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Position sizing & risk

Spotting a value bet:
when your estimate differs from the odds

Before choosing a stake, check whether an edge exists. A few calculations can make that question more objective.

Simon Stark|Published 20 June 2026|Approx. 6 min read
01

Definition

What an edge means

An edge, often called value, exists when your estimate of the win probability is higher than the probability already implied by the offered odds or price. Without that difference there is no mathematical reason to enter, however safe the opportunity feels.

02

Calculation

The implied probability of odds

For decimal odds, implied probability is simple:

Implied probability = 1 / oddsExample: odds 2.50 → 1 / 2.50 = 40% implied probability

If you estimate the true probability at 48%, you have found a mathematical edge. That edge exists independently of the result of this single opportunity.

03

Expected value

One result proves nothing

One win does not prove an edge and one loss does not disprove it. Expected value asks what the average payout would be if you could repeat the same opportunity many times. A positive value can coexist with individual losses, so record your hit rate across comparable cases rather than judging isolated outcomes.

04

Next step

What to do after finding an edge

A confirmed edge does not tell you how much to stake. The article How much should you stake when you have an edge? and the Kelly criterion cover that decision.

Odds and margin

The displayed probability is not automatically fair

With several possible outcomes, add the implied probabilities of all offered odds. They often exceed 100%; that excess is the market margin.

Market margin = sum of implied probabilities − 100%Example: 52% + 31% + 22% = 105%, a 5 percentage-point margin.

Do not compare your estimate blindly with a rounded percentage. First ask whether the market price needs to be adjusted for margin and whether fees or commissions are included.

Expected value

The edge appears per unit staked

Odds of 2.50 mean a net profit of €1.50 per euro staked if the outcome wins. At a 48% win probability:

Expected value = p × b − q 0.48 × 1.50 − 0.52 = €0.20 expected profit per €1 stakedThis is not a promise for the next attempt. It describes the average of many comparable attempts.

With several outcomes, include every possible result. For an investment that may mean price gain, loss, dividend, fees and different sale dates. Give each outcome a probability and financial result; the logic stays the same.

Documentation

Your own probability needs a method

The greatest uncertainty is not in the formula but in your probability estimate. If you write 48%, explain why: which data, which comparable cases and what would falsify the estimate?

  • Before the result: Record probability, odds and reasoning.
  • After many cases: Compare forecast and actual hit rate.
  • Separate errors: Was the analysis poor or was one outcome simply different?
  • No hindsight correction: Do not adjust the probability afterwards to fit the result.

Only when the edge is documented repeatedly does position sizing become the next useful question.

Uncertainty

An edge is a range, not a magic number

A best estimate of 48% does not mean the true probability is exactly 48%. It may be 45% or 51%. In the 2.50 example, break-even is 40% before costs. At 48% there is an 8 percentage-point margin; at 41% it is much smaller. Fees, taxes or a price error can erase a small edge completely.

Safety margin = own probability − fair break-even probability 48% − 40% = 8 percentage points before further costs and model errorsThe smaller the gap, the more reliable the data and execution must be.

Turn an edge into a position size

Enter win probability and payout ratio, then calculate the suggested stake.

Open the Kelly calculator

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