You see a price written as “5/1 against” and you’re not sure what it means. You’re not the only one. This expression comes from fractional odds, the format most common in UK horse racing and older sportsbooks, and it confuses a lot of bettors that only used decimal odds prior to that.

This post explains what “odds against” means, how to read it, and how it connects to the probability behind any price.

The Definition

“Odds against” means the outcome is expected to happen less than half the time. In fractional odds, you’ll see this written as something like 5/1, 10/1, or 7/2. The first number is bigger than the second.

The opposite is “odds on.” That’s written as 1/5, 2/7, or 4/9, the first number is smaller than the second. Odds on means the outcome is expected to happen more than half the time; this is the format you’ll usually see on a strong favorite.

So “against” and “on” aren’t just labels. They tell you whether a price is describing an underdog (against) or a favorite (on).

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How to Read the Fraction Itself?

A fractional price like 5/1 means: for every 1 unit you stake, you win 5 units if you win the bet, plus you get your original stake back.

• First number: Your profit

• Second number: Your stake

Read the fraction as “5-to-1,” which is what 5/1 means out loud. A bigger gap between the two numbers means a bigger underdog. 5/1 is a fairly plain underdog. 20/1 is a long shot.

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Turning “Against” Into a Real Probability

Every price, in any format, implies a probability. For fractional odds, the formula is:

Probability = Second Number ÷ (First Number + Second Number)

• Our example (5/1 against):

Probability = 1 ÷ (5 + 1) = 1 ÷ 6 = 16.7% aprox

That means the market predicts this outcome happenning roughly 1 time in 6.

• Comparing to an odds-on price (1/4):

Probability = 4 ÷ (1 + 4) = 4 ÷ 5 = 80%

The market thinks this one happens 4 times out of 5.

That’s the whole point of the “against” and “on” split. It’s a simple visual way to tell whether you’re looking at a long shot or a strong favorite before you even do the math.

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Converting to Decimal

Modern platforms like BetUnfair show their odds in decimal odds by default because they’re easier to compare and calculate with. To convert a fractional price to decimal:

Decimal Odds = (First Number ÷ Second Number) + 1

• 5/1 against becomes (5 ÷ 1) + 1 = 6.00 in decimal.

• 1/4 odds on becomes (1 ÷ 4) + 1 = 1.25 in decimal.

Once you’re in decimal, the probability formula gets simpler too:

Probability = 1 ÷ Decimal Odds

Check it against the fraction: 1 ÷ 6.00 = 16.7%, the exact same number we got above. Both formats are just two ways of writing the same real probability.

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Why This Matters for Value Betting

Knowing a price is “against” tells you it’s an underdog. It doesn’t tell you whether it’s a good bet. A bet only has value when the probability a price implies is lower than the real chance of that outcome happening, and that comparison works exactly the same whether the price started as a fraction or a decimal.

This is where converting matters in practice. If you’re comparing a UK book using fractional odds against a sharp book quoting decimal, you need both numbers in the same format before you can tell if there’s a real gap between them.

BetUnfair’s Odds Converter automates this process instantly, decimal, fractional, American, and implied probability all in a single input. You don’t need to calculate by hand every time you compare a price.

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FAQ

What does “odds against” mean in betting?

It means the fractional price has a bigger first number than second number (like 5/1), which implies the outcome happens less than half the time.

What is the opposite of odds against?

Odds on, written with a smaller first number (like 1/4), meaning the outcome is expected to happen more than half the time.

How do I turn odds against into a probability?

Divide the second number by the sum of both numbers. For 5/1, that’s 1 ÷ (5 + 1), or about 16.7%.

Is odds against the same as decimal odds above 2.00?

Close. Any fractional price where the first number is bigger than the second converts to a decimal price above 2.00, and both describe a probability under 50%.

Why do some bookmakers still use fractional odds?

It’s a holdover from UK horse racing, where fractional pricing has been standard for over a century. Decimal odds are simpler to calculate with, which is why most modern platforms default to them.

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Try It Yourself

Start your free 5-day BetUnfair trial at betunfair.io and use the Odds Converter to switch any price between fractional, decimal, American, and implied probability in one click.

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