Your spread is a bet. Price it like one.
The chain already told you the odds: every delta is the market's own probability. Type your legs and the price on your ticket, and actualodds hands you the fair value, the dollar EV, and the quarter-Kelly stake before you touch Review & Send.
Free and instant, every number computed in your browser.
Fees & contract math
The math, in plain sight
No vol surface, no Greeks soup. The deltas you already stare at are probabilities, and once you read them that way, pricing a spread takes one multiply.
Delta is a probability
A 0.05-delta call has about a 5% chance of expiring in the money. The market printed that number for you, next to the bid. Read it off the chain, and if you think the market has it wrong, type your own number over it.
The move: treat the chain as a devigged line you didn't have to build.
Fair value is one multiply
Average a spread's two deltas and multiply by the width. That's the credit at which the trade is dead even. (0.05 + 0.04)/2 × 5 = 0.225. A condor is two spreads, so price each side and add.
The move: every cent collected past fair is edge; every cent short of it is juice you paid.
EV$, then EV%
EV$ is your price minus fair, times 100, minus fees. EV% divides that by the max loss the broker locks up, because that's your stake. Risking $456 to make $44 is laying −1036, and the calculator shows you the fair line next to it.
The move: judge a spread the way you judge a bet, edge over stake.
Kelly, on the real payoff
Kelly runs on the actual payoff curve, partial losses between the strikes included, and finds the growth-optimal fraction of your roll. Full Kelly trusts your inputs completely; deltas don't deserve that trust, so the default stakes a quarter.
The move: size first, then decide. A +EV trade at the wrong size is still a losing strategy.
Questions traders actually ask
Is delta really the probability of expiring in the money?
Close enough to trade on, with a known lean. The exact ITM probability is N(d2) while delta is N(d1), and the gap between them grows with IV and time. For calls, delta runs a touch above the true probability; for puts, |delta| runs a touch below it, so put sellers are the ones the shortcut flatters. On a 3-day 0.04-delta put the difference is under a point; on a high-IV monthly it's real. If your platform shows the true ITM probability, type that instead of delta and the math is exact.
Why is EV% divided by max loss?
Because max loss is your stake. It's the buying power the broker locks up the moment you're filled, the same way a sportsbook holds your wager. A bettor quotes edge as EV over the amount risked, and options deserve the same honesty: +$21 on $456 at risk is +4.7%, whatever the credit was.
Where are theta, vega, and days to expiration?
Expiration math doesn't need them. This tool prices the payoff at expiration against the probabilities buried in the deltas, and no Greek changes that number. The Greeks describe the ride between now and then, which matters for management and for closing early, and that's a different question than whether the price you're getting beats fair.
Can I price naked calls, naked puts, and strangles?
Yes. Add the legs and the math adapts. A short put's max loss is real and exact (the stock going to zero, the same number a cash-secured put ties up), so EV%, breakevens, and Kelly all still work. A net short call side has no max loss at all: EV$ still computes, but EV% and Kelly go blank because there's no honest denominator to divide by. And on any open side, the tail EV leans on a one-band model that's kinder than real fat tails, so the result carries a warning that says exactly that.
Why quarter Kelly instead of full?
Full Kelly is only optimal when your probabilities are exact, and yours come from deltas with quote noise on top. Overbetting Kelly hurts more than underbetting it. A quarter keeps most of the growth with a fraction of the drawdown, and it's the same default the devig calculator uses for sports bets.
What about earnings trades?
Load the iron butterfly example: it's a real GOOGL earnings-week fly, and the credit lands within pennies of fair value. That's what an efficient market looks like on a liquid name. An edge on earnings means believing the implied move is wrong, so if that's your view, put your own probabilities in the delta boxes and let the calculator price your disagreement.