What is trading?
Using the margin calculator
Learning objectives
Explain what a margin calculator does and why each of its inputs matters
Interpret the output correctly, including why required margin is not the same as your risk
Apply the right order of operations: decide risk first, size second, check margin last
What the calculator is actually for
A margin calculator does one job. You describe a position, and it tells you how much of your balance opening that position will tie up.
That's the whole tool. It answers two practical questions before you commit: can I open this at all, and what's left in my account afterwards?
The three inputs, and why each one matters
Every margin calculator asks for roughly the same three things.
Your account type and currency. Margin rates aren't universal. They vary between account types at the same broker, so the calculator needs to know which set of rules applies to you. Your account currency matters too: if you're trading an instrument priced in another currency, a conversion is happening inside the calculation, and the answer comes back in the currency you actually hold.
The instrument. This is the input that changes the answer most. Margin requirements vary enormously across asset classes. Forex and metals typically sit at the low end, while stock CFDs sit far higher. The same balance can support a large forex position or a much smaller equity one, and the difference isn't marginal, it's often a factor of hundreds.
The position size. Usually in lots. This is the number you're testing.
Notice the direction of that arithmetic. You supply the size, and the calculator returns the margin. Most beginners instinctively want it the other way round, working out the biggest size their balance allows. That instinct is the thing this lesson is really about.
What the output means (and what it doesn't)
The figure that comes back is the amount reserved while the position is open. It's still your money. You simply can't use it for anything else until you close.
It is not your maximum loss. It is not a stop loss, a safety buffer, or any kind of recommendation. Lesson D1.5 made this point about margin generally and it applies with full force here: margin is the deposit needed to open, while your risk is set by the size of the position.
The most common misreading goes like this. "I have $500, the margin on this is $50, so I can open ten of them." Arithmetically that's correct. As a plan it's how accounts get closed out, because it leaves nothing free to absorb the first adverse move.
Three things the calculator won't tell you
- Your risk on the trade. That's your stop distance multiplied by your position size, which is a different sum needing a different tool.
- What the position will cost you. Spread, commission and overnight financing are all separate, and lesson F1.5 covers them.
- Whether the requirement will stay put. Which brings us to the part most people never read.
Margin requirements move
Margin moves in two ways, and both catch people out.
They can rise with your total exposure. Many brokers apply tiered rates, where the margin percentage increases as your overall position value grows, calculated across everything you have open rather than each trade separately. The practical consequence is unintuitive: opening a second position can increase the margin required for the first one.
They can rise around events. Brokers commonly raise margin requirements for a window around major economic releases, and ahead of weekends and public holidays, because those are the moments when prices gap. A position comfortably funded on a Thursday afternoon can be uncomfortably tight on a Friday.
Find your broker's schedule for this and read it once, in advance. It's a poor thing to discover live.
The four numbers on your platform
The calculator predicts before you trade. Your platform reports after. Four figures do the work:
- Used margin. The total reserved by everything you currently have open.
- Equity. Your balance adjusted for unrealised profit and loss.
- Free margin. What's left to open new positions or absorb losses on existing ones.
- Margin level. Equity divided by used margin, shown as a percentage. This is the number that triggers margin calls and stop-outs, so it's the one worth watching.
How to use it
Here's the sequence most beginners follow: open the calculator, find the largest position the balance permits, trade that.
Here's the sequence that works:
- Decide what you're prepared to risk on this trade, as a fixed and small percentage of your account.
- Decide where the stop belongs, based on the chart rather than on your balance.
- Work out the position size that makes steps 1 and 2 both true.
- Then open the margin calculator, confirm you can actually place that position, and check what free margin remains afterwards.
The calculator belongs at step four. It's a feasibility check on a decision you've already made properly. The moment it becomes the thing that decides your position size, you've sized to what you're permitted rather than to what you can afford to lose, and those are very different numbers.
An honest note on small numbers
A calculator that returns a reassuringly small margin figure for a very large position is working correctly. It isn't endorsing the trade.
Where leverage is high, the margin required looks trivial on positions that are far too big for the account behind them. That's the mechanism doing exactly what it's designed to do. A small number in the output box is precisely the moment to slow down, not speed up.
Key takeaways
A margin calculator converts a position size into the amount of your balance you'll need 'in reserve'
Required margin is not your risk and not your maximum loss. Your risk comes from stop distance multiplied by position size
Margin requirements move: many brokers raise them as total exposure grows, and around major news, weekends and holidays
Decide your risk, place your stop, calculate your size, and only then check the margin. Sizing from the calculator means sizing to what you're allowed rather than what you can afford