Tools
Betting calculators: odds converter, margin, arbitrage, EV and Kelly
Five small tools that run in your browser and send nothing anywhere. Each shows its formula, so you can copy it into your own code, and names the API field that gives you the number in a live feed. For removing a bookmaker's margin four different ways, see the fair odds page.
1
Odds converter
Type a price in any one box and the other three fill in. Decimal odds are the payout per dollar staked, including the stake. Fractional odds are profit to stake. American odds show how much a $100 stake wins (plus) or how much you stake to win $100 (minus). Implied probability is the chance the price would be fair at.
implied % = 1 ÷ decimal × 100
In the API: every quote carries decimal_odds and its implied_probability, so the other formats are one conversion away.
A $100 stake at 2.50 returns $250, a profit of $150.
2
Bookmaker margin
Enter every price in one market from one bookmaker. Their implied chances add to more than 100%; the excess is the bookmaker's margin, also called the overround, vig or juice. Lower is better for you.
margin = (1/odds₁ + 1/odds₂ + …) − 1
In the API: every outcome carries its implied_probability, so a book's overround on a market is the sum across its outcomes.
The book keeps about 5.0 cents of every dollar staked across both sides.
3
Arbitrage stake splitter
Take the best price on each outcome, from different bookmakers, and a total stake. If the implied chances add to under 100%, there is a locked-in return and the tool splits the stake so every outcome pays the same amount.
stakeᵢ = total × (1/oddsᵢ) ÷ Σ(1/odds)
In the API: the best price per side is the highest decimal_odds on that outcome ID across the bookmakers, with available: true. The arbitrage page has a scanner.
| Outcome | Price | Stake | Pays |
|---|
4
Expected value
A bet has positive expected value when its price is longer than the true chance deserves. Give it a fair price (from a sharp book or an exchange with the margin removed) and the price you are offered, and it returns the edge per dollar and over a stake.
EV = (1 ÷ fair) × price − 1
In the API: Pinnacle or the Betfair Exchange with the margin removed makes the fair price; the +EV page has a scanner.
5
Kelly stake
The Kelly criterion sizes a bet from your edge and the price so the bankroll grows fastest over many bets. Full Kelly swings hard, so most people stake a fraction of it; a quarter to a half is common.
kelly = (p × (odds − 1) − (1 − p)) ÷ (odds − 1)
p is the fair chance (1 ÷ fair price). A negative result means no bet.
Questions
About these tools
Do these calculators send my numbers anywhere?
No. They are a few lines of JavaScript that run in your browser. Nothing is stored and nothing is sent.
Where do I get a fair price for the EV and Kelly tools?
From a sharp bookmaker or an exchange, with the margin removed; the fair odds page shows four ways. In the API, Pinnacle (Pro) and the Betfair Exchange (Rookie and up) come back beside every Australian book, ready for it.
Why does the arbitrage tool say there is no arbitrage?
Because the implied chances of the prices you entered add to 100% or more. A true arbitrage needs the best prices from different bookmakers to add to less than 100%, and they rarely sit there for long.
Is Kelly safe to use?
Full Kelly assumes your fair price is exactly right, which it never is, and it produces large swings. Most people use a fraction and treat the number as a ceiling, not a target. None of this is advice; it is arithmetic.
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