Tamaño de posición: la decisión que importa más que tu pronóstico

Ask a beginner what decides a trade's outcome and they point at the forecast: where is the price going? Ask a professional and you hear something less exciting: how much do I lose if I am wrong? That inversion — fixing the size of the potential loss before thinking about the win — is the closest thing trading has to a foundation. This sheet gives you the whole framework: the three inputs that determine position size, the formula that connects them, that formula worked three times on real numbers, and the streak arithmetic that shows why small risk per trade is not caution. It is survival.

The inversion that defines professionals

Most trades begin in the wrong order. The trader finds a setup, feels some level of conviction, and picks a size that matches the feeling: half a lot because the chart “looks strong”, a full lot because the last two trades worked. However good the analysis was, the size came from emotion — and size chosen by emotion is largest exactly when confidence is highest, which is not when the odds are best. Confidence and accuracy are far more loosely connected than they feel from the inside.

The professional order runs the other way. First: how much of the account may this trade lose? That is a policy decision, made once, in calm conditions — commonly 1% of equity or less. Second: where is this trade wrong? That is a chart decision — the ▲stop-loss level that invalidates the idea. Only then, third: what size makes those two answers consistent with each other? Position size is the output of the process. It is never an input.

Notice what is absent from that sequence: the forecast. Sizing assumes any single trade can lose, because any single trade can. A genuinely decent method still produces five losers in a row from time to time — a later sheet in this region works out exactly how often. Sizing is the decision that determines whether that sequence is an annoyance or an ending, which is why it matters more than the forecast it ignores.

Three inputs — and nothing else

▲Position sizing needs exactly three numbers. Anything else that volunteers itself as an input — conviction, recent results, the urge to win back yesterday — is contamination.

• ▲Equity — the current value of the account, open positions included. Use today's number: not the original deposit, not the best the account ever looked.
• Risk percentage — the fraction of equity one losing trade is allowed to cost. The common professional range is 0.5% to 2%; this survey uses 1% in every worked example.
• Stop distance — the gap, measured in pips, between the entry price and the stop-loss that proves the idea wrong.

The third input is the one beginners resist, because it makes the stop a prerequisite. You cannot size a trade without knowing where it is wrong: no stop, no stop distance, and the formula refuses to run. That refusal is a feature, not a gap. A companion sheet covers where the stop itself belongs; this one assumes you can measure the distance to it.

The formula, worked three times

One mechanical detail first: pip value, what one pip of movement is worth per ▲lot. For pairs quoted in US dollars — EUR/USD, GBP/USD — it is $10 per pip per standard lot, the anchor used across this survey. For other pairs it moves with the exchange rate, which is what the ▲pip value calculator is for. With that number in hand, the formula is one line.

Example one: a $10,000 account, 1% risk, a EUR/USD setup with a 40-pip stop. A quarter lot moves $2.50 per pip, so 40 pips against the position costs $100 — the planned 1% and not a cent more. The symmetry matters just as much: the same 40 pips in your favour earns the same $100 before costs. Sizing does not cap the upside. It calibrates both directions to the account.

Example two: identical account, identical risk, but a GBP/USD idea whose invalidation sits only 25 pips away. The tighter stop permits a larger size — 0.40 lots — and the loss if the stop is hit is still $100, with a $100 gain on the same move in your favour. This is the lesson intuition misses: stop distance and position size trade off automatically. A wider stop does not mean more risk; it means less size at the same risk.

Example three: a $2,500 account, 1% risk ($25), a USD/JPY setup with a 50-pip stop. With the pair near ¥150, one pip per standard lot is worth about $6.70, and the formula prints 0.0746 lots. Round down to 0.07 — always down, never up. Rounding up risks more than the policy allows; rounding down risks slightly less. And when the result lands below your broker's minimum size, the answer is not “trade the minimum anyway”. The answer is that this trade, with this stop, is too large for this account. Skip it, or find a setup whose stop sits closer.

Why 1% survives a losing streak — shown, not asserted

Every method produces losing streaks; the only question sizing answers is what a streak costs. The table assumes ten consecutive losses, with the risk amount recomputed from current equity before each trade — which is how percentage sizing actually behaves in an account.

Read the last column slowly. The 1% account is down 9.6% — unpleasant, and recoverable with the same arithmetic that lost it. The 10% account is down 65% and must now nearly triple just to reach its old level. Same method, same streak, same market: the only difference between those rows is the sizing policy chosen before any of it happened.

This is also why the risk percentage is set once and then left alone. Raising it after wins and raising it after losses are both popular, and both destructive: after wins it builds the largest position of the month right before the streak arrives; after losses it turns a routine drawdown into a deep one. Compared with this single account-level choice, the ▲risk-reward ratio of any individual setup decides very little — that ratio gets its own sheet next.

Notice one quiet property of the table: because the risk amount is recomputed from current equity, the dollar risk shrinks automatically as the account falls — $100, then $90, then $81 — and grows again only as the account recovers. The method applies the brake exactly when the account is weakest, with no judgement required. The opposite scheme — increasing size after losses to win it back faster — does the reverse, and it is how routine streaks become account-ending ones.

The standard objection is that 1% feels slow, and it is true that nothing about it is exciting. But the comparison that matters is not 1% against 10% on a winning week — it is the two policies across the same losing streak, which is the comparison the table just made. The larger number does not produce a faster version of the same journey. It produces a different journey with a meaningful chance of ending early, and an ended account compounds at exactly zero.

The contract with yourself

The framework has one operating rule: the same formula, every trade, no exceptions. Not “except when the setup is obvious” — the obvious ones carry the same uncertainty wearing better clothes, and an exception made for confidence reintroduces exactly the emotional sizing the formula exists to replace. A sizing rule with exceptions is not a rule. It is a mood with paperwork.

The good news is that the contract is cheap to keep, because the framework runs as a fixed pre-trade ritual: stop level first, distance measured, equity checked, formula run, size written down — then, and only then, the entry. Perhaps thirty seconds in total. The order matters as much as the steps: a size calculated after the position is open is not a calculation, it is an alibi. And the habit forms fastest where errors cost nothing.

Check it yourself