = net proceeds on the Sell Short trade, in SEK
• Omkostnadsbelopp (acquisition cost) = net SEK cost of the buy or buys that restored the borrowed shares
• Vinst/Förlust = Ersättning − Omkostnadsbelopp

Borrow fees are not added to omkostnadsbelopp on that K4 line; they are deducted separately.

Important: Sweden's default cost basis method is the average cost method. This means the acquisition cost (omkostnadsbelopp) is calculated as the average cost of all shares bought to cover the short, not the cost of a specific lot. You do not need FIFO lot tracking for normal short sale cases.

If you already owned the same shares when you shorted

The acquisition cost for that quantity is the average cost basis of your already-owned shares, and taxation on that quantity happens immediately when the short sale is made. The later purchase that restores the borrow is not a second taxable sale; it is the operational restoration of the borrow and may instead become the cost basis of a new long holding.

For most short sales, the average cost method is sufficient and no per-lot tracking is required. The only exception is the late-return rule:

If you hold short positions for more than one year, the late-return exception requires knowing the exact sale date and proceeds of each individual short sale. For everything else, the average cost method plus the chronological matching of Sell Short and cover Buy trades is sufficient and correct under Swedish tax rules.

Because your trades are likely denominated in a foreign currency (e.g. USD), the K4 must be prepared in SEK. The rules are:

• Acquisition costs are translated to SEK using the rate on the purchase date.
• Sale proceeds are translated using the rate on the disposal date.
• Where actual currency exchange has occurred, use the actual exchange rate. Otherwise, use the daily mid-rate published by Riksbanken.

The clean way to handle short sales is to rebuild the stock chronologically from your broker records:

1. Start with your opening position before the trading session.
2. Place every trade for that ticker in exact timestamp order and maintain a signed share balance.
3. When the balance is positive, ordinary Sell trades are long-share sales and use the Swedish basis rules for long holdings.
4. When the balance goes negative, you have entered or increased a short. Buys while the balance is negative are cover buys until the borrowed-share obligation is back to zero.
5. Use the net cash amount on each trade row, not just price × quantity. Broker fees create small differences that belong in the tax math.

Once reconstructed, the share transaction goes on K4 section A if the shorted instrument was shares. The e-service often already contains the share name, quantity sold, and proceeds, but you still need to supply the correct omkostnadsbelopp. Borrow fees belong separately under point 8.1, not inside the K4 share basis.

The documents that matter most are the trade confirmations and the full account activity extract with exact timestamps. You need the broker notes for your purchases and sales and an account extract covering the activity between the purchase dates and the last sale, because the basis calculation depends on the actual sequence of transactions.

For borrow costs, the aggregate monthly Stock Borrow Fee appears on the account statement. That is the fee you would normally trace for the separate Swedish deduction under point 8.1, rather than folding it into the share acquisition cost on K4.

Consider a scenario where you trade 6,000 shares. The decisive fact is whether you were already holding the same stock when the Sell Short happened, and which later buy actually restored the borrow:

Both answers are consistent with Swedish tax law depending on the time ordering and inventory state. Without the exact timestamp order and a running share balance, you cannot know which branch applies. The actual K4 line must still be stated in SEK, with proceeds and acquisition cost each translated using the correct exchange-rate rule for their own trade date.

Swedish tax law treats short sales under three distinct rules depending on your situation:

1. Normal short sale (true short): Gain = short-sale proceeds − cover acquisition cost (calculated using the average cost method — Sweden's default). Realized in the year the shares are returned to the lender.
2. Already-owned shares at time of short: The short sale is treated as an immediate disposal of your existing long position. The acquisition cost is the average cost basis of those already-owned shares. Taxation happens in the short-sale year, not the cover year.
3. Late-return exception: If borrowed shares are not returned by 31 December of the year after the short sale year, the full proceeds are taxable in the original sale year. This rule is tied to the specific short-sale event and requires per-lot tracking.

In all cases, borrowing fees are not part of the acquisition cost on K4. They are deducted separately as capital expenses at point 8.1.