Position sizing: the arithmetic, not the advice
How many contracts follows from three numbers. What those numbers should be is not something a website can tell you.
Position sizing is usually presented as wisdom. Most of it is arithmetic, and separating the two is useful — the arithmetic is the same for everyone, and the inputs are not.
The calculation
If you know how much you are willing to lose on a trade, and how far away the invalidation point sits, size is determined:
For a stock, value per point is one share per point, so it reduces to shares = amount at risk divided by the distance in dollars. For futures, use the contract's tick value.
Nothing in there is a judgement. Given the three inputs, the answer follows.
Where volatility enters
If the invalidation distance is set in ATR terms rather than fixed points, size adjusts automatically as conditions change — a wider stop in volatile conditions means fewer contracts for the same money at risk.
This has a property worth naming: it means your position shrinks exactly when the market gets more violent, without you deciding anything. Whether that behaviour suits you is a preference. The arithmetic just produces it.
The part that is not arithmetic
How much to risk per trade is not a question this article can answer. It depends on your account, your income, your obligations, how many positions you hold at once, what you can tolerate, and what you are actually trying to do. We do not know any of that, and nobody who does not know it should be giving you a number.
What can be said: the number should be chosen before you are in a position, written down, and not revised because a particular trade feels compelling.
Two consequences people miss
Correlated positions are one position. Sizing each of three related positions separately means the combined exposure is three times what the arithmetic suggested. See picking your instruments.
Losses compound against you. Losing a fixed percentage repeatedly requires a larger percentage gain to recover, and the gap widens as the drawdown deepens. This is arithmetic too, and it is the reason risk limits exist at all. See drawdown.
What the chart contributes
One input: the invalidation distance. A chart can show where a read stops being valid. It cannot tell you what fraction of your account belongs on that trade, because it does not know your account exists.