Polymarket Combo Calculator: Probability, Payout & Quote Comparison
Add Combo legs, measure the chance they all win, see the chance one fails, estimate an independence payout, and compare a manual Polymarket quote honestly.
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It looks like you're trying to calculate a parlay. Would you like mathematically honest help?
CowDog Confidence Rating: 72.90% (He is a dog)
CowDog notes: 90% × 90% × 90% ≠ 270%.
The calculator multiplies leg probabilities to estimate the chance every leg wins if the legs are independent. It also reports the chance at least one fails, an approximate gross payout at that benchmark, and how a manually entered Combo quote differs in probability points.
It does not fetch a live RFQ quote. Phase 0 confirmed that Polymarket's public API exposes Combo-compatible markets, while public requester quote integration remains unavailable in the documented API.
How does the Combo probability calculation work?
For three legs at 90%, 80%, and 70%:
0.90 × 0.80 × 0.70 = 0.504 = 50.4%
The chance at least one fails is:
1 − 0.504 = 0.496 = 49.6%
If the joint chance really were 50.4%, fair decimal odds before costs would be about 1 ÷ 0.504 = 1.984×. A $10 stake corresponds to about $19.84 gross. This is probability arithmetic, not a promise that a market maker will quote 50.4¢.
Why does the actual quote differ?
The Combo arrives through an RFQ auction among connected market makers. A quote can reflect:
- correlation between legs;
- the executable bid/ask of each underlying market rather than a displayed midpoint;
- position size and available liquidity;
- rapid price movement during the quote window;
- the market maker's ability to hedge the exact package;
- spread and protection against adverse selection.
That is why the tool says “above/below the independence benchmark,” not “overpriced,” “underpriced,” “fee,” or “edge.” Those stronger words require a better estimate of true joint probability than multiplying whatever numbers happened to be visible.
What should you enter for each leg?
Use a probability relevant to the side you selected. If YES costs 62¢ at the executable ask, 62% is a more realistic input for a prospective buy than a 59% displayed midpoint you cannot actually purchase.
For a NO leg, use the NO side's relevant price/probability rather than automatically subtracting a rounded YES display from 100. The order book can have spread; executable YES and NO prices are not required to present a frictionless classroom exercise.
For a single prediction-market payout, the Polymarket probability calculator shows shares and pre-fee profit. For category-dependent taker costs, the Polymarket fee calculator uses the current official formula.
What does correlation do to Combo math?
Independence means learning that one leg won does not change the probability of another. Real-world events often refuse to sign that agreement.
If two legs tend to happen together, multiplying their standalone probabilities can understate their joint chance. If they work against each other, multiplication can overstate it. Some combinations can be logically impossible even though both standalone market prices are above zero.
Do not convert difference into confidence
A quote 4 probability points below the independence benchmark is not proof of a bargain. The quote may understand the relationship between legs better than the benchmark does. CowDog notes that a spreadsheet becomes dangerous at precisely the moment it starts flattering its owner.
Why is there no automatic live quote?
Polymarket's official Combo API documentation currently provides an unauthenticated catalog of markets that may serve as legs. The public RFQ specification documents authenticated maker quote actions. The requester flow is described, but requester integration is marked “Coming Soon.”
Showing a number assembled from single-leg prices and calling it live would be technically convenient and editorially false. So the quote box is manual. You bring the executable number Polymarket showed you; the calculator explains the comparison.
Is this the official Polymarket Combo payout?
No. It is an independent calculator. The benchmark assumes independent legs and full execution at the entered values. Your actual RFQ quote controls the trade available to you.
Why does 90% × 90% × 90% equal 72.9%?
Each multiplication preserves only 90% of the probability remaining from the previous leg: 0.9 × 0.9 × 0.9 = 0.729. The chance one or more legs fails is 27.1%.
Can I use midpoint prices as inputs?
You can, but then the result is a midpoint benchmark. It may not describe the prices you can execute, particularly when spreads are wide or size is meaningful.
Does the payout include fees?
No. The result is a probability-derived gross benchmark. It does not model the RFQ package, Predictions category fees, spread, or slippage.
Multiplication is merciless, which is why we hired it. It's just business.
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This article is educational and satirical content from Business Dog. It is not financial, legal, or tax advice. It's just business.