Position sizing with leverage: the wrong calculation

The two calculations

There are two separate questions when you open a position, and they use different inputs.

**How much will I lose if this goes wrong?** The answer is the distance to your stop, multiplied by the value of that distance for your position size. Neither the leverage setting nor the margin requirement appears anywhere in this calculation.

**How much margin will the broker hold?** The answer is the notional value of the position divided by the leverage ratio. This is a constraint on whether the trade is possible, not a measure of what it risks.

A worked example makes the separation concrete. One lot of gold with a contract size of 100 units and a value of 100 per 1.0 of price movement. Price falls 20.00. The loss is 20.00 × 100 = 2,000 — identical whether the account is set to 1:1, 1:100 or 1:500. What changes is only the margin held: at 1:100 with gold at 2,400 the notional is 240,000 and the margin is 2,400; at 1:500 it is 480.

The leverage setting changed the margin by a factor of five and changed the risk not at all.

Why sizing to leverage goes wrong

If you treat leverage as a sizing input, the logic runs: higher leverage means I can afford a bigger position, so I should take one. That reasoning is true about margin and false about risk.

The failure is not subtle. Two accounts, one at 1:100 and one at 1:500, holding the same instrument and the same signal. Sized to margin, the second account takes a position five times the size of the first. If price moves against both by the same amount, the second loses five times as much. Nothing about the trade changed; the broker's margin policy changed, and the account's risk changed with it.

There is a second-order effect that makes it worse. Brokers offering very high leverage are often the ones whose instruments have wider spreads and thinner liquidity, so the account taking the largest position is also paying the highest cost per unit. The two effects compound in the same direction.

The habit to build instead: choose the position size first, from the stop and your risk tolerance, and then check whether the margin fits. Leverage becomes a filter on which trades are possible, never an input to how large they should be.

How leverage does legitimately matter

It is not irrelevant — it constrains things in three ways that are worth understanding.

**It limits position size in practice.** If correct sizing from your risk rule exceeds what your margin allows, the trade is not available at that size. This is most relevant for strategies with very wide stops relative to their risk budget.

**It determines margin call timing.** Lower leverage means more margin held, which means you hit a margin call later for the same adverse move — but also that more of your equity is tied up and unavailable for other positions.

**It interacts with gaps.** A stop is an instruction, not a guarantee. On a weekend gap or a fast market, the fill can be far from your level, and the loss is the actual move times the size. Leverage does not cause this, but high leverage makes a given gap more likely to be fatal, because the position it permits is larger relative to the account.

A useful way to hold it: leverage decides how big a position you *can* hold; your risk rule decides how big you *should*. The second number should always be the smaller one, and if it is not, the answer is a smaller position, not more margin.

The correct order of operations

1. Decide your risk per trade as a percentage of the account. One percent is a common institutional-style starting point.

2. Convert that to money: account × risk%.

3. Measure the distance from entry to stop in price terms.

4. Convert that distance to money per lot using your instrument's specification — the value of one lot per one unit of price movement.

5. Divide: position size = money at risk ÷ money risked per lot.

6. Only now check margin. If the position requires more margin than you have available, reduce the size or skip the trade — do not increase leverage to make it fit.

Step six is the one people run first, which inverts the whole procedure. Sizing from leverage means starting from the constraint and working backwards to a risk number you never chose.

Where the confusion comes from

Platforms encourage it. A leverage setting sits in the account configuration, position size sits in the order ticket, and both are expressed as numbers you choose. The interface implies a relationship that the mathematics does not contain.

Marketing encourages it more. High leverage is advertised as access — the ability to trade larger positions with less capital. That is an accurate description of what leverage does to margin, and a misleading description of what it does to risk.

And there is a genuine reason the two feel connected: on a small account, leverage is usually what makes a correctly sized position possible at all, because the minimum lot may require more margin than the account holds. That is a real constraint. It just does not change the fact that the size should be chosen from risk and then checked against margin, rather than chosen from margin.

What this does not settle

It does not tell you what risk per trade to use. That depends on your drawdown tolerance and on the risk of ruin arithmetic, which is a separate question with its own answer.

It does not account for correlated positions. Sizing five trades correctly at 1% each and then discovering they are all the same bet means your real risk is closer to 5%.

And it does not make a strategy profitable. Correct sizing determines whether an edge gets to compound; it does not create one. A strategy with no edge loses more slowly at a smaller size, which is worth something but is not the same thing.