Kelly Criterion Betting: How Much to Stake on a Value Bet

Finding a value bet is half the job. The other half is the stake, and that's where a real edge is won or thrown away. The Kelly criterion turns a bet's edge into a stake, and OddsFantasy gives you both numbers it needs on every row of the value bet finder.

This guide takes one real value bet, Norwich City to beat Watford on Kalshi, listed by the OddsFantasy +EV finder on 30 October 2026, and sizes it step by step in the free Kelly calculator.

What is the Kelly criterion?

The Kelly criterion is a staking formula published by John Kelly in 1956. It tells you what share of your bankroll, the money you've set aside for betting, to put on a bet with positive expected value. Written with the numbers OddsFantasy shows:

• Full Kelly fraction = EV ÷ (decimal odds − 1)
• Stake = bankroll × Kelly multiplier × EV ÷ (decimal odds − 1)

It stakes more when the edge is bigger, and less when the odds are longer, because long odds lose more often. With no edge, its answer is not to bet.

The two numbers Kelly needs

• EV %: the average profit per dollar staked over many bets like this one. An EV of 2% means each dollar is expected to make about 2 cents on average.
• Decimal odds: what one dollar returns if the bet wins, stake included. On Kalshi and Polymarket a winning share pays $1, so the odds are $1 ÷ the share price: a 50-cent share is odds of 2.00.

The OddsFantasy +EV finder shows both on every value bet: the EV % on the left, the odds on the right. On Kalshi and Polymarket, the odds it lists already have the taker fee taken out.

Worked example: Norwich City v Watford

On 30 October 2026 the finder listed Norwich City to beat Watford, a match in the Championship (the second division of English football), at:

• odds of 1.596 on Kalshi
• an EV of 1.59%
• fair odds of 1.571, from Pinnacle's prices with the margin removed (1.596 ÷ 1.571 − 1 = 1.59%)

Sized on a $1,000 bankroll:

• 1 (full Kelly): $26.68 · 2.67%
• 0.25 (quarter Kelly): $6.67 · 0.67%

Both stakes are the Kelly calculator's own outputs.

How to size a bet with the Kelly calculator

1. Find the bet. Open +EV in the OddsFantasy menu and pick a value bet. Read its EV % (1.59) and odds (1.596).
2. Check where the EV comes from. Click the row: a table opens with every bookmaker's price, and OddsFantasy's fair odds on the top row (1.571). The EV is the gap between the odds on offer and those fair odds.
3. Fill in the calculator. Open **Calcs → Kelly calculator** and type Expected Value 1.59, Odds 1.596, Kelly Multiplier 0.25 and Bankroll 1000.
4. Read your stake. Press Calculate Wager. The Amount to Wager is $6.67.

If you want to check a fair price by hand first, the devig guide shows how the fair odds are worked out.

Mistake 1: staking full Kelly

With the multiplier at 1, the same bet's stake is $26.68, four times as much. Full Kelly is only right when the EV is exact, and an EV is always an estimate. If the real edge is half your estimate, full Kelly stakes double the right amount, and in Kelly's own model staking double the right amount stops the bankroll growing at all, even with a real edge. Quarter Kelly gives up some growth for far smaller swings, and it leaves room for the estimate to be wrong.

Mistake 2: typing the price instead of the odds

The calculator's Odds box wants decimal odds. This bet cost about 63 cents a share on Kalshi, fee included, and typing 0.63 gets "Please enter valid values for all fields.", because decimal odds are always above 1. Convert the price first ($1 ÷ price) or copy the odds straight from the finder.

When Kelly needs a smaller stake

• Several bets at once. Kelly sizes one bet at a time. Bets open together draw on the same bankroll, so use a lower multiplier when many are running.
• A shaky estimate. The less sure you are of the fair odds, the smaller the multiplier.
• A losing run. Kelly doesn't stop bets losing; it controls how much a run of losses costs. Re-size from the bankroll you have now, not the one you started with.

For the other staking plans and how they compare, see bankroll and stake management. To place value bets on Polymarket from one screen, use the OddsFantasy trading terminal.

Trade responsibly

Only stake what you can afford to lose, and check that Kalshi or Polymarket is available where you live.