Position sizing translates a risk plan into a share count. It does not make a setup good; it limits how much the planned loss would be if the invalidation level is reached under the assumed fill.
The Core Calculation
Start with three inputs:
- Planned entry: the price used for the calculation, not a guaranteed fill.
- Invalidation: the level where the original trade thesis no longer applies.
- Loss budget: the maximum planned loss for this idea within the broader daily and portfolio limits.
Per-share risk = absolute difference between planned entry and invalidation.
Initial share count = loss budget divided by per-share risk.
For an illustrative example, a $75 loss budget and $0.50 of per-share risk produce an initial calculation of 150 shares. That is only a starting point. Round down and apply the constraints below before treating the number as feasible.
Constraints the Formula Does Not Capture
- Slippage and gaps: a stop order does not guarantee execution at the stop price.
- Spread and liquidity: thin trading can make the assumed entry or exit unrealistic.
- Buying power: notional position value must remain inside account and broker limits.
- Portfolio exposure: several positions in the same sector can behave like one larger risk.
- Event risk: earnings, economic releases, halts, and overnight holds can change the loss distribution.
- Short-sale mechanics: borrow fees, recalls, squeezes, and gap risk require separate controls.
Use the Invalidation to Size the Trade
Choosing a share count first and moving the stop until the loss fits reverses the process. The invalidation should come from the setup definition. If that distance creates an impractical position, reduce the loss budget, wait for a different structure, or skip the idea.
Plan at the Portfolio Level
A single-trade budget can still be too large when combined with open positions. Review total planned loss, correlated symbols, sector concentration, and any shared catalyst. Daily loss limits are operational boundaries, not predictions about how much the market can move.
Review Planned Risk Versus Realized Loss
After the trade, compare the planned entry, invalidation, and loss budget with actual fills and realized results. Repeated differences can point to spread assumptions, late entries, gaps, partial exits, or data-quality problems. Do not hide those differences by rewriting the original plan.
How MAC Terminal Guardrails Helps
Guardrails accepts manual account, entry, stop, and risk inputs and calculates a planned position size. It also helps record daily risk boundaries. MAC Terminal is not connected to a brokerage for order enforcement, so it cannot prevent an oversized order, guarantee a stop fill, or liquidate a position. Verify the calculation and current broker constraints before acting.
Educational example only, not individualized financial advice. Choose risk limits that reflect your circumstances and remember that actual losses can exceed a planned stop.