Yesterday, I met someone who could make a profit of $30 from trading, but when I checked their account, there was only $4 left. Why is that? The answer is the Swap, which is often overlooked.

For most traders, we think about Spread and Commission, but what is Swap? It’s a hidden cost. If you don’t understand it well, it will eat into your profits without you even realizing it.

Basically, Swap is the fee for holding an order overnight. It’s like interest generated from borrowing money—but what makes it more complex is that it comes from the difference in interest rates between two currencies.

When you trade EUR/USD, for example, you are “buying” euros and “borrowing” dollars to pay for them. The euro has an interest rate of about 4%, but the dollar is higher than 5%. That difference is what turns into the Swap you have to pay.

But it’s not always a net negative. If you trade Sell EUR/USD, you “borrow” euros and “hold” dollars. In that case, you can “earn” Swap plus, because the dollar pays higher interest.

So why do most people end up losing on Swap? The answer is simple: brokers are middlemen. They add a management fee to the actual Swap rate. So even if, in theory, you should receive a positive Swap, the broker might make it negative—or reduce it until it’s almost gone.

What you need to watch out for is the 3-Day Swap. When you hold an order overnight from Wednesday to Thursday, you get charged 3x the Swap. Why? Because the Forex market is closed on Saturday and Sunday, but interest keeps accruing every day. Brokers have to collect the weekend’s charges and account for them on the next business day.

Swap calculations aren’t that hard. If the broker tells you it as a percentage per night (e.g., -0.008%), multiply it by the full value of the position—not the Margin you put up. Suppose you trade 1 Lot EUR/USD at a price of 1.09. The full value is $109,000. Swap -0.008% per night equals about $8.72 per night. And if that night is a 3-Day Swap, multiply it by 3.

The scary part is that with high Leverage, Swap may eat into your Margin quickly. If the market is flat and the price doesn’t move, but the Swap is negative and charges you every day, you might soon get hit with a Margin Call.

But Swap isn’t only about risk—it also creates opportunities. Carry Trade is a strategy that takes advantage of positive Swap. You “borrow” a currency with a low interest rate (such as Japanese Yen) and “buy” a currency with a high interest rate (such as Australian Dollar) in order to receive positive Swap every day. For example, with Buy AUD/JPY, if the Long Swap is positive, money gets added to your portfolio every night.

But be careful: Carry Trade comes with risks. If AUD/JPY drops sharply, the losses from the exchange rate could be greater than the Swap profits you’ve accumulated over a year.

One option I find interesting is a Swap-Free account (Islamic Account). Some brokers offer this service: there is no Swap charged at all, no matter how long you hold an order. This is ideal for Swing Traders or Position Traders who need to hold orders for weeks or months. The trade-off is that the Spread may be slightly wider.

In summary, what is Swap? It’s a cost you need to understand well, not just a floating number on the platform. For short-term traders, it may not have much impact, but for those holding orders for months, the effect can be massive. Choose a broker that is transparent about these fees, and plan your trading carefully—so you don’t get hit by hidden costs that eat your profits.