Calculator - odds

Odds calculator and converter: decimal, fractional, American, probability

Enter a price in any format and the calculator gives the other two formats, the implied probability and the return on your stake, including an exchange bet after commission. The formulas it uses are below, each with a worked example.

checked 11.11.2026by Casinos Geek editorial team

Fractional to decimal

Fractional odds show profit relative to stake: 5/2 means €5 profit for every €2 staked. Decimal odds show the total return per €1, stake included.

Decimal = (numerator / denominator) + 1

5/2 = 2.5 + 1 = 3.50
4/6 = 0.667 + 1 = 1.67
Evens (1/1) = 2.00

Going back: fractional = decimal - 1, written as a fraction. 2.75 - 1 = 1.75 = 7/4.

Which format to use

Irish and UK bookmakers traditionally quote fractional odds, especially in racing. Exchanges quote decimal, and so do most betting apps if you change the setting. Decimal is the easiest to work with: multiply by the stake to get the return, and divide 1 by it to get the probability.

American odds

American (moneyline) odds use 100 as the pivot. A plus sign shows profit on a 100 stake; a minus sign shows the stake needed to win 100.

+250 to decimal: 1 + 250 / 100 = 3.50
-150 to decimal: 1 + 100 / 150 = 1.67
Decimal 2.00 or higher to American: +(decimal - 1) x 100. So 3.50 = +250.
Decimal below 2.00 to American: -100 / (decimal - 1). So 1.67 = -150 (rounded).

Implied probability

Every price implies a chance of the outcome happening. It is the price's break-even point: bet at 3.50 a thousand times, and you need to win more than 28.6% of them to come out ahead.

Implied probability = 1 / decimal odds

3.50: 1 / 3.50 = 28.57%
1.67: 1 / 1.67 = 59.88%
Fractional a/b: b / (a + b). For 5/2: 2 / 7 = 28.57%.

Common prices in all three formats

Fractional	Decimal	American	Implied probability
1/2	1.50	-200	66.67%
4/6	1.67	-150	60.00%
Evens	2.00	+100	50.00%
6/4	2.50	+150	40.00%
2/1	3.00	+200	33.33%
5/2	3.50	+250	28.57%
3/1	4.00	+300	25.00%
4/1	5.00	+400	20.00%
5/1	6.00	+500	16.67%
10/1	11.00	+1000	9.09%

Conversion table of common prices between fractional, decimal, American odds and implied probability — Six common prices in all formats.

Decimal prices are rounded to two places, which is why 4/6 shows as 1.67 and 59.88% when worked from the rounded decimal, but exactly 60% from the fraction (6 / 10).

The overround on a three-way market

Add up the implied probabilities of every outcome in a market. If the prices were fair, the total would be 100%. A bookmaker's prices add up to more, and the excess is the overround, the margin it builds in. The prices below are illustrative.

Stacked bar of a three-way market adding up to 104.81%, an overround of 4.81% — Illustrative prices; proportional removal is an estimate.

Home 2.10, Draw 3.40, Away 3.60

Home: 1 / 2.10 = 47.62%
Draw: 1 / 3.40 = 29.41%
Away: 1 / 3.60 = 27.78%
Total: 104.81%. Overround = 4.81%.
Remove it proportionally: Home 47.62 / 1.0481 = 45.43%, fair odds 2.20. Draw 28.06%, fair 3.56. Away 26.50%, fair 3.77.

Proportional removal is the simplest method and assumes the margin is spread evenly. In practice the margin may not be spread evenly, so treat the fair odds as an estimate. The useful part is the comparison: a market at 102% costs you less than one at 108%.

Exchange returns after commission

An exchange takes no margin in the price. Instead it charges commission on your net winnings in a market. To compare an exchange price with a bookmaker price, convert it into effective odds.

Profit on €10 at 3.00 with 0%, 2% and 5% exchange commission — Commission comes off the profit, not the stake.

Back, net profit = stake x (odds - 1) x (1 - c)

Effective odds = 1 + (odds - 1) x (1 - c)

Lay, net profit if the selection loses = lay stake x (1 - c); liability = lay stake x (odds - 1)

€10 back at 3.00 with c = 2%: profit €19.60, effective odds 2.96.
Same bet with c = 5%: profit €19.00, effective odds 2.90.
€10 lay at 3.00 with c = 2%: liability €20; net €9.80 if the selection loses.

The 2% rate is the one Betdaq published for Irish residents before it stopped trading; 5% is its rate for other countries. For live exchanges and the rates we have confirmed, see our exchanges page.

Other calculators

Each-way calculator: win part, place part, place terms, dead heats and Rule 4.
Accumulator calculator: multiplied odds, void legs, and how margin compounds across legs.

Questions

How do I convert fractional odds to decimal?

Divide the first number by the second and add 1. 5/2 becomes 2.5 + 1 = 3.50; 4/6 becomes 1.67.

What does an implied probability of 28.57% mean?

It is 1 / 3.50: the share of bets at that price you must win just to break even, before any margin.

What is a normal overround?

It varies by bookmaker and market. Calculate it yourself: add 1/odds for every outcome; anything above 100% is margin.

How do I compare exchange odds with bookmaker odds?

Convert the exchange price to effective odds: 1 + (odds - 1) x (1 - commission). At 2%, 3.00 is worth 2.96.

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changelog

11.11.2026 First published.