Before opening a Bull Put Spread, there’s a question that matters more than how much premium you’re going to collect: what’s your real probability of keeping it? The answer isn’t in your gut — it’s a number you can calculate precisely before moving a single dollar. It’s called probability of profit, or POP.
What POP is and where it comes from
POP (Probability of Profit) estimates, using current market data, the likelihood that the trade ends in profit at expiration. In a Bull Put Spread, you sell a put and buy a lower one for protection — you collect the premium now, and you keep it if the price stays above the short strike at expiration.
The key piece of the calculation is the delta of the strike you sell. Delta isn’t just “price sensitivity” — it also works as a direct approximation of the probability that option ends up in the money.
A put sold at delta 0.20 has, roughly, an 80% probability of expiring worthless (in your favor). It’s not an exact rule — it also depends on the skew of the price distribution — but it’s the starting intuition every options seller uses.
The problem with calculating it by hand
Exact POP isn’t simply “1 minus delta.” It depends on the current price, both strikes, days to expiration, and implied volatility — four variables that interact with each other in a non-linear way. Doing it by hand, spread by spread, is slow and error-prone.
An example with the calculator
Picture a stock trading at €100. You sell the 95 put and buy the 90 put for protection, both at 30 days. With 25% implied volatility, here’s what you see instantly in our free options calculator:
What you see instantly, without calculating anything by hand
Probability of profit (POP) for the full spread
Net credit you collect when opening the trade
Maximum loss if the price falls below the long strike
Exact break-even: the price below which you start losing
You can move the expiration or the implied volatility and watch POP and break-even change in real time — exactly what you need to compare before deciding on a strike, without opening a spreadsheet.
Why this matters more than it seems
A Bull Put Spread with a 90% POP and a small premium can be mathematically worse than one with a 75% POP and a bigger premium, depending on how much you risk for every dollar you can make. POP alone isn’t the whole story — you need to look at it together with the risk/reward ratio — but it’s the first filter, and the fastest one to calculate with a tool.
This is, in fact, the base structure we teach in the ECO Workshop: define your risk before opening the trade, not after.
Conclusion
POP turns a decision many people make “by feel” into a concrete number, calculable before risking a dollar. In a Bull Put Spread, cross-referencing delta, credit, and break-even by hand is tedious and error-prone; with the options calculator you have it instantly, letting you compare different strikes in seconds and choose with data, not intuition.
