Expected Value (EV) in Prediction Markets: How to Calculate It

How to Calculate Expected Value Before Any Prediction TradeExpected value (EV) is the single number that tells you whether a trade is worth entering at all. The formula is simple: (your probability × the profit if right) − (probability of being wrong × the loss if wrong). A positive result means you have an edge; a negative one means the payout does not justify the risk, no matter how "certain" the event feels.This guide walks through the formula with a full worked example built on reading prices as probabilities, when positive EV misleads you, and the mistakes that quietly break the calculation. Practise on PolySouq — the leading Arabic prediction market platform — with a balance you fund in USDC on sign-up and real financial risk.What expected value is, and why it comes before every decisionExpected value is the average outcome of a decision if you repeated it hundreds of times under identical conditions. It is not a prediction about any single trade — you can lose a positive-EV trade and win a negative-EV one — but over a run of trades it is the thing that decides your results.In prediction markets on PolySouq each side of a market shows a percentage — its share of the money staked — and that percentage is the market's probability for the event. That makes EV far easier to compute here than in equities: the probability and an estimated payout are displayed for you, and all you need is your own estimate to compare against them.The formula in one lineSay you stake S on YES, the payout if YES wins is R (stake included, an estimate until the market closes), and your own probability estimate is Q:EV = (Q × (R − S)) − ((1 − Q) × S)Which simplifies to EV = (Q × R) − S, and it is zero at your break-even probability, S ÷ R. In other words, your edge is the gap between your probability and your break-even probability — not just the percentage on screen, because the 10% commission on the losing pool puts break-even a little above it. Estimate 70% against a 62.5% break-even and your EV is positive. The rule follows: only enter when your estimate beats break-even by a margin worth having.A full worked example: 00 on a market showing 60% YESA market asks whether Brent crude closes above a given level at month end. The YES pool holds $6,000 and the NO pool $4,000, so the market shows 60% YES. After reviewing production data, inventories and producer statements, you put the true probability at 70% and stake 00 on YES.The maths: if YES wins, you get about 100 × (1 + 4,000 ÷ 6,000 × 0.90) = 60 — a $60 profit; if NO wins you lose the 00. Break-even is 100 ÷ 160 = 62.5%, above the 60% shown because of the commission. So (0.70 × 60) − (0.30 × 100) = 42 − 30 = +2 per 00 staked — a reasonable edge, though only an estimate: the pools keep moving until the market closes, and your own stake shifts them too. Had your estimate been 60%, EV would be (0.60 × 60) − (0.40 × 100) = −$4 — a market that looks fair on screen but loses money once the commission is counted. Neither figure is a guaranteed profit.Edge: the gap between your estimate and break-evenEdge is your entire justification for being in a market. The displayed percentage is not arbitrary; it is the distilled view of everyone who has staked money, which is why it is accurate more often than not. Disagreeing with it is a claim that you know or read something better than the crowd — and that claim needs backing. It also has to be big enough to clear your break-even, which sits above the displayed percentage.So ask before every trade: why am I right and the market wrong? If you cannot name something specific — fresher data, a sharper reading of the settlement rule, a source others do not follow — your supposed edge is probably overconfidence rather than information.Steps to calculate EV before entering

Read the settlement rule first: what is the official source, and exactly when does it resolve? An estimate without the rule is worthless.

Set your estimate before looking at the displayed percentage: looking first anchors your number to the market without you noticing.

Start from the base rate: how often has a similar event happened historically? That is your starting number before any adjustment.

Adjust only for genuinely new information: one headline rarely moves a probability from 30% to 70%.

Compute break-even and EV: stake ÷ estimated payout is your break-even probability; your estimate × the payout, minus the stake, is your expected value — an estimate, since the payout moves until the market closes.

Require a minimum: many traders will not enter unless their estimate beats break-even by 7–10 percentage points, to absorb their own estimation error.

When positive EV is still a bad decision

When the position is too large: positive EV does not help if one trade empties your balance before the law of large numbers can work for you.

When liquidity is thin: one large stake can swing the pool split and eat the whole edge before settlement.

When the estimate rests on a single source: an edge built on unconfirmed news is not an edge, it is doubled risk.

When the horizon is very long: a market resolving months out ties up your stake until settlement — funds you could deploy in clearer opportunities.

When you repeat the same underlying bet: five markets that all hinge on one cause are really one position at five times the size.

Common mistakes that break the EV calculation

Confusing a "big payout" with "good value": a 10× payout breaks even only at 10%, so it is expensive if the true probability is 4%.

Rounding probabilities to 0% or 100%: rare things happen, and certain things sometimes miss the resolution deadline.

Counting the upside without the downside: EV always subtracts the losing branch, and dropping it makes every trade look like a winner.

Revising the estimate after entering to justify the position: that is confirmation bias, not analysis — see trading psychology.

Ignoring the cost of time: a 5-point edge in a market resolving in six months is weaker than it looks.

Practise the calculation on PolySouqThe fastest way to internalise expected value is to repeat it on real markets with small, fixed stakes. On PolySouq you fund your account in USDC after a free sign-up — real financial risk — so you can log your estimate, the displayed percentage and the estimated payout before every trade, then compare after settlement to see whether your estimates are genuinely calibrated or simply optimistic.As results accumulate, the quality of your calculations shows up directly in your leaderboard ranking — a far more honest measure of performance than a personal impression.The bottom line: one number separates guessing from analysisA disciplined trader does not ask "will this happen?" but "is my true probability above or below my break-even probability?" That small shift in the question is the whole difference. Calculate EV before every trade, insist on a sufficient edge, and size the position according to sound capital management.Trading prediction markets on PolySouq is lawful and legitimate, and it is halal: no riba, no leverage, settlement by an official source, and decisions driven by information and probability rather than chance.Frequently asked questionsWhat is expected value in prediction markets?It is the average return a decision produces if repeated many times. You calculate it as your probability × the profit if right, minus your probability of being wrong × the loss if wrong: with a 70% estimate, 00 on YES at an estimated 60 payout is worth about +2 in expected value.How do I calculate expected value step by step?Read the settlement rule, set your probability estimate before looking at the displayed percentage, start from the historical base rate, adjust only for genuinely new information, then multiply your estimate by the estimated payout and subtract your stake (or compare your estimate with your break-even: stake ÷ payout). A positive result means an edge.What is the difference between the displayed percentage and expected value?The displayed percentage is each side's share of the money staked — the crowd's current view. Expected value is the gap between your own estimate and your break-even probability (stake ÷ estimated payout), which sits above the displayed percentage because of the 10% commission on the losing pool. The percentage is given; the expected value is your judgement about it.Does positive expected value guarantee a profit?No. It only guarantees a statistical advantage over a long run of trades, and you can lose many positive-EV trades along the way. That is why trade count and careful position sizing matter. The payout shown before settlement is also an estimate that moves as money enters, so your EV moves with it.How large should the gap be before entering?There is no absolute rule, but many traders require their estimate to beat break-even by at least 7–10 percentage points, because smaller gaps are swallowed by estimation error and thin liquidity.What are the most common probability mistakes?Treating a big estimated payout as automatically good value, comparing your estimate with the displayed percentage instead of your break-even, ignoring the historical base rate, overreacting to a single headline, and revising the estimate after entering in order to justify the position.Do I need advanced maths to calculate EV?No. One multiplication and one subtraction is enough. The hard part is not the arithmetic but estimating the probability honestly, which improves with repetition and post-settlement review.How can I practise EV — and your capital is at risk?Sign up free on PolySouq, fund your balance with USDC on Polygon (from 0), and log your estimate alongside the displayed percentage and estimated payout before each trade. Trading is in real money — and your capital is at risk.Start trading related markets

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Disclaimer: Prediction markets are a legal and legitimate way to trade information about the outcomes of future events. However, trading carries risk and you may lose the full amount you trade — so only trade what you can afford to lose. This content is educational and is not financial or investment advice.