Exchanges use decimal odds, UK bookmakers still often show fractions, and US sites use American (moneyline) odds. Type in any box and the others update.
Decimal = fractional + 1 (5/2 → 3.5)
American: +(decimal − 1) × 100 when decimal ≥ 2; −100 ÷ (decimal − 1) below 2
Probability = 100 ÷ decimal
American: +(decimal − 1) × 100 when decimal ≥ 2; −100 ÷ (decimal − 1) below 2
Probability = 100 ÷ decimal
The converter also shows the nearest valid Betfair price, because not every decimal is on the exchange ladder.
Questions
Why is evens 2.0?
Decimal odds include your stake: £1 at evens returns £2, being £1 profit plus the £1 stake.