Odds Conversion

Three formats, one price. Being able to move between them quickly matters mainly when comparing operators across markets, since a US book and a UK book will quote the same bet in different notation.

The conversions

Fractional → Decimal: (numerator ÷ denominator) + 1

• 5/1 → 5 + 1 = 6.00
• 6/4 → 1.5 + 1 = 2.50
• 1/2 → 0.5 + 1 = 1.50

Decimal → Fractional: subtract 1, express as a fraction

• 3.50 → 2.5 → 5/2
• 1.40 → 0.4 → 2/5

Decimal → American:

• If 2.00 or above: (decimal − 1) × 100 → 3.00 becomes +200
• If below 2.00: −100 ÷ (decimal − 1) → 1.50 becomes −200

American → Decimal:

• Positive: (odds ÷ 100) + 1 → +150 becomes 2.50
• Negative: (100 ÷ |odds|) + 1 → −150 becomes 1.67

Reference table

Fractional	Decimal	American	Implied
1/5	1.20	−500	83.3%
1/2	1.50	−200	66.7%
4/5	1.80	−125	55.6%
10/11	1.91	−110	52.4%
1/1 (Evens)	2.00	+100	50.0%
6/4	2.50	+150	40.0%
2/1	3.00	+200	33.3%
4/1	5.00	+400	20.0%
10/1	11.00	+1000	9.1%
50/1	51.00	+5000	2.0%

The one to memorise

Decimal 2.00 = Evens = +100 = 50%. Everything else orients around it. Below 2.00 is odds-on and shown as a negative American number; above 2.00 is odds-against and shown as positive.

If you only internalise one conversion, make it decimal to implied probability, divide 1 by the odds. That single division is what turns a price into a question you can evaluate.