Expected Value Is the Only Number That Actually Matters in Prediction Markets
People argue endlessly about which prediction market has the better interface, the deeper liquidity, or the friendlier fees. Those things matter at the margins. But the number that actually determines whether you make money over time is expected value; and a surprising number of active bettors have never actually calculated it.
Expected value is the average outcome of a bet if you ran it a very large number of times. It is not what happens on any single bet; a coin flip that pays 3 to 1 has great expected value even though you will lose half the time. What matters is whether the payout is fair compensation for the probability you are taking on. If you are consistently finding bets where the payout overcompensates you for the risk, you will make money. If you are consistently finding the opposite, you will lose money slowly and wonder why.
The FatNarwhal Expected Value tool makes this calculation instant. Give it your probability estimate, the payout structure, and your stake, and it tells you your EV, your edge over the market, and your expected profit per dollar wagered.
How to Use It
Head to fatnarwhal.com/expected-value and set up a position.
Say there is a market asking whether the Fed will cut rates at the next meeting. You think there is a 45% chance of a cut. The market on Polymarket is pricing it at 38%. The payout is $1 per share, and shares are trading at $0.38.
Enter your probability (0.45), the market price (0.38), and your stake. The tool works out your edge (your probability minus the market's implied probability), your expected profit if you are right about the true probability, and your expected value per dollar risked.
In this case your edge is 7 percentage points and your EV is positive. The market is underpricing this outcome by your estimate, so every dollar you put in has positive expected value. The tool shows you this clearly so you can make the decision with eyes open.
Change the numbers and watch what happens. Drag your probability estimate down toward the market price and the EV collapses. Push it below the market price and the EV goes negative; now the market is overpricing the outcome relative to your view and you either pass or consider betting the other side.
The Math Behind It
Expected value is a classic piece of probability theory and the formula is simple:
Where p is your estimated probability the event resolves Yes, profit_if_win is what you clear if it does (payout minus your stake), and the second term is your loss if it resolves No.
If EV is positive you have a positive-expectation bet; if EV is negative you do not. Your edge over the market is just the difference between your probability estimate and the market's implied probability. A market priced at $0.38 implies a 38% probability, so if you think the true probability is 45% your edge is 7 percentage points.
The key insight is that edge times volume equals profit over time. Find small edges consistently across many markets and let the law of large numbers do its job. That is the entire strategy of serious prediction market traders.
When to Use It and When Not To
Use it every single time before you place a prediction market position. There is genuinely no reason not to; it takes fifteen seconds and grounds the decision in math instead of feeling. Use it to compare multiple open positions and prioritize the ones with the highest edge per dollar. Use it to check whether a market you find intellectually interesting is actually worth trading.
The tool will not help you if your probability estimate is bad. Garbage in, garbage out; if your 45% estimate is actually closer to 35% then your "positive EV" trade is negative in reality. The math is only as good as your probability model.
Also worth noting; this is a single-bet EV calculator. It tells you whether the bet has positive expected value, not how much to bet. For that you want the Kelly Criterion tool, which sizes your position to maximize long-run growth given your edge.
Try It
Find a market on any prediction platform where you have a view that differs from the consensus. Go to fatnarwhal.com/expected-value, plug in your estimate and the market price, and see what the numbers actually say.
The point of prediction markets is to find spots where you see something the market does not. This tool tells you whether you have found one.