The formula
shares = (account × risk%) ÷ |entry − stop|
A $25,000 account risking 1% has $250 to lose. With an entry at $48.50 and a stop at $46.00, the risk per share is $2.50, so the position is 100 shares — $4,850 of stock to risk $250.
Two things follow from that example, and both are specific to equities.
Whole shares, and why we round down
Most brokers trade whole shares. A calculation returning 103.7 has to become 103 or 104, and the direction matters more than it looks.
Rounding down keeps the actual risk below the limit. Rounding up — including the very natural urge to make it "a nice round 110" — puts it above, every time, on every trade. Individually it is a rounding error; across a year of trades it is a risk rule that was never actually enforced.
The calculator rounds down and shows the resulting risk on its own line, so the number you are actually taking is visible rather than assumed.
Fractional-share brokers remove this constraint entirely, which is genuinely useful for small accounts — it is often the only way to take a wide-stop position at a 1% risk rule without the size collapsing to zero.
The constraint that does not exist in forex or crypto
A cash account cannot buy more stock than it has money.
There is no leverage by default, so the position size is capped twice: once by your risk rule, and once by the cash available. On a low-priced share with a tight stop, the risk rule can happily return a position worth more than the whole account.
When that happens, the calculator says so rather than returning an impossible number. The resolutions are a margin account, which means borrowing to hold a position that was sized for a much smaller risk, or accepting a smaller position and a risk below target. The third option — widening the stop until the size fits — moves the exit to where the arithmetic is comfortable rather than where the idea is wrong, and that is the one to avoid.
Gaps, and what the stop does not protect
A stock stop is a resting order, and overnight the market does not exist to fill it. Earnings, guidance and sector news are priced in at the open, and a stock that closed at $46.50 can open at $41.
This means the risk figure here is the planned loss, not a guaranteed one — a distinction that matters far more in equities than in continuously traded markets. Anyone holding through an earnings date is accepting a gap risk the position size did not account for, and the usual answers are a smaller position into the event or no position at all.