Calculate a position size
For a contract that pays $1 if your selected outcome wins and $0 otherwise. Enter your own probability and the price of that same outcome. Results update as you type.
Selected outcome: Yes. Use your probability for this same outcome.
- Selected bankroll allocation
- 5%
- Position budget
- $50.00
- Whole contracts
- 100
- Stake including entry fee
- $50.00
- Net profit if correct
- $50.00
- Maximum loss
- $50.00
- Full Kelly allocation
- 20%
- Break-even probability
- 50%
| Assumption | Estimated result |
|---|---|
| 55% probability | 2.5% of bankroll ($25.00) |
| 60% probability | 5% of bankroll ($50.00) |
| 65% probability | 7.5% of bankroll ($75.00) |
Whole-contract quantity rounds down. Fee rounding at the venue can change the actual cost. The estimate assumes holding to a $1/$0 settlement; exit costs, correlated positions, taxes and model error are not included. Your probability estimate can be wrong, and full Kelly can produce large losses.
Use the calculator above to compare a probability estimate with the all-in cost of a contract that pays $1 if correct and $0 otherwise. It returns a position budget and a whole-contract quantity. It does not place an order or access a trading account.
How the calculation works
Let p be your probability estimate as a decimal and c the entry price plus entry fee, in dollars per contract. The net odds per dollar risked are b = (1 - c) / c. The full Kelly fraction of bankroll is f = (p - c) / (1 - c), clamped at zero when the estimated edge is not positive. Kelly's criterion maximizes expected logarithmic growth under its betting assumptions.[1]
Half Kelly uses half that fraction; quarter Kelly uses one quarter. With a hypothetical $1,000 bankroll, 60% probability and a 50-cent cost, full Kelly allocates $200 and quarter Kelly allocates $50. At that price, the quarter-Kelly budget buys 100 whole contracts. If the outcome loses, the $50 is lost.
The calculator includes the entered entry fee in both cost and maximum loss. It assumes no additional settlement charge and rounds quantity down. Confirm the final cost in the provider's order preview, because real fee rounding or quantity rules may differ.
Interpret the result
The probability is your estimate, not a fact verified by this tool. Full Kelly can allocate a large portion of the bankroll and cause substantial drawdowns. A fractional allocation reduces the model's exposure but does not correct a bad forecast. Correlated positions, withdrawal delays, exit fees and personal cash needs require separate consideration.
Read position sizing with Kelly for the derivation and a sensitivity example, or portfolio construction to examine related exposures.
References
- J. L. Kelly Jr. A New Interpretation of Information Rate. Bell System Technical Journal 35(4), 917-926, July 1956. DOI: 10.1002/j.1538-7305.1956.tb03809.x.