Expected Value and Kelly Stake Calculator
You already have a number in your head for how often this wins. Type it in next to the price, and see what that number is actually worth before you stake anything on it.
Type your price and your own number
Odds in, your own probability in, and the maths runs below as you type.
Reading the expected value and the Kelly stake
Two numbers go in. The odds you can actually get, and your own estimate of how often the selection wins. Nothing about the second one comes from this page: it is yours, typed in, and the whole result stands or falls on it.
Tick the exchange box and the odds get one adjustment first: a commission rate comes off the winning side, turning your price into an effective price, a little lower than the one quoted. Leave the box unticked and the effective odds are simply the odds you typed.
Expected value, EV, multiplies your probability by that effective price and takes off 1. A positive figure means the price pays more than your own number implies it should; a negative one means the opposite, which is the whole test behind what this site calls value betting. Weather forecasters live and die by a version of this test. A forecaster who says 70% for rain needs it to rain on roughly seven days out of ten they said that, across thousands of forecasts, or the number is wrong and anyone can check. Nobody grades a betting probability that way. That gap is exactly what this calculator cannot close for you.
A positive EV feeds the Kelly fraction, which turns an edge into a stake size: a bigger edge earns a bigger stake, always as a share of a bankroll, never a fixed amount. Full Kelly grows a bankroll fastest on average and swings hardest on the way there; half Kelly trades away some of that average growth for a noticeably smoother run, which is why it loads as the default below. Quarter Kelly goes further the same way again.
This calculator turns two numbers into a stake size: the odds you can get, and your own estimate of how often the pick wins. Expected value (EV) shows whether that price pays more than your number implies, as a percentage. When EV is positive, the Kelly fraction sizes a stake from it, scaled to half by default to smooth the ride; tick the exchange box first if the price you were quoted already has exchange commission built in. At odds of 2.10 and a 55% estimate with no exchange commission, EV comes out at +15.5% and half Kelly suggests staking about 7.05% of a bankroll. None of this checks whether the 55% was right. The calculator sizes a stake for an edge you believe you have. It does not confirm that one exists.
Two examples, worked all the way through
Both use invented numbers. Nobody quoted either price. Type them into the calculator above and it should return exactly these figures.
| Case | Odds and probability | Exchange commission | Effective odds | EV | Full Kelly | Half Kelly |
|---|---|---|---|---|---|---|
| A | 2.10 at 55% | none | 2.10 | +15.5% | 14.09% | 7.05% (about £70.45 on £1,000) |
| B | 2.20 at 50% | 2%, exchange ticked | 2.176 | +8.8% | 7.48% | 3.74% (about £74.80 on £2,000) |
Case A is the calculator's own starting point: no exchange box ticked, so the effective odds are just the 2.10 typed in. A probability of 55% against a price of 2.10 gives an expected value of +15.5% per unit staked, a full Kelly fraction of 14.09%, and half Kelly at 7.05%; on an assigned bankroll of £1,000 that half-Kelly share comes to £70.45, which is exactly half of the £140.90 that full Kelly would suggest on the same bankroll, because the calculator scales the cash figure by the same multiplier it applies to the percentage, rather than rounding each one separately and letting the two drift apart.
Case B ticks the exchange box, with a commission rate of 2%, Betfair's own Basic package example on a £100 net profit (check the current terms for the rate that actually applies to your account and region). A quoted price of 2.20 becomes an effective 2.176 once that exchange commission is taken off. Against that lower price, a 50% probability gives EV of +8.8%, a full Kelly fraction of 7.48%, half Kelly at 3.74%, and about £74.80 on an assigned £2,000 bankroll.
Where this calculator stops
No stake yet. Just a number, and a decision still waiting on you.
The maths above treats every bet as its own isolated event, matched in full, at the price you typed, with nothing else touching it. Real betting rarely stays that clean.
A cheap sanity check before trusting a probability you typed above: compare it with a price from an operator that can actually back the word up. Not every book that claims to be sharp can show its working, and a market that has done its own careful pricing is a sturdier benchmark than a hunch.
Reach the exchange through a broker instead of a direct Betfair account, and a second exchange commission usually applies on top of anything modelled here, set by the broker's own product rather than by Betfair. BetInAsia and MadMarket both sell a product called Sharp Exchange, built on Betfair: MadMarket's own page states a flat 3% on winning bets, not for you if you are in the UK, the USA or Australia, all on MadMarket's own restricted list; BetInAsia's current pages print no number at all for that same rate, so ask before you rely on one. AsianConnect reaches the same kind of market through PIWIX and OrbitX, both charged at 3% on winning bets by AsianConnect's own help centre, though PIWIX drops lay betting on horse racing specifically. Sportmarket's FairExchange charges 3% on winning bets only, and excludes the UK, the USA and France on its own terms, the same countries it excludes for the rest of the platform (check local law either way, since a broker's own list is not the only thing that decides whether betting on an exchange is allowed where you live).
None of that changes the maths above, only what lands in an account afterwards. How the four brokers compare for exchange access sets out the routes in full, and opening whichever account this maths assumes you can reach covers the account itself, step by step.
This is one of several small calculators here. Splitting a stake across two or three overlapping prices covers the gap that opens between operators, checking how much margin sits inside a set of odds comes before you compare them at all, and settling an Asian handicap line comes once the match is over. The rest of the free tools for exchange bettors are listed together in one place.
Questions people ask
What counts as a good Kelly fraction to use?
There is no single right answer. Full Kelly grows a bankroll fastest on average and swings hardest along the way; half Kelly, the default here, gives up some of that average growth for a noticeably smoother run, which is why many staking plans use it; quarter Kelly goes further again. All three depend on the probability you typed being close to right.
Does a positive result here prove I have found a value bet?
No. It proves that your own probability, multiplied through the effective price, comes out above zero. That is an assumption you are testing, not a confirmed edge, for the reason further up this page: your own read on a match is not automatically sharper than the price the market has already settled on. The useful next step is to stress-test the probability itself, not to stake straight away.
Why does ticking the exchange box change the odds?
An exchange takes a commission from winning bets, so the price you can actually bank is a little below the one quoted. The box turns your odds into that effective price first, then runs the same expected value and Kelly maths on the lower figure. The exchange commission field opens at 2%, Betfair's own Basic package example on a £100 net profit; your own rate depends on your account and region, so check the current terms.
What happens if my probability estimate is wrong?
Everything downstream moves with it. A probability nudged a few points in either direction can flip a positive expected value to a negative one, or the other way round, because the calculator has no way to check the number against reality. Treat the result as a test of one specific guess, not a verdict on the bet.
Can I use this as a Kelly criterion calculator for betting exchanges specifically, not just bookmakers?
Yes. Tick the exchange box, type in the commission rate that actually applies to your account rather than the example 2%, and the effective odds, expected value and Kelly stake all follow from that adjusted price.