How position sizing works in futures
Position sizing answers one question: given where my stop is, how many contracts can I hold so that a full stop-out costs no more than my risk budget? The budget is a percentage of the account, usually between 0.5% and 2%. The cost of a stop-out per contract is the stop distance converted to dollars, plus the commission you pay to get in and out. Divide the first by the second and round down.
Rounding down is the important part. A result of 2.8 means 2 contracts, not 3. Trading the third contract means risking more than you decided to risk, and the whole point of a sizing rule is that it is decided before the trade, not during it.
Worked example
$50,000 account, 1% risk, so the budget is $500. Trading NQ with a 10 point stop: 10 × $20 = $200 per contract, plus $4 round-turn commission, $204. $500 ÷ $204 = 2.45, so 2 contracts, $408 at risk. On MNQ the same stop costs $20 + $4 = $24 per contract, so 20 contracts, $480 at risk, which is closer to the budget. That is what micros are for.
When the answer is zero
If the calculator says 0 contracts, the stop is too wide for the budget. Three honest fixes: tighten the stop to a level that still makes sense on the chart, lower the risk per trade, or move to the micro contract. The dishonest fix is to trade one contract anyway and tell yourself the stop is "mental." That is how small accounts become smaller accounts.
Slippage and gaps
A stop order is not a guarantee of your price. In fast markets, at the open, and around news, fills can be several ticks worse than the stop level. Build that in by either adding a few ticks to the stop distance you enter here or by using a lower risk percentage. Overnight positions through the 5:00pm ET break and over weekends carry gap risk that no stop protects against; the margin page covers what that costs.