How Position Sizing Works

Position sizing answers a single question: if this trade hits its stop loss, how much money leaves the account? Entry, stop and target are decisions about the market. Volume is the one input on the ticket that is purely about you — and it is the only one that controls what a losing trade costs.

The reliable way to choose it is to work backwards. Fix the acceptable loss first, then solve for the volume that produces exactly that loss at the stop. Five quantities connect in a chain:

Each link is plain arithmetic. The first two fix how much may be lost; stop distance and pip value translate price movement into money; the final division turns it all into a volume.

The denominator is the cost of the full stop distance for one standard lot. On EUR/USD with a USD account, one pip on 1.00 lot is worth $10, so a 25-pip stop costs $250 per lot. With a $100 budget, the account can afford 100 ÷ 250 = 0.40 lots. Halve the stop to 12.5 pips and the same budget affords 0.80 lots; double it to 50 pips and it affords 0.20.

One requirement is easy to miss: every term must be expressed in the same currency. That is where the only genuinely fiddly part of the calculation lives.

Pip value starts in the quote currency — the second currency of the pair. One pip on one standard lot is always 0.0001 × 100,000 = 10 units of the quote currency (1,000 units for JPY-quoted pairs, where a pip is 0.01 — see the pip guide for the conventions). Whether those 10 units are already your money depends on the account currency.

Because the conversion uses a live exchange rate, pip value in account currency drifts as prices move. Platforms and calculators handle this automatically, but it explains why the “same” position can carry slightly different risk on different days.

A common shortcut is to trade the same volume on everything — say 0.50 lots, every trade. The problem is that the formula then runs forwards instead of backwards: money at risk becomes a by-product of stop distance rather than a decision. Two equally sensible stops of different widths produce very different losses.

Same account, same fixed size — yet trade B risks three times more, simply because its stop is wider. Percent-risk sizing inverts the relationship: the wider stop gets a smaller position, and every losing trade costs roughly the same fraction of the account. Over a long series of trades, that consistency is what makes statistics such as drawdown and expectancy meaningful — a mixed history of 1% and 3% losses is much harder to interpret.

Brokers accept volume only in increments: a minimum lot (often 0.01), a volume step (often 0.01) and a maximum. The raw formula output is almost never exactly on a step, so it has to be rounded — and the safe direction is down, because rounding up risks more than the budget allows.

• $100 ÷ (27 pips × $10) = 0.3704 lots → rounds down to 0.37 on a 0.01 step.
• After rounding, recompute the actual risk: 27 × $3.70 = $99.90 — slightly under budget, as intended.
• If the calculated size falls below the broker’s minimum volume, the trade cannot be taken at that risk level — the options are a smaller budget, a structurally tighter setup, or no trade.

Position size determines what a trade loses if the stop is hit. Margin determines whether the broker lets you open the position at all. They are computed from different inputs and answer different questions.

Notice that leverage never appears in the sizing formula. It changes how much margin a position locks up, not what the stop loss costs — a distinction unpacked in the margin and leverage guide. A trade can pass the risk check and still fail the margin check on an account with many open positions, so both constraints have to clear before the order is placed.

The same arithmetic — with the pip-value conversion handled for any pair and account currency — runs in the free Position Size Calculator. And the reverse check is just as useful: reviewing the percentage each past trade in your own MetaTrader history actually risked shows quickly whether sizing was a plan or an accident.