Buyer guide
Odds formats explained: decimal, fractional and American
Odds come in three common formats, and a product that shows prices has to handle all of them cleanly. This guide explains decimal, fractional and American odds — how they relate, how to convert, what implied probability means — and why the format your feed delivers matters.
6 min read
Ask for a price and you might get 2.50, 6/4, or +150 — three ways of saying the same thing. For anyone building a product on odds, understanding the three formats, and how a feed hands them to you, avoids a class of subtle bugs.
This guide covers what each format means, how to convert between them, and why the format your feed delivers is part of how clean your integration is.
Decimal odds
Decimal odds express the total return per unit staked, including your stake. 2.50 means a winning 1-unit bet returns 2.50 (1.50 profit plus your 1 back). It is the format betting exchanges use and the one most APIs return, because the maths is simple: return = stake × decimal odds.
Decimal is the easiest format to compute with, which is why it is the natural internal representation for a feed and for any calculation you do on top.
- Total return per unit staked (stake included).
- 2.00 is even money; below 2.00 is odds-on; above is odds-against.
- The exchange and API standard, and the simplest to compute with.
Fractional odds
Fractional odds — 6/4, 2/1, 10/11 — express profit relative to stake. 6/4 means a 4-unit bet profits 6 (returning 10). They are the traditional UK high-street format and what many UK bettors still read most naturally.
To convert fractional to decimal, divide and add one: 6/4 = (6 ÷ 4) + 1 = 2.50. To go back, subtract one and express the result as a fraction.
American odds
American (moneyline) odds use a positive or negative number based on 100. Positive (+150) is the profit on a 100 stake; negative (-200) is the stake needed to profit 100. So +150 means a 100 bet profits 150, and -200 means you stake 200 to profit 100.
To convert to decimal: for a positive number, (american ÷ 100) + 1; for a negative number, (100 ÷ |american|) + 1. So +150 = 2.50 and -200 = 1.50.
Implied probability
Every price implies a probability: the break-even chance at which the bet is fair. For decimal odds it is simply 1 ÷ decimal — 2.50 implies 40%. Across all outcomes in a market these implied probabilities sum to more than 100%; the excess is the bookmaker's margin (the overround).
Implied probability is the bridge between odds and models: value betting compares a bookmaker's implied probability against your own estimate, and arbitrage looks for a set of prices whose implied probabilities sum to under 100%.
Why the feed's format matters
If different books hand you different formats, or the same book changes format, you inherit a conversion-and-consistency problem before you can compare anything. A feed that normalises every book to one consistent format removes that entirely: comparison becomes a direct number-to-number lookup.
Decimal is the sensible normalised format — it is exact, easy to compute with, and trivial to convert to fractional or American at your presentation layer if your users prefer them. Normalise once on the way in; format once on the way out.
At a glance
| Criterion | What to look for |
|---|---|
| Decimal | Total return per unit (2.50)Exchange/API standard; the simplest to compute with. |
| Fractional | Profit relative to stake (6/4)The traditional UK format many bettors read. |
| American | Moneyline based on 100 (+150 / -200)US-market convention; the sign shows favourite or underdog. |
| Implied probability | 1 ÷ decimalThe bridge to models, margin and arbitrage maths. |
| Feed normalisation | One consistent format across every bookMakes comparison a direct lookup, not a conversion project. |
Step by step
- 01
Fractional → decimal
Divide the fraction and add one: 6/4 → (6 ÷ 4) + 1 = 2.50.
- 02
American → decimal (positive)
Divide by 100 and add one: +150 → (150 ÷ 100) + 1 = 2.50.
- 03
American → decimal (negative)
Divide 100 by the absolute value and add one: -200 → (100 ÷ 200) + 1 = 1.50.
- 04
Decimal → implied probability
Take the reciprocal: 1 ÷ 2.50 = 0.40, i.e. a 40% break-even chance.
Key takeaways
- Decimal (2.50), fractional (6/4) and American (+150) are three ways to write one price.
- Decimal is the exchange/API standard and the simplest to compute with.
- Implied probability (1 ÷ decimal) is the bridge to models, margin and arbitrage.
- A feed that normalises every book to one format turns comparison into a lookup — normalise in, format out.
Where OddsRelay fits
OddsRelay normalises odds to one consistent decimal format across every book, so comparison is a direct number-to-number lookup and there is no per-book format juggling. You convert to fractional or American at your presentation layer if your users prefer them — the maths above is a one-liner. Normalise once on the way in; format once on the way out.
Questions
Which odds format does OddsRelay use?
The feed normalises every book to one consistent decimal format — the exchange and API standard, and the simplest to compute with. You can convert to fractional or American at your presentation layer with a one-line formula.
How do I convert fractional to decimal?
Divide the fraction and add one: 6/4 becomes (6 ÷ 4) + 1 = 2.50. To convert American, for a positive number divide by 100 and add one (+150 → 2.50); for a negative, divide 100 by its absolute value and add one (-200 → 1.50).
What is implied probability?
The break-even chance a price implies: 1 ÷ decimal odds. 2.50 implies 40%. Across a market the implied probabilities sum to over 100% — the excess is the bookmaker's margin. It is the bridge between odds and probability models.
Keep reading
Odds comparison
60+ UK books in one normalised feed — bet365 included, coverage maintained.
Matched-betting data
Oddsmatcher-ready rows — back/lay paired, rated, gated, bet365 included.
Trading data
Market and exchange reference data for pricing, value detection and trading models.
Choosing a provider
The eight criteria that actually separate odds feeds — coverage, freshness, schema, support and more.
Coverage, freshness & reliability
How to verify the three claims every odds feed makes — and the proof to demand for each.
Matched vs arb vs value
Three strategies, three data profiles — what a feed must deliver for each, and how to tell them apart.
Developers
A predictable REST feed you integrate in an afternoon — no SDK, no collection to run, no matching engine.
Affiliates
Accurate best-price and comparison data to drive conversions — without a data-collection team.
For odds comparison
What a comparison site needs — breadth, normalisation, accuracy — and where OddsRelay fits.
Odds & betting glossary
Put the criteria to the test.
Start a free trial of the full UK feed, bet365 included, and judge it against everything in this guide.