Value betting and expected value, explained properly

What a fair price is

A value bet is a bet where the odds you get are higher than the true chance of the outcome justifies. Over many bets, that gap is your profit. Unlike a sure bet, you only bet one side, so any single bet can and often will lose.

The hard part is knowing the true chance. Nobody knows it exactly, but some prices are much better estimates than others. Sharp bookmakers such as Pinnacle, and the Betfair exchange on liquid markets, take big bets from informed players and move their lines quickly. Their prices, especially close to kick-off, are the best public estimate of the real probability.

The reference is only as good as the market behind it. A Premier League match an hour before kick-off has a very efficient sharp price. A lower-league game two days out, or an exchange market with a few hundred euro matched, does not. Be more sceptical of edges found against thin or early lines, and demand a bigger margin before betting them.

So the standard approach is simple: take the sharp price, strip out its margin to get a fair price, and look for soft bookmakers offering more than that.

Removing the margin

A bookmaker's implied probabilities add up to more than 100%. The excess is its margin, also called the overround or vig. To get fair probabilities you remove it.

The multiplicative method is the simplest. Divide each implied probability by the total:

fair p = (1/odds) / Σ(1/odds)
fair odds = 1 / fair p

The power method instead finds an exponent k so that Σ(1/odds)^k = 1. It takes slightly more margin off longshots than favourites, which matches how bookmakers tend to price. For two near-even outcomes the methods give almost the same answer. For a 1.20 favourite against an 8.00 outsider, the difference matters. The no-vig calculator does both.

Expected value

Once you have a fair probability p, the expected value of a bet at decimal odds is:

EV = p × odds − 1

The result is profit per unit staked. An EV of 0.05 means you expect €5 per €100 on average, over a very large number of similar bets.

Real edges are usually smaller, 2% to 5%. Anything far above that deserves a second look: the sharp line may have moved, it may be a different market, or the soft price may be an error that gets voided.

Variance: why 100 bets prove almost nothing

This is the part most people underestimate. Take flat €10 stakes at odds of about 2.00 with a 4% edge. A 4% edge at 2.00 means a true win probability of 52%. Each bet either returns +€10 or −€10, and the spread of results over n bets is roughly:

SD of profit ≈ stake × odds × √(n × p × (1 − p))

After 100 bets, the noise is two and a half times the signal. After 1,000 bets, a genuinely profitable bettor still has roughly a 1 in 10 chance of being down. Only after several thousand bets does the edge reliably dominate. Higher odds make this worse, because each result swings further.

Practically, that means you need a bankroll that survives the bad runs (see the bankroll guide) and a better way to judge yourself than short-term profit.

Kelly and fractional Kelly

The Kelly criterion sizes each bet to maximise long-run growth:

Kelly fraction = (p × odds − 1) / (odds − 1)

In the example above: 0.075 / 1.15 = 6.52% of bankroll. That assumes your fair probability is exactly right, which it never is. Full Kelly with an overestimated edge leads to large drawdowns, so most value bettors use a quarter or less, here about 1.6%, and cap any single stake. The Kelly calculator works this out for any fraction.

Closing line value: the best short-term evidence

Because results are so noisy, serious bettors track closing line value. Compare the odds you took with the sharp fair price just before the event starts:

CLV = your odds / closing fair odds − 1

If you took 2.15 and the fair closing price was 2.05, your CLV is +4.9%. The closing line includes all the information the market had, so consistently beating it is strong evidence of a real edge, often visible within a few hundred bets, long before profit is.

The flip side: beating the closing line is also exactly what soft bookmakers watch for. Expect restrictions sooner or later, and read how bookmaker limits work.

Sure betting vs value betting

Many people use both: arbs for steady turnover, value bets for higher expected return. Either way, losing runs are part of betting. Only stake what you can afford to lose.