Methodology
- Use American odds when reading US books and props.
- Use decimal odds to compute payouts quickly: stake × decimal = total return.
- Use fractional odds for UK horseracing and exchange-style markets.
- Use implied probability to compare prices against fair-model estimates.
The four formats are equivalent
Every American price has a single decimal and a single implied probability. The converter handles edge cases like +100 (decimal 2.00, 50%) and -110 (decimal 1.91, 52.38%) that trip up manual conversions.
- American +200 = decimal 3.00 = 33.33% implied
- American -150 = decimal 1.667 = 60.00% implied
- American +100 = decimal 2.00 = 50.00% implied
- Fractional 11/10 ≈ decimal 2.10 ≈ American +110
When to switch formats
Decimal is easiest for EV math because total return is just stake times decimal. Implied probability is the right form for comparing against any fair-price estimate. American is fastest for shopping US books because the eye gets used to the +/- signs.
- Decimal: payout calculations and parlay multiplication
- Implied: comparing book price to model fair price
- Fractional: cross-checking UK or Australian books
- American: US sportsbook UI and prop research
Side-by-side format reference
Same price, four different reads.
| American | Decimal | Fractional | Implied |
|---|---|---|---|
| -200 | 1.500 | 1/2 | 66.67% |
| -110 | 1.909 | 10/11 | 52.38% |
| +100 | 2.000 | 1/1 | 50.00% |
| +200 | 3.000 | 2/1 | 33.33% |
When in doubt, convert everything to implied probability before comparing.
Responsible-use note
Analytics should support disciplined decision-making, not guaranteed outcomes. Bet only where legal, never risk money you cannot afford to lose, and use limits before volume increases.
FAQ
Which odds format is best for sports betting math?
Decimal is best for payout and parlay math. Implied probability is best for comparing prices against any fair-value estimate, including model projections and no-vig closings.
How do I convert American odds to implied probability?
For negative odds: |odds| / (|odds| + 100). For positive odds: 100 / (odds + 100). The calculator handles both and returns the result as a percent.
Are fractional odds still used in the US?
Mostly for horseracing pari-mutuel pools. American odds dominate US sportsbook UIs; decimal dominates the rest of the world.
