Margin is the deposit your broker locks up to support a leveraged position. Used margin grows with every open trade and shrinks free margin.
When to use it
Before scaling up or layering positions, especially on news days when stop-outs cluster.
Formula
Required Margin = (Trade Size × Contract Size) / Leverage, then converted to account currency.
Worked example
1 standard lot EUR/USD at 1:30 leverage in a USD account: margin = 100,000 / 30 = $3,333.
What is a margin call?
A broker-issued warning that your free margin is too low. Hit the maintenance level and the broker auto-closes positions to protect itself.