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Odds converter and bookmaker margin calculator

Enter decimal odds for each outcome of a market to compare formats and see how much margin the bookmaker builds in.

Set to 0 for a two-way market (tennis, over/under).

Bookmaker margin (overround)
4.81%
Sum of implied probabilities minus 100%. Lower is better for bettors: sharp books run 2–4%, many soft books 5–8%.
Outcome 1 (home)
47.6% implied
Fractional 11/10 · American +110 · fair odds without margin 2.20
Outcome 2 (draw)
29.4% implied
Fractional 12/5 · American +240 · fair odds without margin 3.56
Outcome 3 (away)
27.8% implied
Fractional 13/5 · American +260 · fair odds without margin 3.77

Implied probability

Decimal odds of 2.50 mean an implied probability of 1 ÷ 2.50 = 40%. Add up the implied probabilities of all outcomes in a market and you get more than 100% — the excess is the bookmaker’s margin, also called overround or vig.

Fair odds

Removing the margin proportionally gives “fair” odds: what the price would be if the bookmaker took nothing. Comparing fair odds across books shows who prices a market more sharply.

FAQ

How do I convert decimal odds to American odds?
For decimal odds of 2.00 or more: (decimal − 1) × 100, with a plus sign. Below 2.00: −100 ÷ (decimal − 1).
What is a normal bookmaker margin?
Sharp, high-limit books often run 2–4% on main markets; recreational books and niche markets commonly carry 5–10% or more.
Why does margin matter for operators?
Margin is the operator’s theoretical hold on a market. Too high and players move to competitors or odds comparison sites; too low and trading risk grows.

Estimates for planning only, not financial or legal advice. Card-scheme thresholds as publicly described in November 2026; your acquirer’s contract and the schemes’ current rules prevail.

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Odds converter: decimal, fractional, American odds and bookmaker margin