How to compute it
Take a two-way market at 1.90/1.90. Implied probabilities: 1/1.90 + 1/1.90 = 52.6% + 52.6% = 105.3%. The margin is about 5.3% — that is what the book keeps on balanced action regardless of the result.
Why it matters
The margin is the price of every bet you place. At 5% margin a coin-flip outcome is paid 1.90 instead of the fair 2.00, so a bettor with no edge loses the margin on average. Sharp books such as Pinnacle run low margins; retail books run noticeably higher ones, especially on 1X2.
Removing the margin (devig)
To recover the fair probability, divide each implied probability by their sum: 52.6% / 105.3% = 50%. This «devig» step is the starting point of any value calculation. The margin calculator does it for any 2- or 3-way market.