Size from the stop, not from conviction

Most beginners decide how much to buy first and figure out where the stop should go afterwards, if at all. Do it the other way round. The stop comes first, the risk budget comes from the account, and the position size is whatever those two numbers produce.

The formula

position size = dollars at risk ÷ (entry price − stop price)

For a long trade, the denominator is entry minus stop. For a short, it is stop minus entry. Either way it is the distance, per share or unit, between where you get in and where you admit you were wrong.

One trade, three stops

Account $10,000, risk 1%, so $100 at risk. Entry at $50.00.

Same account, same risk, same entry. The stop distance alone moved the position from $1,000 to $10,000. That is why "how much should I buy?" has no answer until you know where the stop is.

Note what the tight stop did: it put the entire account into one stock in exchange for the right to be wrong by only 1%. That is legal and often stupid, for a reason the next section covers.

Key idea: Position size is an output, not an input. Choose the stop based on the chart, choose the risk based on the account, and let the arithmetic tell you how much to buy.

Where the stop actually goes

The stop should sit where the trade idea is wrong, not at a round number of dollars. If you are buying a pullback to support at $48.40, the idea is wrong somewhere below $48.40, so a stop at $47.90 makes sense and a stop at $49.50 does not, regardless of what that does to your position size.

If the honest stop is so far away that the resulting position is tiny, that is information: the setup is not good enough relative to its risk, or the market is too volatile for your account right now. Skip it. Do not move the stop closer to make the position bigger; you have just converted a good trade with a small size into a bad trade with a large size.

Cap the position regardless of the stop

The formula assumes your stop fills where you put it. Stocks gap overnight. Futures gap on the open. Crypto and forex slip badly during news. So add a second rule that ignores the stop entirely:

no single position larger than X% of the account

A common cap for a beginner is 20% to 25% of the account in any one stock position, and lower in anything that can gap hard (small caps, earnings, illiquid crypto).

See what it prevents. Take the 200-share, $10,000 position from the table above. The stop is at $49.50, planned loss $100. The company reports bad news overnight and opens at $40.00. The stop fills at $40.00:

loss = 200 × ($50 − $40) = $2,000 = 20% of the account, twenty times the planned risk.

With a 25% cap the position would have been 50 shares. Same gap: 50 × $10 = $500, a 5% loss. Still five times the plan, still painful, but survivable.

Conviction is not a size input

Every trader feels more sure about some trades than others. The temptation is to size the sure ones up. Two problems.

First, most traders' "sure" trades do not win more often than their ordinary ones. If you keep a log (Module 3), you can check this; the usual finding is no difference or a slightly worse result, because conviction often comes from a strong narrative rather than a strong setup.

Second, even if conviction were informative, the streak math does not care. A trade at 5% risk that loses is a 5% loss whether you were 60% sure or 95% sure.

If you want to express conviction, do it inside a narrow band: 0.5% for a marginal setup, 1% for a standard one, never above your plan's maximum. The difference between 0.5% and 1% is meaningful over a year. The difference between 1% and 5% is the difference between a drawdown and a disaster.

Use the calculator

The position size calculator does this arithmetic for stocks, futures, forex and crypto, including contract multipliers and pip values. Use it until the numbers are automatic, then keep using it anyway, because the error you are guarding against is a fat-finger on a tired afternoon, not a lack of understanding.

Try it: Take a chart you would consider trading. Mark the entry and the point where the idea is clearly wrong. Compute the position using $100 of risk. Then compute the position using a stop half as far away and note how the position doubles. Ask which stop you would honestly have used a month ago.

Recap

• Shares or units = dollars at risk ÷ distance from entry to stop.
• The stop goes where the idea is wrong; the size follows from that, never the reverse.
• A tight stop creates a large position; cap any single position at a fixed share of the account to survive gaps.
• Conviction belongs inside a narrow band (0.5% to 1%), not as a multiplier.
• Use the position size calculator every time; the goal is to remove arithmetic errors, not to prove you can do arithmetic.