Vertical Spreads as Prices on Probabilities: Reading Credit Against Width

Most traders look at a vertical spread and see two numbers: what they collect and what they can lose. There is a third number hiding in plain sight, and it is the one that tells you whether the trade makes sense. Divide the spread's price by its width and you get a rough estimate of the probability the market is charging for. Once you read verticals that way, credit spreads stop looking like income and debit spreads stop looking like lottery tickets. Both become what they are: a price on a probability.

The rules first

Price divided by width is the market's rough probability that the spread pays out in full. For a credit spread, that is the chance you take the maximum loss; for a debit spread, the chance you collect the maximum gain.
Your break-even win rate is max loss divided by width. If you risk 3.75 on a 5-point spread, you need to win at least about 75% of the time just to break even.
Credit or debit is a label, not an edge. A credit put spread and a debit call spread at the same strikes and expiration have nearly the same payoff. Choose between them on execution, carry and assignment, not on how the cash feels.
Size by the max loss, every time. The width minus the credit, or the debit paid, is the number you must be able to accept in full.
You only have an edge if your probability differs from the market's. If you cannot say why, you are paying the spread and the commission for a fair coin.

Why price over width works

Picture a very narrow vertical. At expiration it is worth either nothing or its full width, with only a sliver of prices in between. That makes it behave like a bet that pays the width if the stock finishes past the strikes and nothing if it does not. A fair price for that bet is the width times the probability of winning it. Turn that around and price divided by width is the probability the market is pricing in.

Real spreads are a few points wide, so some outcomes land between the strikes and pay part of the width. The ratio is an approximation, not an exact figure. It also uses the market's risk-neutral odds, which build in skew and the premium sellers demand for taking risk, so it is not a forecast. It is still the most useful single number you can put on a vertical before you trade it.

Worked example: a credit put spread

Take a stock at 100, with every number here an illustrative example, not a quote. Sell the 95 put and buy the 90 put, same expiration, for a credit of 1.25. The spread is 5 points wide.

Maximum gain: the 1.25 credit, if the stock finishes above 95.
Maximum loss: 5.00 minus 1.25 = 3.75, if the stock finishes below 90.
Breakeven at expiration: 95 minus 1.25 = 93.75.
Price over width: 1.25 / 5.00 = 25%. The market is charging as if there is roughly a one-in-four chance of a full loss.
Break-even win rate: 3.75 / 5.00 = 75%. Win three times for every loss and you are roughly flat before costs.

Now move the short strike farther out. Sell the 90 put and buy the 85 put for 0.50. Price over width drops to 10%, so the trade wins more often. But the max loss rises to 4.50 against a 0.50 gain, and the break-even win rate climbs to 90%. You need nine wins to pay for each full loss. Neither trade is better by default. The farther strike is not safer; it trades a smaller chance of losing for a much larger loss when it happens.

Worked example: the same strikes as a debit

Now buy the 90 call and sell the 95 call on the same stock and expiration. This debit call spread pays 5.00 if the stock finishes above 95 and nothing below 90, the same shape as the credit put spread above. Ignoring interest and dividends, put-call parity says the two should be priced so the payoffs line up: if the put spread collects 1.25, the call spread should cost about 5.00 minus 1.25 = 3.75.

Credit put spread: collect 1.25, risk 3.75.
Debit call spread: pay 3.75, can make 1.25.

Same risk, same reward, same probability. In practice small differences come from interest on the cash, dividends before expiration, early assignment on American-style options, and the bid-ask spread on each leg. Those are real reasons to prefer one version on a given day. "I like getting paid up front" is not one of them.

When reading the ratio helps most

Comparing strikes. Line up several spreads and write each one's price over width next to it. You will see quickly where you are paying up for safety and where the credit barely covers the risk.
Checking a view. If you think a drop below 95 has a 10% chance and the market prices it at 25%, selling the spread is consistent with your view. If you think it is 30%, it is not, however attractive the credit looks.
Around events. Before earnings or a major data release, the ratio usually rises because larger moves are priced in. Higher credits then reflect higher odds of loss, not free money.
Debit spreads. A 1.50 debit on a 5-point call spread is a 30% price. Buy it only if you believe the chance of finishing past the upper strike is meaningfully better than that.

Common mistakes

Treating a high win rate as an edge. A spread that wins 90% of the time at a 90% break-even win rate is a fair bet, not a good one. Costs then make it slightly negative.
Reading the short strike's delta as an exact probability. Delta is a handy rough guide to the chance of finishing in the money, but it is not the same thing, and it ignores the long leg. Price over width uses the whole spread.
Sizing by the credit. The credit is the best case. Size by width minus credit, which is what a bad week actually costs.
Ignoring the middle. Between the strikes, losses build point by point. Decide in advance where you close or roll rather than waiting to see if price comes back.
Forgetting assignment. On American-style equity options, a short leg that goes deep in the money can be assigned early, especially before a dividend. European-style cash-settled index options avoid that particular risk.

A short pre-trade checklist

Write down the width, the price, the max gain and the max loss.
Calculate price over width and say in one sentence whether your own estimate of the odds is higher or lower, and why.
Calculate the break-even win rate and compare it with how often trades like this have actually worked for you.
Size the position so the full max loss is an amount you accept.
Check the calendar for earnings, dividends and macro events before expiration, and set your exit before you place the order.

Reading credit against width, choosing between credit and debit, sizing and knowing when to pass are covered in depth in Trading Verticals.