How to use the calculator
- 1
Choose the odds format
Pick American or decimal. Every outcome in one calculation uses the same format.
- 2
Enter the odds for every outcome
Type each outcome's odds, one per row, and add rows for a three-way line or a market with many entrants. The set has to be complete, 2 to 64 outcomes, because the overround is the sum of the whole set.
- 3
Read the fair price
Each row shows the implied probability, the no-vig probability and the fair price in cents. The overround under the table is the margin the set carried.
What each column means
Implied probability is the odds read as a chance, before any margin comes out. -110 is 52.4%, 2.50 is 40%, and +300 is 25%.
Add the implied probabilities of every outcome and a set with no margin sums to 100%. The amount above 100% is the overround, the margin priced into the odds. -110 on both sides of a two-way line sums to 104.8%, an overround of 4.8%.
The no-vig probability divides each implied probability by that total, so the set sums to 100% again. Every outcome gives up the same share of the margin, the proportional method that fair-line calculators use. Removing the margin adds no information: the no-vig number is what the odds say once the pricing edge is taken out, not a forecast of the result.
The fair price is the no-vig probability in cents. A Polymarket contract pays $1 if its outcome happens, so a 60% chance is a fair price of 60¢. That is the number to hold beside the price a market shows.
Odds formats the calculator reads
American odds show the payout against 100. A negative number is the amount that returns 100 in profit: -150 implies 150 / 250, or 60%. A positive number is the profit on 100: +200 implies 100 / 300, or 33.3%. The calculator accepts -100 and below, and +100 and above, out to 100,000 either way.
Decimal odds show the total return per unit, stake included. The implied probability is 1 divided by the odds: 1.91 is 52.4% and 4.00 is 25%. The calculator accepts any decimal above 1.00.
A Polymarket price needs no conversion. 62¢ is a 62% chance, and the Yes and No asks of a liquid market sum to a cent or two above $1: the spread does the job an overround does elsewhere. The guide at 0xinsider.com/learn/why-prediction-market-odds-exceed-100-percent explains where that gap comes from and how to read a price when it is wide.
Reading a market against the fair price
Convert a line for any game 0xinsider tracks, an NFL Sunday or a CS2 series, then open the same game on 0xinsider.com/sports. A market priced under the fair price is cheaper than the line says the outcome is worth; one priced over it is dearer. The gap is a starting point: a line carries the information of the venue that set it, and a Polymarket price carries its spread and the taker fee. The guide at 0xinsider.com/learn/sports-betting-analytics-guide walks through implied probability, hold, expected value and closing line value with the formulas.
The other read on a game is who holds each side. The sports boards show the share of graded wallets on each outcome and how the smart-money split compares with the price, so a fair price and a wallet split can be read together on one page. 0xinsider.com/learn/how-to-find-sharp-money-polymarket-sports covers how that split is built and what it has predicted.
Frequently asked questions
What does a no-vig calculator do?
It reads each outcome's odds as an implied probability, adds them up, and divides each one by the total so the set sums to 100%. The amount removed is the overround, the margin priced into the odds. The result is the fair price each outcome carries once that margin is out.
How do I convert American odds to a probability?
For negative odds, divide the odds by the odds plus 100: -110 is 110 / 210, or 52.4%. For positive odds, divide 100 by the odds plus 100: +250 is 100 / 350, or 28.6%. The calculator does both and then removes the overround across every outcome you enter.
How do I convert decimal odds to a Polymarket price?
Divide 1 by the decimal odds to get the implied probability, then read it in cents: 2.10 is 47.6%, a price of 47.6¢. Paired with 1.80 on the other side, the two sum to 103.2%, and the no-vig price of the 2.10 side is 46.2¢.
What is overround?
Overround is the amount by which the implied probabilities of every outcome sum past 100%. It is the margin built into the odds; sportsbooks also call it hold or vig. On Polymarket the same cost appears as the spread between the best bid and the best ask plus the taker fee, and the Yes and No prices of a liquid market sum close to $1.
Does the calculator handle more than two outcomes?
Yes. Enter 2 to 64 outcomes in one format. A three-way soccer line (home, draw, away) or a tournament winner market with many entrants works the same way: every outcome's implied probability is divided by the total of the whole set.
Is the fair price a prediction?
No. The fair price is the entered odds with the margin removed in proportion. It carries whatever the source of the odds knew and nothing more. 0xinsider's read on a game comes from the graded wallets holding each side, a separate signal shown on the sports boards.
Does the calculator use live Polymarket prices?
No. It converts the odds you enter. To compare with a live market, open the game on the sports boards, where each market's price, volume and smart-money split are shown.
Read next
- Why Prediction Market Odds Can Add Up to More Than 100%
Where the overround comes from on Polymarket and how to normalize a price.
- Sports Betting Analytics: The Numbers That Matter and How to Read Them
Implied probability, hold, expected value and closing line value, with the formulas.
- Implied Probability
The glossary entry: price as a forecast, and the spread and fee inside the number.
- How to Find the Sharp Money on a Polymarket Game, Step by Step
The graded-wallet split the sports boards show beside every market price.