How to Work Out a Bet and Calculate Its Value, Step by Step
Researched and drafted with AI assistance from published sources; published by LokeNessiSport Editorial Team · Last updated: 6 July 2026
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Gather the three numbers you need
Every value calculation uses exactly three inputs: the decimal odds on offer, your own honest estimate of how likely the outcome is, and the stake you would place. Write all three down before you touch a calculator. For this walkthrough, use a running example: the odds are 3.00, you rate the outcome at 40%, and your stake is £10. If you cannot state your probability estimate as a number, stop here — without it there is nothing to calculate, and you are guessing rather than valuing.
Work out the payout in any odds format
Before judging value, know exactly what a bet pays. Decimal odds: total return = stake × odds, so £10 at 3.00 returns £30 (£20 profit plus your £10 stake). Fractional odds: profit = stake × the fraction, so £10 at 2/1 makes £20 profit, again £30 back in total. American odds: for a plus price, profit = stake × the number ÷ 100, so £10 at +200 makes £20; for a minus price, profit = stake × 100 ÷ the number, so £10 at −150 makes £6.67. All three formats can describe the same bet — 3.00, 2/1 and +200 are identical — so convert everything to decimal before comparing prices. Each-way, accumulator and Asian handicap bets settle by their own rules, so check the bookmaker's settlement terms for those.
Turn the odds into implied probability
Implied probability is the chance the bookmaker's price is built around. Convert it with one sum: implied probability = 1 divided by the decimal odds. For odds of 3.00 that is 1 ÷ 3.00 = 0.333, or 33.3%. This is the number your own estimate has to beat for the bet to be worth considering. Keep in mind the raw implied probabilities across a whole market add up to more than 100% because they include the bookmaker's margin, so the true margin-free figure is slightly lower — but the raw number is fine for a first-pass check.
Set your own probability (and be honest about it)
Your estimate should come from evidence you can defend — quality-weighted recent form, confirmed team news, venue and context, and sensible base rates — not from wanting the bet to be good. In the example, you have settled on 40%. The single most common way people fake value is by nudging this number upward until the maths agrees with the bet they already wanted. Resist that. If anything, estimate conservatively: the market is a strong opponent and is usually closer to the truth than you are.
Compare: is there value at all?
Put the two probabilities side by side. Your estimate is 40%; the implied probability is 33.3%. Because your estimate is higher than the implied probability, the bet passes the value test and is worth calculating further. If your estimate had been lower than 33.3%, the bet would have negative value and the correct action would be to stop and pass — no further sums needed. Most bets fail at this step, and passing on them is the discipline, not a missed opportunity.
Work out the profit if the bet wins
For decimal odds, profit if the bet wins equals stake × (odds − 1). In the example that is £10 × (3.00 − 1) = £10 × 2.00 = £20. So a winning £10 bet returns £20 profit (£30 back in total, being your £10 stake plus £20 profit). You need this figure for the expected-value sum in the next step. Note that the amount you lose if the bet fails is simply your £10 stake.
Calculate the expected value in pounds
The expected value formula is: EV = (probability of winning × profit if it wins) − (probability of losing × stake). Your probability of winning is 0.40, so your probability of losing is 1 − 0.40 = 0.60. Plug in the numbers: EV = (0.40 × £20) − (0.60 × £10) = £8.00 − £6.00 = +£2.00. A positive expected value means that, if your 40% estimate is accurate, this same bet repeated many times would average a £2 profit per attempt. Any single attempt can still lose the whole £10.
Convert the EV into an edge percentage
To compare bets of different sizes, express the EV as a percentage of the stake: edge = EV ÷ stake. Here that is £2.00 ÷ £10 = 0.20, a 20% edge. Percentages let you rank opportunities and spot when an edge is too thin to bother with once you account for the chance your estimate is slightly off. A large printed edge often just means your probability estimate is too optimistic — treat unusually big edges with suspicion, not excitement.
Strip the margin to find the fair price
The implied probabilities across a whole market add up to more than 100% because the bookmaker's margin is baked in. To see the margin-free figure, add up every outcome and divide each one by the total. Take a three-way market of 3.00, 3.60 and 2.40: the implied probabilities are 33.3%, 27.8% and 41.7%, a total of 102.8%, so the margin is 2.8%. Divided by 1.028, the 3.00 outcome's fair probability is 32.4% and its fair price is 1 ÷ 0.324 = 3.08. Your 40% estimate now has to beat 32.4% rather than 33.3% — slightly easier — and the stripped figure is the honest one to compare with, because it is what the market believes once its fee is removed.
Compare two prices for the same bet
Value is not only about whether to bet, but where. The same selection at 3.20 instead of 3.00 changes the expected value directly: at 3.20 the profit if the bet wins is £10 × 2.20 = £22, so EV = (0.40 × £22) − (0.60 × £10) = £8.80 − £6.00 = +£2.80, against +£2.00 at 3.00. That is 40% more expected profit for the identical selection and the identical risk, with no extra forecasting skill. Checking two or three bookmakers before every bet is the cheapest edge in betting; the routine is in our tutorial on comparing sports betting odds at /tutorials/how-to-compare-sports-betting-odds.
Decide, size, and record
If, and only if, the expected value is positive, the bet is a candidate. Size it modestly and consistently, within what you can afford to lose, and never increase a stake because the edge looks large or because you feel sure. Then log everything: the odds you took, your estimated probability, the closing odds at the event's start, and the result. Over dozens of bets that record is the only honest way to tell whether your estimates are genuinely beating the market or simply running hot. If the EV was zero or negative, there is nothing to record because there was no bet to place.
Responsible gambling
No calculation removes the risk of betting. Expected value is a decision aid, not a promise: positive-EV bets lose regularly, and your stake is always at risk. Only ever bet money you can afford to lose, set deposit and time limits before you start, and treat betting as entertainment rather than a way to make money. LokeNessiSport is an independent information platform for adults aged 18 and over and does not accept bets; whether online betting is available and lawful depends on where you live, so check your local rules and eligibility first. If gambling is causing harm, free confidential support is available at BeGambleAware.org and in our responsible gambling resources at /responsible-gambling.