Recently, someone asked me what exactly rollover is and why it matters for their forex trades. It turns out it's one of those concepts that many traders ignore until they see an unexpected charge in their account.

Basically, rollover is the extension of a position beyond a trading session. When you hold a currency pair overnight, you're doing a rollover. It's also called a swap in some contexts. It sounds complicated, but it actually works like this: each currency has its own annual interest rate. When you trade with leverage, your broker lends you money in one currency so you can trade the other.

Now, here’s the interesting part. Depending on which currency pair you're trading, that rollover can be a cost that deducts from your profits or a benefit that adds to them. It all depends on the interest rate differential between both currencies. If you go long and the base currency has a higher interest rate than the secondary, you receive a benefit. If it's the other way around, you pay a cost. With short positions, it works inversely.

Let’s take a practical example. Imagine we trade GBP/USD because we expect the pound to appreciate. We buy one lot and hold it until tomorrow. The calculation would be: take the annual interest rate differential, divide it by 360 days, multiply by the size of the position, the opening price, and the number of days we hold the trade. In this case, if the pound’s rate is 0.75% and the dollar’s is 1.00%, the negative differential results in a cost that is deducted from our profits.

Understanding what rollover is really important if you're a swing trader, meaning if you hold positions for days, weeks, or months. If you're one of those who close everything before the session ends, this doesn’t affect you. But if your style is more long-term, you need to know this.

There are two main factors that affect rollover. First, the monetary policy of central banks. If the European Central Bank or the Federal Reserve change their interest rates, that directly impacts what you pay or receive. Second, inflation. When central banks raise rates to combat inflation, rollover becomes more significant.

An interesting strategy is to look for currency pairs where the base currency is appreciating AND has a higher interest rate. Look at what happened with USD/JPY since March 2022: the dollar was appreciating while U.S. rates were rising. That meant gains both from price movement and rollover. The opposite happened with GBP/USD: the pound was depreciating and had lower rates, so shorting allowed you to profit on both fronts.

This rollover concept isn't exclusive to forex. It also applies when trading CFDs on other assets like stocks, commodities, and indices. With CFDs, you have the advantage of trading up or down without restrictions, but remember that leverage amplifies both gains and losses. If you hold a CFD beyond a session, you'll always pay rollover, unless it’s a forex CFD where it could be in your favor.

Many traders underestimate the impact of rollover on their results. It’s a detail that easily goes unnoticed but can make the difference between a profitable year and an average one. If you understand it well and use it strategically, it becomes another ally in your trading.