Size Is the Decision

Traders spend their days on entries. They compare charts, read threads and argue about levels. Then, at the moment it matters, they pick a position size by feel.

This is backwards. The market decides whether you are right. Size decides what that verdict costs. It is the one input in the whole trade that is entirely yours.

Start from the loss

The cleanest method is called fixed-fractional sizing. You begin with what you are willing to lose, not what you hope to make.

Say your book is $100,000 and you decide to risk 1% of it on a trade, which is $1,000. Your analysis puts the stop 5% below your entry. The position size follows:

Position size = amount at risk ÷ distance to stop

$1,000 ÷ 5% = $20,000.

If the stop were 10% away, the position would be $10,000. A wider stop means a smaller position, and the amount at risk stays the same. The method does the thinking for you, and it behaves the same on a good day and a bad one.

What leverage actually changes

Leverage is widely misunderstood. On a $20,000 position with a 5% stop, you risk $1,000 whether you post $20,000 of margin or $4,000. The leverage does not change the risk to the stop. The position size does.

What leverage changes is how close liquidation sits. Roughly speaking, at 10x an adverse move of a little under 10% can end the position. At 25x, it is under 4%. So the real danger is that high leverage tempts you to use the freed-up capital for a bigger position, and that gaps or wicks can reach your liquidation price before your stop can act.

Survive the streak

Losing streaks are not a sign that something has gone wrong. They are normal. Over a thousand trades with a coin-flip hit rate, the odds of ten straight losses at some point are better than one in three.

Here is what that streak does at different sizes:

Risk per trade	Book after 10 losses	Gain needed to recover
1%	-9.6%	+10.6%
2%	-18.3%	+22.4%
5%	-40.1%	+66.9%
10%	-65.1%	+186.8%

The same run of bad luck is a bruise at 1% and a career event at 10%. Your method did not change. Only your size did.

A word on Kelly

The Kelly criterion gives the theoretically optimal fraction of capital to stake when you know your edge. For a bet that pays even money, it is 2p - 1, where p is your probability of winning. At p = 0.55, Kelly says to stake 10%.

There are two reasons to treat this as a ceiling, not a target. First, you do not know p. You have an estimate, and if your true hit rate is 0.50 rather than 0.55, the correct size is zero. Second, full Kelly brings brutal drawdowns even when it is correct. Betting half of it gives you about three-quarters of the growth with half of the volatility, and it leaves room for being wrong about your edge. Many professionals use a quarter or less.

The discipline

Write down your risk per trade. Write down the most you will have at risk across the whole book at one time, remembering that correlated positions count as one. Then let those numbers choose your size, every time, without negotiation.

The conviction you feel about a trade is information about you, and only sometimes about the trade. Size is where the two should be kept apart.