Expected value (EV) is your average profit or loss per bet if it were repeated endlessly. On Betfair you have positive EV when the true probability of an outcome is higher than the price implies. EV = (probability of winning x profit) minus (probability of losing x stake). Long-term results follow EV, not luck.
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This sub belongs to our Betting Exchange Concepts pillar. If you only ever truly understand one idea from this whole site, make it this one. Expected value is not an advanced technique you graduate to — it is the foundation under every other thing a profitable trader does. Backing, laying, scalping, greening up: all of them are just machinery for capturing positive EV and discarding negative EV.
What expected value actually is
Expected value is the average result of a bet if you could place it an infinite number of times. That's the whole definition. A bet with positive EV makes money on average; a bet with negative EV loses money on average. The result of any single bet is mostly luck. The result of ten thousand bets is almost entirely EV. Your job as a trader is to place bets and trades that have positive EV, over and over, and let the law of large numbers do the rest. You will never control whether one trade wins. You completely control whether your average trade has an edge.
This reframes everything. A trade that lost money can still have been a good, +EV trade — you just hit the unlucky side this time. A trade that won can have been a bad, −EV gamble that happened to land. Beginners judge decisions by outcomes; professionals judge decisions by EV and accept that outcomes are noisy in the short run. Getting this distinction into your bones is what stops you from chasing losses and abandoning good methods after a bad run.
The formula, in plain numbers
For a simple back bet, expected value is: EV = (P_win × profit) − (P_lose × stake), where P_win is your honest estimate of the probability the bet wins. Suppose you back a selection at 3.0 for £10. The price 3.0 implies a probability of 1 / 3.0 = 33.3% (this is implied probability). If your own assessment says the true chance is actually 40%, then:
- Profit if it wins: £20 (2.0 x £10)
- EV = (0.40 × £20) − (0.60 × £10) = £8 − £6 = +£2
That +£2 is your edge per £10 bet, on average. Place that same +EV bet a thousand times and you expect roughly +£2,000, give or take variance. The entire skill of betting is making P_win — your probability estimate — more accurate than the market's. If your estimates are worse than the market's, your EV is negative no matter how clever your staking.
Why the exchange makes EV findable
On a traditional bookmaker, the built-in margin (the overround) means almost every price is −EV before you start — you're fighting a headwind. The Betfair Exchange is different in a way that matters enormously: prices are set by other bettors, not by a house margin, so they cluster much closer to true probability. The only cost is commission on net winnings. This means genuine +EV opportunities actually exist on the exchange — when the crowd misprices something, you can take the other side at a fair price. It also means you can lay, betting that something won't happen, which doubles the places you can hunt for value. The exchange doesn't hand you EV; it removes the structural barrier that makes EV nearly impossible at a bookmaker.
The situation: a well-backed favourite had been hammered into 1.90 (implied 52.6%) on what looked to me like name recognition rather than form — it was stepping up in trip on ground it had never handled.
My estimate: I had its true chance at around 42% (a fair price nearer 2.38). If I'm right, laying at 1.90 is clearly +EV.
The trade: I laid £40 at 1.90 (liability £36). EV check: (0.58 × £40 win) − (0.42 × £36 liability) = £23.20 − £15.12 = +£8.08 expected on the lay.
What happened: the horse won. I lost the £36 liability. And that's the point — it was still a good trade. One outcome tells you nothing; I've made that same lay shape hundreds of times and the +EV ones print over the sample even though plenty lose individually.
EV vs variance: why +EV still loses sometimes
This is where most people quit. A +EV approach does not win every bet, or even every week. Variance — the natural swing of results around their average — can bury a positive edge for surprisingly long stretches. You can be a genuinely +EV trader and still have a losing month; it happens constantly and it is mathematically normal. The traders who survive are the ones who can tell the difference between “I'm running below EV due to variance” (keep going) and “my edge has actually disappeared” (stop and reassess). That judgement requires records, honesty, and a big enough sample. It also requires bankroll management sized so that variance can't bust you before your edge pays out.
Positive EV only helps if your probability estimates are genuinely better than the market's. Most people overrate their own estimates, which means their “+EV” bets are actually −EV and they lose. Be brutally honest about whether you really know more than the thousands of others pricing the same market. Most Betfair traders lose money; assume you're average until your records prove otherwise.
Where people get EV wrong
- Judging by outcomes. “That lost, so it was a bad bet.” No — judge the decision by EV, not the result.
- Overconfident probabilities. If your P_win is just optimism, your EV maths is fiction. The market is hard to beat.
- Ignoring commission. Your real edge is after commission. A 2% edge can vanish at a 5% rate on high-churn trading.
- Chasing. Raising stakes after losses to “get it back” doesn't change EV; it just increases variance and risk of ruin.
Using EV day to day
You don't run the formula on every trade in real time — that's impractical mid-ladder. What you do is internalise the habit of asking, before every position: is the price I'm taking better or worse than the true probability as I see it? If better, you have edge; if you can't honestly say it's better, you're gambling, not trading. Over time the EV mindset reshapes which markets you enter (only ones you understand), how you size (bigger when the edge is clearer), and when you walk away (when you have no view). Combine it with a disciplined psychology so variance doesn't push you into −EV decisions, and with the price-movement reads that tell you when the crowd is mispricing. That's the whole job: find +EV, protect it from yourself, repeat.
EV applies to trading, not just betting
So far the examples have been straight bets, but the EV lens is even more powerful applied to trading positions. When you scalp a tick on the ladder, you're making a tiny +EV bet that the price will tick your way more often than against, after the spread and commission. When you green up, you're converting an uncertain +EV position into a certain smaller one — sometimes correct, sometimes leaving EV on the table out of fear. Even a stop-loss is an EV decision: cutting a position whose EV has turned negative because the market told you your read was wrong. Once you see every click as adding or subtracting expected value, your whole approach sharpens. Bad trades stop being “unlucky” and start being either “−EV decisions” (fix the process) or “+EV decisions that lost” (do nothing, keep going). That single reframe is worth more than any indicator.
It also tells you where not to trade. If you have no genuine probability edge over the market in, say, a tennis match you don't understand, then by definition your EV there is at best zero and, after commission, negative. The disciplined answer is to not trade it — sit out and wait for a market where you actually know more than the crowd. Most of the money lost in trading is lost in markets where the trader had no edge but couldn't resist the action.
EV is a habit of thought before it's a formula. Start asking “is this price better than the true chance?” on every position.
Concepts Pillar Open Betfair Account →FAQ
What is expected value in betting? It's the average profit or loss a bet would produce if repeated endlessly. Positive EV means you profit on average; negative EV means you lose on average. Long-run results track EV, not luck.
How do I calculate EV on Betfair odds? EV = (probability of winning x profit) minus (probability of losing x stake). Use your honest probability estimate, and compare it to the price's implied probability (1 divided by the decimal odds). If your estimate is higher than implied, the back bet is +EV.
If a +EV bet loses, was it a bad bet? No. A single result is mostly luck. A +EV bet that loses was still the right decision; you hit the unlucky side. Judge decisions by EV, outcomes by a large sample — never judge a decision by one outcome.
Why do I lose even with positive EV? Variance — the natural swing of results — can bury a real edge for weeks. Positive EV wins over a large sample, not every bet. You also need accurate probability estimates and bankroll management so variance can't bust you first.
Does commission affect my EV? Yes. Your real edge is after commission on net winnings. On high-frequency trading a small EV edge can be eaten entirely by a 5% rate, so factor it into every calculation.