Model and input conventions
The signed intrinsic value is max(upper strike − terminal price, 0) minus max(lower strike − terminal price, 0). Premium paid equals the long-put premium less the short-put receipt. Quantity scales both legs equally in underlying units; the fixed fee is then deducted once. The lower strike must be strictly below the upper strike. No stock holding is included in this strategy.
Worked numerical example
Buy a 100-strike put for 10 and sell an 80-strike put for 3, using one underlying unit and no fees. The net debit is 7. A terminal price of 93 breaks even. At 80 or below the signed option payoff is 20 and total P&L is 13. At 100 or above both intrinsic values are zero and the entire 7 debit is lost.
Read the scenario table
Read across rows around each strike to locate where the payoff slope changes. Below the short-put strike, gains on the long put are offset by growing obligations on the short put. Increase fixed costs to see the break-even move lower; sufficiently high costs can eliminate any nonnegative break-even price. The table exports actual arithmetic values rather than sampling a supposed probability distribution.
Where the model stops
The results describe expiry values, not interim option marks. Before expiry, time value and volatility can change the spread’s price independently of the selected terminal scenario. European-style cash settlement is assumed; American exercise, assignment financing and unbalanced fills are omitted. A loss bound in the payoff diagram does not calculate the cash or margin a venue may require while the position remains open.