Conversion calculators
Nine calculators for the math that comes up when you are converting a promo or shopping a line. Nothing here needs an account and nothing here is gated. Type in a box and the answer is already there.
Prediction market price to American odds
Kalshi and Polymarket quote a contract as a price. A sportsbook quotes the same thing as a line. Type either one here and you get the other. Paste it however you have it: 63, 63%, 63c or $0.63 all read the same.
This is the raw conversion with no vig in it. A sportsbook adds its margin on top, so the line a book actually posts on the same event will be worse than this number. That gap is the point: when a book beats it, you have found something.
Odds format converter
American, decimal, fractional and implied probability are four ways of writing the same price. Change any box and the other three follow.
Fractional odds come off a fixed ladder of denominators, so the fraction shown is the closest one a board would actually print rather than an exact ratio with a four digit denominator.
No vig odds and hold
A book prices both sides so the numbers add up to more than 100%. The extra is its cut. Put both sides in and this strips the cut out, leaving what the book really thinks the chances are.
The juice is split evenly across the sides here, which is the standard method and is right for two way markets. On a long field with a heavy longshot it is not, so do not read this as a fair price on a 20 runner futures board.
Expected value
What a bet is worth on average, given what you think the real chance is. A prediction market price is a good source for that chance, which is what makes this pair well with the calculator at the top.
Hedge calculator
You have a live bet and you want the same money whichever way it lands. This sizes the other side. Both legs here are your own cash. If the first leg is a bonus bet, use the bonus bet calculator at the bottom instead, because a bonus bet does not give the stake back.
Arbitrage calculator
When two books disagree enough, you can back both sides and win either way. Put the best price on each side in, plus what you want to put at risk in total, and this splits it.
Prices move while you are placing the second leg, and books limit or void accounts that do this often. Treat the number as the best case, not the expected case.
Parlay odds calculator
Multiply legs into one price. Fill in as many as you have and leave the rest blank.
This treats every leg as independent. Legs from the same game are not, and a book prices a same game parlay on its own correlation model, so the ticket you see there will not match this number.
Kelly stake calculator
How much to put on a bet you think is priced wrong. Bigger edge means bigger bet, and Kelly is the math that says how much bigger.
Full Kelly assumes your chance estimate is exactly right. It almost never is, and being a little wrong at full Kelly is how people go broke while still picking winners. Quarter is the setting most people should stay on.
Bonus bet conversion calculator
A bonus bet is not cash. If it wins, the book keeps the stake and pays only the winnings, so a $200 bonus bet is worth well under $200. This works out the bet on the other side that leaves you the same money either way.
What is your promo actually worth?
A $200 bonus bet is not $200. Put in what you have and what you can cover it at.
The book's money. If it wins you keep the winnings, not the stake.
That is what the math says. Real prices move while you are placing, and an exchange takes a cut on the cover leg, so your number lands close to this rather than exactly on it. Anything at or above 70% on a bonus bet is a good one.
The math is the easy half. Finding a cover price that actually leaves you that much, on a book you can still bet at, in your state, is the part we do for you.
Try it on a real promoWhy a prediction market price is worth converting
Kalshi and Polymarket run as exchanges. People trade against each other and the price settles where the money balances, so the number on the screen is close to what the market really thinks. A sportsbook adds its own margin on top of its number, which is why both sides of a coin flip pay -110 instead of +100.
Convert the exchange price and you have a fair line to hold the book against. When the book is better than the fair line, you are getting the good side of the bet. When it is worse, the difference is the book's cut and you are paying it.
Worth saying plainly: none of these calculators finds you a price. They do the math on prices you already have in front of you. Finding the price is the part we do.