Most blown accounts aren't blown by a bad call. They're blown by a position that was far too big for the account in the first place. Margin is the number that decides whether a trade fits, and it takes about ten seconds to work out once you know the formula.
Here's how to do that sum before you place the order, with examples built around UK accounts and the leverage limits UK retail clients actually trade under.
The margin formula, stripped back
Every margin calculation comes down to two figures: the notional value of the position, and the leverage available on that instrument.
Margin required = Notional position value ÷ Leverage ratio
Notional value is the full size of the position — what you would be controlling if you had to buy it outright. For an index CFD, it's your stake per point multiplied by the index level. For shares, it's the number of shares multiplied by the price. For a currency pair, it's the number of units of the base currency.
Leverage is written as a ratio: 30:1, 20:1, 5:1. Divide 100 by the ratio and you have the margin rate as a percentage. At 30:1 the rate is 3.33%, at 20:1 it's 5%, at 5:1 it's 20%. Both routes give the same answer, so use whichever your platform shows you: notional ÷ leverage and notional × margin rate are the same calculation.
Three worked examples
An index CFD
Say the FTSE 100 is quoted at 8,000 and you buy one CFD at £1 per point. Notional value: £8,000. Major indices sit under a 20:1 cap for UK retail clients, so margin = £8,000 ÷ 20 = £400. Now double the stake to £2 per point and the notional becomes £16,000, with margin of £800. Double the index level instead — say a market trading at 16,000 — and you get the same £800. Size and price both move the number, and they move it in the same direction.
A major currency pair
One standard lot of GBP/USD is 100,000 units of sterling. Because the base currency is GBP and your account is in sterling, notional is simply £100,000 and no conversion is needed. Major pairs carry a 30:1 cap, so margin = £100,000 ÷ 30 = £3,333. A mini lot of 10,000 units needs £333; a micro lot of 1,000 units needs £33. This is why FX traders often think in lots rather than in pounds.
US shares
You buy 200 shares at $50. Notional: $10,000. Individual equities sit at 5:1, so margin = $10,000 ÷ 5 = $2,000. If your account is in sterling, that $2,000 is converted at the prevailing rate, and the sterling figure will drift as GBP/USD moves. A rough rate is fine for planning; the platform uses the live one at the moment of the fill.
When your account currency isn't the same as the trade
This is where most quick calculations go wrong. If you trade a US share, a German index or an Australian currency pair from a sterling account, the notional value has to be expressed in pounds before you divide by leverage. Roughly: value in your account currency = value in the instrument's currency ÷ the relevant FX rate. The margin isn't hedged against that rate either. A position in dollars funded from a sterling account carries a currency risk on top of the market risk, and both land in your equity figure.
Margin used, free margin and margin level
The deposit frozen when you open a trade is margin used. Your remaining balance is free margin, and that's what's available for the next position. Your margin level is equity divided by margin used, expressed as a percentage. That figure is what brokers watch: as it falls towards their threshold, they'll restrict new positions and eventually close trades to protect the account.
Keep one thing straight. Margin is a deposit, not a maximum loss. A £400 margin requirement on a £8,000 position means the position can lose far more than £400 before you close it — which is exactly why position size matters more than the deposit figure. UK firms offer negative balance protection to retail clients, so you shouldn't end up owing money after a gap, but that protection is a backstop, not a strategy.
What pushes your requirement up or down
Four things move the number, and only one of them is under your control:
- The leverage cap for that asset class. For UK retail clients, the FCA sets maximum leverage on these products: 30:1 for major currency pairs, 20:1 for non-major pairs, gold and major indices, 10:1 for other commodities and minor indices, 5:1 for individual shares, and 2:1 for cryptocurrencies. Professional classification changes the caps.
- The broker's own rate. Firms can apply tighter margins than the cap, and rates often differ between CFDs and spread bets. Check the instrument's rate on the platform rather than assuming.
- Your position size. The lever you pull. Halve the stake and you halve the margin — and halve the profit or loss per point.
- Market price. If price moves against you, your notional value grows and so can the margin needed to hold the position.
A pre-trade checklist
- Open the instrument's details and note the exact margin rate — not the headline leverage figure.
- Calculate notional value in your account currency, converting if needed.
- Divide by leverage, or multiply by the margin rate, to get margin required.
- Compare that with your free margin. If it takes more than a small fraction of it, the trade is too big.
- Assume the trade goes badly. Work out the loss at your stop distance per point or per share, and check it against your account. If that loss would sting, cut the stake now rather than later.
- Add the cost of the spread and any overnight financing, which aren't part of margin but do come out of equity.
Do the sum before the click
Margin isn't a number the platform decides on your behalf. It's the arithmetic of notional value and leverage, and it tells you in advance whether a trade is sized sensibly. Run the calculation, keep the requirement to a fraction of your free margin, and let the stop — not the margin call — decide when the trade ends. If you're unsure whether leveraged trading suits your financial circumstances, take regulated independent advice before committing capital.
Photo: TheDigitalWay / Pixabay
