Sizing from the stop

Two numbers come before size:

1. How much you will accept losing if this idea is wrong. That is a dollar (or percent-of-account) decision you make while you are calm. It should be an amount you can live with, not an amount you hope you will not have to pay.
2. Where the idea is wrong — the invalidation, and the stop just beyond it. That distance is measured in price, and it comes from the chart or the thesis, not from a target profit.

Position size is then the output. In plain language: size equals the risk budget divided by the loss per unit if the stop is hit. If the stop is farther away, the position has to be smaller to keep the same budget. If you refuse to shrink the size, you are no longer keeping the budget. You are changing the risk after the fact.

The habit this prevents

A common failure looks like this: you want a certain number of coins or shares because the idea “feels big.” Then the proper stop is uncomfortably far, so you tighten it to a round number that has nothing to do with the thesis. Or you leave the wide stop and hope. Both moves disconnect size from invalidation.

Do it in the other order and the discomfort shows up early, which is the point. A wide, honest stop with a small size is a valid trade shape. A tight, invented stop with a large size is often a way to keep the size you wanted.

Fees, slippage, and humility

The simple formula assumes you exit at the stop price. In a real market, fees and slippage can make the loss a little larger, and a gap can make it a lot larger. Budget a little room, and never treat the calculated loss as a guaranteed maximum. The next lesson is about accepting that fact, not about pretending you can eliminate it.

This site has a risk calculator if you want to practice the arithmetic. It will not tell you whether an idea is good. It will only turn a budget and a stop distance into a size.

There is no published edge here, and a correctly sized trade can still lose. Sizing is how you stay in the game long enough to have a series, not a way to make a loss feel like a win.