Fractional odds (also called British odds) are written as a/b — for example 5/2, 11/10, or 4/6. They state profit relative to stake, not total return. At 5/2, every £2 staked pays £5 profit if you win; your stake comes back on top, so total return is £7 on that £2.
UK and Irish books (and horse racing globally) still default to fractions. Most other markets use decimal. Same price either way — different notation. Full format map: sports betting odds explained.
How to read a/b
| Piece | Meaning |
|---|---|
Numerator (a) |
Profit units if you win |
Denominator (b) |
Stake units required for that profit |
| Evens | 1/1 (sometimes written Evens) — profit equals stake; decimal 2.00 |
Profit and return are different:
profit = stake × (a / b)
return = stake + profit = stake × (1 + a/b) = stake × (a + b) / b
Mixing them up is how soft guides invent fake payouts. Return always includes stake; profit does not.
Worked example — longer price (dog)
Illustrative only — not a tip and not a live market. Home win offered at 14/5.
decimal equivalent = 1 + 14/5 = 3.80
£50 stake → profit = 50 × (14/5) = £140
→ return = 50 + 140 = £190
Worked example — shorter price (favourite)
Away win offered at 5/12:
decimal equivalent = 1 + 5/12 ≈ 1.4167
£50 stake → profit = 50 × (5/12) ≈ £20.83
→ return ≈ £70.83
A short fraction (a < b) means a strong favourite on that board — you risk more relative to the profit you stand to make.
Convert fractional ↔ decimal ↔ American
Work in decimal when you can: implied probability is 1 / decimal, and comparing books is cleaner. Formulas below are exact for a given fraction; books still quote on ticks, so the posted fraction may not match the exact decimal you saw elsewhere.
Fractional → decimal
decimal = 1 + (a / b) = (a + b) / b
| Fractional | Decimal | Notes |
|---|---|---|
| 1/4 | 1.25 | short fav |
| 4/6 (= 2/3) | ≈1.6667 | UK often writes 4/6, not 2/3 |
| 5/6 | ≈1.8333 | |
| 1/1 (Evens) | 2.00 | |
| 6/4 (= 3/2) | 2.50 | |
| 5/2 | 3.50 | |
| 5/1 | 6.00 | |
| 10/1 | 11.00 |
Decimal → fractional
(a / b) = decimal − 1
Reduce to lowest terms, or map onto the book’s tick ladder (UK boards use a fixed set of fractions — see table below). Example: decimal 3.50 → 3.50 − 1 = 2.50 → 5/2.
American (moneyline) from the same price
Anchored on a $100 reference (same as moneyline):
if decimal ≥ 2: American = +100 × (decimal − 1) # underdog / plus money
if decimal < 2: American = −100 / (decimal − 1) # favourite / minus money
| Fractional | Decimal | American |
|---|---|---|
| 1/4 | 1.25 | −400 |
| 4/6 | ≈1.6667 | −150 |
| 1/1 | 2.00 | +100 |
| 5/2 | 3.50 | +250 |
| 5/1 | 6.00 | +500 |
Check: −150 means risk $150 to win $100 profit → decimal 1 + 100/150 ≈ 1.6667 → 4/6. +250 means risk $100 to win $250 → decimal 3.50 → 5/2.
Implied probability
implied = 1 / decimal = b / (a + b)
At 5/2: 2 / (5+2) ≈ 28.57%. That is what the price implies — not a claim that the selection truly wins that often. Soft books add overround; raw implieds across a full market sum to more than 100%.
Common UK ticks (quick reference)
Books do not offer every mathematical fraction. They jump on a ladder. Approximate decimal for common ticks:
| Fractional | ≈ Decimal | Fractional | ≈ Decimal |
|---|---|---|---|
| 1/5 | 1.20 | 5/4 | 2.25 |
| 2/9 | ≈1.222 | 11/8 | 2.375 |
| 1/4 | 1.25 | 6/4 | 2.50 |
| 2/7 | ≈1.286 | 13/8 | 2.625 |
| 1/3 | ≈1.333 | 7/4 | 2.75 |
| 4/11 | ≈1.364 | 9/5 | 2.80 |
| 2/5 | 1.40 | 15/8 | 2.875 |
| 4/9 | ≈1.444 | 2/1 | 3.00 |
| 1/2 | 1.50 | 9/4 | 3.25 |
| 8/15 | ≈1.533 | 5/2 | 3.50 |
| 4/7 | ≈1.571 | 11/4 | 3.75 |
| 8/13 | ≈1.615 | 3/1 | 4.00 |
| 4/6 | ≈1.667 | 7/2 | 4.50 |
| 8/11 | ≈1.727 | 4/1 | 5.00 |
| 4/5 | 1.80 | 5/1 | 6.00 |
| 5/6 | ≈1.833 | 6/1 | 7.00 |
| 10/11 | ≈1.909 | 8/1 | 9.00 |
| 1/1 | 2.00 | 10/1 | 11.00 |
Two-decimal decimal odds rarely land exactly on a tick. When a book shows 2.48 decimal and 6/4 fractional for the “same” selection, prefer the longer of the two after converting — coarse rounding is a real cost over a large sample.
Why most value bettors work in decimal
- One number for return.
return = stake × decimal. No separate “add stake back” step. - Implied % is trivial.
1 / decimal. - Cross-book compare. Shopping 2.55 vs 2.48 is obvious; comparing 6/4 vs 11/8 mid-session is slower and easier to misread.
- Tick rounding. Fractional boards often shave you onto a shorter tick than the true decimal equivalent elsewhere.
Set the book’s display to decimal if you can. Use fractions only when the board forces them (many UK racing screens).
Fractional odds and +EV
A price is +EV when your estimated win probability p beats the price:
EV = stake × (p × decimal − 1) > 0
Example: you estimate 32% (p = 0.32) and the book still offers 5/2 (decimal 3.50):
EV = £100 × (0.32 × 3.50 − 1) = £100 × 0.12 = £12
→ +12% of stake in expectation if p is right. Settlement is still either +£250 profit or −£100. One result does not prove the edge. Process: value betting, expected value (EV). Stake sizing: bankroll / Kelly.
Next step once you can read the board: same maths on 1X2 and handicap markets. Light note — tools such as Trademate Sports can surface +EV prices against a reference line once you convert formats yourself. No invented ROI; the conversion and EV maths above are the product.
FAQ
What are fractional odds?
UK-style betting odds written as a/b. The fraction is profit:stake. At 5/1, £1 stake wins £5 profit (plus stake back). Also called British odds.
How do you calculate winnings from fractional odds?profit = stake × (a/b); return = stake + profit. At 5/2 with a £20 stake: profit = £50, return = £70. Do not call the return “profit”.
How do you convert fractional odds to decimal?decimal = 1 + a/b. Example: 5/2 → 1 + 2.5 = 3.50. Evens (1/1) → 2.00.
How do you convert fractional odds to American?
Convert to decimal first, then: if decimal ≥ 2, American = +100 × (decimal − 1); if decimal < 2, American = −100 / (decimal − 1). So 5/2 → 3.50 → +250; 4/6 → ≈1.6667 → −150.
What does 4/6 mean?
Same as 2/3: stake 6 to win 4 profit (or stake 3 to win 2). Decimal ≈ 1.6667; American −150. UK boards often print 4/6 rather than the reduced 2/3.
Are fractional odds better than decimal?
No — same price, different display. Decimal is usually cleaner for implied probability and shopping books. Fractional is traditional on UK racing boards; switch format in account settings when you can.