Risk and position size: the arithmetic that keeps you in
Being right is not the same as surviving. A trader who is correct six times out of ten and risks a third of their balance each time will still be wiped out by the first two losses in a row, and two losses in a row is not bad luck. It is ordinary.
Position size is the part of a decision you control completely. The market decides whether you were right; you decide how much it costs to be wrong. The practice simulator is useful mainly because it lets you work through that arithmetic with a balance you do not need.
Quick answer
Decide the money you are willing to lose before you enter, then divide it by the distance between your entry price and the price at which you are wrong. The result is your position size. If that size feels too small to be worth it, the trade is too large for your balance, not too small for your ambition.
The calculation, once
Risk amount divided by stop distance gives size. A worked example with round illustrative numbers: a balance of 1,000, a willingness to lose 10 on one trade, and a stop 5 away from entry. Ten divided by five is two units. If the stop were 2 away, the same 10 of risk would allow five units.
Notice what the formula does. A wider stop makes the position smaller, not the risk larger. People who set stops by feel and size by confidence reverse this, and the reversal is what turns a manageable loss into a session-ending one.
These are illustrative numbers, not a recommendation. No percentage of a balance is safe in general, because the correct fraction depends on how often you are wrong and how large the losses are when you are. The arithmetic is the durable part; the number is yours to set.
Why small losses are the point
Losing half a balance requires doubling it just to return to where you started. Losing a tenth requires an eleventh. The asymmetry of percentage losses means the first job is never to take a large one, and this is arithmetic rather than caution.
Put plainly: a loss of a tenth needs a gain of about a ninth to recover. A loss of a third needs a gain of a half. A loss of a half needs a gain of a hundred per cent. The sequence is not linear, and the last entry is the one that ends accounts.
A useful consequence: if your plan needs a large position to produce a meaningful result, the plan depends on being right, and being right is the part you cannot control. That is the definition of a fragile position.
The three numbers to write down first
- Your balance. Not the balance you expect to have, and not one that includes money you cannot lose. If the money is needed for rent next month, it is not part of the balance.
- The loss you can accept. Defined in money, before entry, without reference to how confident you feel about this particular trade — confidence is highest exactly when the risk is being ignored.
- Where you are wrong. A price at which the reason for the trade no longer holds. Not a price that feels uncomfortable, and not a round number chosen because it is round.
If any of the three is undefined, the position has no size, only an amount. That distinction is the whole point of the exercise.
The mistake that hides the risk
The most common version is moving the stop after entry. The stop was the thing that defined the size, so moving it makes the size larger after the decision was made, which is exactly the sequence the arithmetic exists to prevent.
The second is adding to a losing position. It is defensible as a stated plan with a stated total risk, and it is indefensible as a reaction to the price moving against you, because the reaction is the point at which your judgement is least reliable.
The third is sizing by what a trade might return. This inverts the logic: the return is uncertain and the stop distance is known, so building the position from the uncertain quantity means the risk is unknown.
What this does not do
Sizing does not make a bad idea good. It bounds the cost of being wrong; it does not improve the odds of being right. A negative-expectation strategy with disciplined sizing loses more slowly, which is a real benefit only if it buys you time to find a better strategy.
It also cannot help against gaps and liquidity events, where the price moves past your stop. The protection is a limit on ordinary losses, not a guarantee against every loss, and a plan that assumes the stop is always honoured is a plan written for a market that does not always exist.
Where the position size comes from the balance rather than the entry price, the working is set out in investing basics for students. If the motive for the trade is recovering a previous loss, that is a separate problem and money and mental health names it plainly.
Common questions
How do I calculate a position size?
Divide the money you are willing to lose by the distance between your entry price and the price at which you are wrong. A wider stop makes the position smaller, not the risk larger.
Why do small losses matter more than being right?
Because losses and gains are asymmetric. A loss of a tenth needs a gain of about a ninth to recover, and a loss of a half needs a gain of a hundred per cent. Avoiding a large loss is arithmetic rather than caution.
What does position sizing not protect against?
Gaps and liquidity events, where the price moves past your stop. It bounds ordinary losses; it is not a guarantee that the stop is always honoured.