yes, your combo probably sucks

I am a professional predictions markets analyst/trader, and I often hop on the r/predictionmarkets subreddit (first mistake) to see if I can help out.

The most common thing I see is people hoping to win big on a long parlay where they can put down 10 dollars and hypothetically win back 100x. I cannot explain how bad of a decision this is, but alas, here we are… So I am going to attempt to.

It’s always the same shape: five or six legs, ten bucks down, a screenshot of the payout with too many zeros, and a comment section hyping them up to make the bet.

Combo/Parlay Calculator

Parlays have two problems. One, it only takes one leg going wrong and you lose everything, so you’re actually better off just winning a single market and rolling that money into the next one. Two, you’re probably getting fucked on the price anyway. A sportsbook, a market maker, someone like me, whoever’s on the other side of that combo, prices it below the true joint probability. That gap is their edge, not yours.

So here’s a cheap way to check the math before you take the combo deal on Kalshi or Polymarket. Chain your probabilities together below and it spits out two numbers: your honest joint probability, and the fair payout that number is actually worth. Compare that against what the platform’s offering. If theirs is lower, that’s the house’s cut, and now you can see exactly how big it is.

Change any value. The result never climbs above the smallest factor, because a joint probability is a chain: each event carves a slice out of what the ones before it left standing. Every factor you add drags the total down, usually by more than it feels like it should.

Independent, or does it need the last one first?

The only judgment call is whether each event stands on its own or depends on the ones before it.

• On its own (independent). The event doesn’t care what happened earlier, so you enter its plain probability. Two fair coins: P(heads) · P(heads) = 0.5 · 0.5 = 0.25.
• Given the prior (conditional). The event only makes sense because the earlier ones happened, so you enter P(this | that): what it’s worth once the chain has already gotten this far. You can only pass the interview if you got the callback.

Why the answer feels too low

It usually isn’t. Your gut wants to average the steps instead of multiplying them, so three plausible-sounding 50/50s feel like “still about half,” not 12.5%, one in eight. Stack five “probably” (70%) steps the same way and you’re under a coin flip before you’ve noticed. The napkin’s job is to make that shrinkage visible before you’ve talked yourself into a number, so a plan that needs six things to go right gets priced like one, not like six separate maybes.

Don’t do this

Seriously, don’t parlay, or bet combos. That’s how you get screwed. If you’re going to anyway, at least run the numbers first so you’re not getting screwed twice: once by the odds, once by your own math. And please don’t gamble more than you’re willing to lose.

Next time I see that screenshot on r/predictionmarkets, this is what I’m linking. You should too. Consider linking this your one good deed for this year. Help your friend save some dollars And build the chain before you tap place bet.