The one number that says if a bet is any good

Strip betting down to its core and you arrive at a single question: over the long run, does this wager make money or lose it? The tool that answers it is expected value, or EV. It’s the average result you’d get if you could place the same bet thousands of times. Positive EV means a bet that profits over time; negative EV means one that bleeds money. Nearly every sound idea in betting — line shopping, no-vig fair prices, beating the closing line — is ultimately just a way of finding positive EV. Understand this, and the rest clicks into place.

The formula, in plain terms

EV combines two things: how likely you are to win, and how much you win or lose in each case.

EV = (probability of winning × amount won) − (probability of losing × stake)

If the answer is positive, the bet makes money on average. If it’s negative, it loses. That’s the whole idea.

A worked coin example

Start with something clean. A friend offers you a bet on a fair coin: stake £10, and if it lands heads you win £11 (getting your £10 back plus £11 profit). Tails, you lose your £10.

  • Probability of winning (heads): 50%
  • Amount won: £11
  • Probability of losing (tails): 50%
  • Stake lost: £10

EV = (0.50 × £11) − (0.50 × £10) = £5.50 − £5.00 = +£0.50.

Every time you take this bet, you make 50p on average. You’ll still lose plenty of individual flips — but over many repetitions, this is a money-maker. That’s positive EV.

Now flip the deal: heads wins you only £9, tails still loses £10.

EV = (0.50 × £9) − (0.50 × £10) = £4.50 − £5.00 = −£0.50.

Same coin, but now you lose 50p per bet on average. That’s negative EV — and, not coincidentally, it’s the shape of almost every bet a bookmaker offers, because the overround tilts the payout against you.

Applying it to real odds

Real bets need one extra step: you supply your own estimate of the true probability, then compare it to the odds. Suppose you back a football team at decimal odds of 2.20 with a £10 stake, and you genuinely believe they’ll win 50% of the time.

  • Win: probability 0.50, profit = £10 × (2.20 − 1) = £12
  • Lose: probability 0.50, stake lost = £10

EV = (0.50 × £12) − (0.50 × £10) = £6.00 − £5.00 = +£1.00.

Positive EV: at those odds, for that probability, it’s a good bet. But notice the catch — everything hinges on your 50% estimate being right. If the team really wins only 40% of the time, the same bet flips to negative EV. EV is only as good as your probability estimate. The bookmaker’s price already reflects a sharp estimate plus a margin, which is exactly why beating it is hard.

EV vs. reality

Two honest warnings keep EV from being misused:

  • Positive EV does not mean you’ll win the bet. It means you’ll profit on average, over many bets. Any single result is dominated by variance. Good bets lose; bad bets win. Only the long run reveals EV.
  • Your probability is the weak link. It’s easy to talk yourself into a rosy estimate. The market’s no-vig fair price is a valuable reality check on your own numbers.

Why it’s still worth knowing

EV won’t make betting safe or turn it into income — the margin means most available bets are negative-EV by design, and that’s the honest truth. What EV does is give you a rigorous way to think: to stop betting on gut feeling and start asking whether a price genuinely beats the true odds. Combined with line shopping and bankroll discipline, hunting for positive EV is as close to a real method as betting offers. Do the maths with our tools, sanity-check prices in our reviews, and take the best available line from our best betting sites.

18+. Gambling involves real financial risk. If it stops being fun, take a break — play responsibly.