Calculating the Margin Step by Step

The bookmaker's margin (also called overround or vigorish) is the percentage by which the sum of the implied probabilities of all outcomes in a market exceeds 100%. Here is the exact process.

Step 1: Convert Each Set of Odds into Implied Probability

Use the formula that matches the odds format you are given.

Decimal odds (most common internationally):
Implied probability = 1 ÷ decimal odds

American odds:

• Positive odds (e.g. +150): 100 ÷ (odds + 100)
• Negative odds (e.g. –150): |odds| ÷ (|odds| + 100)

Fractional odds (e.g. 5/2):
Implied probability = denominator ÷ (denominator + numerator)

Step 2: Add All Implied Probabilities Together

Sum the implied probabilities of every possible outcome in the market.
For a two-way market there are two numbers; for a three-way market there are three, and so on.

Step 3: Calculate the Margin

Margin (overround) = (Sum of implied probabilities – 1) × 100

Alternatively:
Margin = (Sum of implied probabilities × 100) – 100

The result is expressed as a percentage.

Worked Examples

Example 1 – Two-way moneyline (decimal odds)
Team A: 1.91
Team B: 1.95

• Implied probability Team A = 1 / 1.91 ≈ 0.5236 (52.36%)
• Implied probability Team B = 1 / 1.95 ≈ 0.5128 (51.28%)
• Sum = 0.5236 + 0.5128 = 1.0364
• Margin = (1.0364 – 1) × 100 = 3.64%

Example 2 – Three-way soccer match (decimal odds)
Home: 2.10
Draw: 3.40
Away: 3.60

• Home = 1 / 2.10 ≈ 0.4762
• Draw = 1 / 3.40 ≈ 0.2941
• Away = 1 / 3.60 ≈ 0.2778
• Sum = 0.4762 + 0.2941 + 0.2778 = 1.0481
• Margin = (1.0481 – 1) × 100 = 4.81%

Example 3 – American odds
Team A: –110
Team B: –110

• Implied probability each side = 110 / (110 + 100) = 0.5238
• Sum = 0.5238 + 0.5238 = 1.0476
• Margin = (1.0476 – 1) × 100 = 4.76%

Quick Reference Formulas

• Two outcomes, equal decimal odds d:
  Margin ≈ (2 ÷ d − 1) × 100
• General case:
  Margin = (Σ 1 ÷ oddsᵢ − 1) × 100

Practical Tips

• Always use the exact odds offered (including any small differences between sides).
• For multi-way markets (outrights, player props with many options), include every listed outcome.
• Rounding to two decimal places for probabilities is usually sufficient for practical purposes.
• Lower margin = better value for the bettor. Comparing margins across books is one of the simplest forms of line shopping.

Key Takeaway

Converting odds → summing the implied probabilities → subtracting 1 and converting to a percentage gives you the exact bookmaker margin on any market. Once you can do this quickly, you can instantly judge how competitive a set of odds is and how high a hurdle your edge must clear.