What position size is, and why it is decided before entry

Position size is the number of lots you open a trade with — what turns an opinion about the market into a specific amount of money exposed to loss. You cannot control where price goes; you control this number completely.

Deciding it before entry means knowing your worst case before you know the outcome. A fixed habit such as “always one lot” makes the loss a function of stop distance instead: a 20-pip stop and a 120-pip stop at the same size are very different losses.

The three numbers: balance, stop distance and pip value

Three linked figures drive the result: the amount at risk (balance × risk percentage), the stop-loss distance in pips, and the pip value per standard lot.

Stop distance × pip value is the cost of one lot if the stop is hit; the amount at risk divided by that cost is the number of lots allowed. A wider stop gives a smaller size, a tighter stop a larger one, and the amount at risk is unchanged in both. Size is the dependent variable; risk is the constant.

Pip value is not one fixed figure: $10 per standard lot on USD-quoted pairs, 1000 ÷ the USD/JPY rate on JPY-quoted pairs, and 10 ÷ the pair’s price where the dollar is the base. Hence the price field in the latter two cases only.

Why 1% to 2% per trade

The reason is arithmetic, not temperament. Losses compound rather than add up, since each is taken from a smaller balance than the last. The table below is calculated cumulatively.

The gap between 1% and 5% is not five times the risk; it is 69 chances to be wrong against 14 — and fourteen losses in a row is no freak event over thousands of trades.

Three worked examples

One — a USD-quoted pair. Balance $1,000, risk 1%, stop 30 pips. Amount at risk = 1000 × 0.01 = $10. Pip value = $10 per lot. Cost of one lot = 30 × 10 = $300. Size = 10 ÷ 300 = 0.033333 lots, executed as 0.03 lots, a real loss of $9.00 rather than $10.00.

Two — a JPY-quoted pair. Balance $5,000, risk 1%, stop 25 pips, USD/JPY at 150. Amount at risk = $50. Pip value = 1000 ÷ 150 = $6.67. Cost of one lot = 25 × 6.67 = $166.67. Size = 50 ÷ 166.67 = 0.30 lots exactly — the full $50.00 loss, nothing rounded away.

Three — a USD-based pair. Balance $2,000, risk 1.5%, stop 40 pips, pair price 1.35. Amount at risk = $30. Pip value = 10 ÷ 1.35 = $7.41. Cost of one lot = 40 × 7.41 = $296.30. Size = 30 ÷ 296.30 = 0.101250 lots, executed as 0.10 lots for a real loss of $29.63.

Why the result is rounded down, never up

Brokers execute in two decimals, so 0.033333 cannot be sent as it stands: the choice is 0.03 or 0.04. Rounding up lifts the first example’s loss to $12.00 — 1.2% instead of the 1% you decided on. Rounding down brings it to $9.00, or 0.9%.

On one trade that is trivial; over hundreds it is a small, regular overshoot of your stated limit. That limit is a ceiling to stay under, not a target to approach from above — which is why the calculator shows both values.

Frequently asked questions

This section works two ways: from levels, you enter a stop and a target and get the ratio; from a risk amount, you enter the amount and a target ratio and get the stop and target. Values are kept between modes and are never cleared when you switch.

What the reward-to-risk ratio is

The reward-to-risk ratio is the division of two distances and nothing more: the target distance from entry divided by the stop distance from entry. Its simplicity is its strength, because it strips the trade of its instrument, its size and its currency, making it comparable with any other trade. A 1:3 trade on gold and a 1:3 trade on a yen pair are equivalent on this one dimension, however much everything else differs.

From the ratio to the breakeven point

The derivation from the ratio to the breakeven point deserves to be shown in full rather than claimed as borrowed. At a win rate w and a reward R times the risk unit, the expected value of a single trade is w times R, minus one minus w times one. Setting that to zero gives w times R plus one equals one, and therefore w equals one divided by one plus R. We found no acceptable reference source publishing this formula, so we show the derivation and attribute it to nobody.

Worked example: a 1:2 ratio

Take a buy at 1.08500 with a stop at 1.08000 and a target at 1.09500 on a pair whose pip size is 0.0001. The risk distance is 0.00500, that is fifty pips, and the reward distance 0.01000, that is one hundred pips. The ratio is 2.00, and the breakeven win rate is one divided by three, that is 33.33%. What that means in practice is that a third of your trades suffices to break even before costs at this ratio — not that a third of your trades will win.

Worked example: 1:1 and below

Compare that with two other cases on the same entry and stop. At a target of 1.09000 the reward distance becomes fifty pips and the ratio 1.00, and the breakeven rate jumps to 50.00%. At a target of 1.08750 the reward distance becomes twenty-five pips and the ratio 0.50, pushing the breakeven rate to 66.67%. So a target half the stop distance obliges you to win two thirds of your trades before costs even enter the calculation.

Worked example: deriving levels from a risk amount

The second mode runs the other way: from the amount to the levels. Take a risk amount of 100 dollars, a pip value of 10 dollars per lot and a half-lot size. The stop distance is 100 divided by 10 times 0.5, that is twenty pips. At a pip size of 0.0001 that distance is 0.00200, so the stop sits at 1.08300 and, at a target ratio of 2.00, the target sits at 1.08900 with a target amount of 200 dollars. This way you start from the amount you accept losing rather than from a level on a chart.

Why the real threshold is always higher

The real breakeven threshold is always higher than the figure shown, without exception. The reason is that the derivation deals with two clean distances and knows nothing of spread, commission, swap or slippage. The effect is not marginal: in the profit and loss calculator we saw a trade with a gross profit of 0.20 dollars and costs of 0.57 dollars, which flipped it to a net loss. The same ratio that looks like 1:2 on paper can be worse than 1:1 after costs at a small size.

The tension between ratio and win rate

Between the ratio and the win rate lies a tension that cannot be ignored. A more distant target raises the ratio and lowers the win rate needed to break even, which looks like pure gain until you notice the other side: a more distant target is reached less often, so your actual win rate falls too. The two numbers cannot be read apart. Raising the target ratio from 2 to 8 lowers the breakeven threshold from 33.33% to 11.11%, but that means nothing if your actual win rate at that target has fallen below 11.11%.

Common mistakes

Four mistakes recur. The first is measuring the ratio from chart prices rather than execution prices, which removes the spread cost from the calculation. The second is moving the stop to improve the ratio instead of adjusting size; the stop belongs where the trade idea is invalidated, and size is the variable you tune. The third is reading the breakeven rate as a performance target when it is an arithmetic floor. The fourth is ignoring costs entirely, which we just saw can flip the result at small sizes. You can also derive your levels from the previous session instead of estimating them.

Frequently asked questions

What is the breakeven win rate and how is it computed?

It is the lowest win rate that makes your long-run result zero at a given reward-to-risk ratio. The derivation is direct: at a win rate w with reward R times the risk, expected value is w × R − (1 − w). Setting that to zero gives w = 1 / (1 + R). At 1:2 that is 33.33%, at 1:1 it is 50%, and at 1:0.5 it rises to 66.67%.

Is the figure shown the win rate I actually need?

No — the real threshold is always higher. The formula ignores spread, commission, swap and slippage, all of which subtract from the winning side and add to the losing one. The figure shown is an arithmetic floor, and the gap between it and the real threshold widens as trades get smaller or holding periods get longer. To price the effect of costs, use the profit and loss calculator with the commission and swap fields.

What is the difference between the two input modes?

Levels mode starts from the chart: you enter the stop and target as prices and get back the ratio and the breakeven rate. Risk-amount mode starts from money management: you enter the amount you accept losing and the target ratio, and get back the stop and target as prices. The first measures a trade you already planned; the second builds one from a capital constraint.

Is a higher reward ratio always better?

The ratio alone is not enough to judge. Raising it lowers the win rate needed to break even, but it also means a more distant target that is reached less often, so your actual win rate falls too. The two figures are linked and neither is read in isolation. This calculator calls no ratio good or recommended, because the judgment depends on your approach and your actual trade record.

Why derive the stop distance from the risk amount rather than the reverse?

Because risk management reasons from capital outward. The amount you accept losing is decided before you look at the chart, and with pip value and trade size known, the stop distance is the only unknown left. The reverse order — picking a stop from the chart and accepting whatever loss follows — makes your capital a function of the chart rather than the other way round. Both modes are available here because both are used in practice.

Does this calculator tell me where to place my stop?

No. In risk-amount mode it computes the distance that produces an amount you set; it does not claim that distance is technically appropriate. Where a stop belongs is an analytical decision based on the market structure in front of you. The calculator only answers ‘if I risk this amount at this size, how far away does the stop sit’. Reconciling the two — the arithmetic distance and the technical one — is normally done by adjusting trade size rather than by moving the stop.

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