How the dropping odds calculator works
- Odds drop: how much the reference price fell, as a percentage of the old price.
- Implied probability, before → after: what each price implies (1 ÷ odds). The shift shows how much the market's opinion moved.
- Fair odds now: the reference price after removing the margin using both sides of the line (power method).
- Edge of your bookmaker's price: your odds ÷ the fair odds − 1. Positive means your bookmaker is paying more than the bet is worth.
Drop % = 1 − odds after ÷ odds before
Edge = your odds ÷ fair odds now − 1
Example: a tennis match
The reference price for Player A falls from 2.40 to 2.05, a 14.6% drop. With Player B at 1.85, the margin-free fair odds for A are 2.113, which means the market now gives A about 47.3% instead of 39.4%. Bookmakers that haven't moved yet are worth very different amounts:
| Your bookmaker's price on A | Edge against fair 2.113 |
|---|---|
| 2.40 (hasn't moved) | +13.6% |
| 2.30 | +8.9% |
| 2.20 | +4.1% |
| 2.10 | −0.6% |
| 2.05 (same as the reference) | −3.0% |
This is the whole logic of dropping odds: the value sits with bookmakers that haven't caught up, and it fades as they do. At 2.10 there is already no value, and betting the dropped price itself (2.05) is a losing bet.
What size of drop matters?
- Small moves (a few percent) are the market breathing; they rarely mean anything.
- Moves of around 10% or more at a high-limit bookmaker are usually informative: news, line-ups or professional money.
- Very large moves can be real news, but also a pricing error or a thin market. Check the market's limit and the other lines before you trust them.
The reference has to be a sharp market. Drops at a recreational bookmaker are not signals; they are copies. Read how to read odds movement and sharp vs soft bookmakers.
Using it with real alerts
Every BetTheDrop alert already shows the price before and after the drop, the fair price and the value of your bookmaker's price in one tap. This calculator lets you check any drop you spot by hand, with the same method.
Limits of the calculation
- Use the same line and period on both sides of the market, and the reference bookmaker's prices for all three numbers.
- For a three-way market (1X2) the margin removal needs the third outcome; use the no-vig calculator or the vig calculator.
- The fair odds are an estimate. A minimum edge, for example +2% to +3%, leaves room for error: see how to find value bets.