Used margin reduces available free margin
Understanding the relationship between used margin and free margin is not merely an academic exercise; it is the core mechanism that determines whether your trading account survives a volatile session or gets stopped out prematurely. When you open a leveraged position in the foreign exchange market, your broker immediately earmarks a specific portion of your account equity as collateral. This earmarked amount is the used margin. The moment that number increases, your free margin—the funds available to open new trades or absorb losses—decreases by an exactly equal amount. This is not a suggestion or a warning; it is a mathematical certainty that governs every leveraged trade you execute.
To grasp why used margin reduces free margin, you must first understand how margin is calculated for each individual trade. Margin is not a flat fee or a cost of trading. It is a security deposit that your broker holds to cover the potential credit risk of your position. The formula is straightforward: margin requirement equals the trade size in units divided by the leverage multiplier. For instance, if you trade one standard lot of EUR/USD, which is 100,000 units, and your broker offers 50:1 leverage, the required margin is 100,000 divided by 50, which equals 2,000 units of base currency. If your account is denominated in USD, that translates to roughly $2,000. That $2,000 is now marked as used margin.
Before you entered that trade, your account had a certain equity balance—let us say $10,000. That entire $10,000 was free margin, meaning you could use it to open other positions or withdraw it. After the trade, your equity remains $10,000, but your used margin is now $2,000. Your free margin is therefore $8,000. This is the critical point: used margin and free margin are two sides of the same coin. They always sum to your total equity. If you open a second trade that requires another $1,500 in margin, your used margin climbs to $3,500, and your free margin drops to $6,500. No additional funds have left your account, but your operational capacity has been severely constrained.
The danger arises when traders fail to recognize that free margin is not just a buffer for new opportunities; it is the only resource available to cover floating losses. If the market moves against you, your equity declines. Your used margin, however, does not change unless the broker adjusts margin requirements due to volatility. When your equity drops to the point where it equals your used margin, your free margin hits zero. At that moment, you cannot open any new trades, and you are one small unfavorable tick away from a margin call or stop-out. This is why experienced forex traders monitor free margin like a heartbeat. A free margin that is too low relative to open positions indicates that even a moderate adverse move could wipe out the account.
Every trade you take consumes a piece of your free margin permanently until that trade is closed. This is not a temporary loan that gets returned immediately. The used margin remains locked until you exit the position. If you are running multiple positions, the cumulative used margin can quickly absorb most of your equity, leaving almost no free margin. For example, a trader with $5,000 in equity who opens three standard lots of USD/JPY with 30:1 leverage would have a used margin of approximately $3,333. That leaves only $1,667 in free margin. If the market moves just 34 pips against each lot, the loss of about $1,020 would drop equity to roughly $3,980, making free margin only $647. The trader is now in a high-risk zone.
Advanced traders use this relationship strategically. They know that free margin is not something to maximize by opening as many positions as possible. Instead, they keep a significant portion of equity as free margin to withstand drawdowns. Some adopt a rule that used margin should never exceed 10 to 20 percent of total equity, ensuring ample free margin for volatility. Others use free margin calculations to determine position sizing for stop-loss distances. If the stop-loss is 50 pips away, the potential loss must be covered by free margin before the trade even begins.
It is also important to understand that used margin is recalculated continuously as the market price moves for certain pairs. For currency pairs where the base currency differs from your account currency, fluctuations in exchange rates can alter the margin requirement in real time. This means that even if you do not open new trades, your used margin can increase, automatically shrinking your free margin. A sharp move in GBP/USD while you hold a GBP-denominated pair in a USD account could tighten your free margin unexpectedly, leaving you with less room for error.
Ultimately, the relationship between used margin and free margin is the single most practical indicator of your account’s health. It tells you whether you have the freedom to act or are boxed into a corner by your own positions. Every time you consider adding a trade, you should first calculate how much free margin will remain afterward. If the answer is dangerously low, you are better off waiting. The goals of safety and profitability are served not by maximizing leverage but by understanding that used margin is a permanent subtraction from your available firepower until you exit.
