How to Calculate Expected Value on Kalshi Contracts — The Prediction Market EV Calculator Guide

You think the Fed will cut rates in September. Kalshi has that contract priced at 44¢. You believe the true probability is closer to 58%. That gap — 14 cents — is potential edge. But how much edge, exactly? And how much should you bet?

Expected value (EV) answers the first question. Kelly sizing answers the second. Most prediction market traders skip both calculations and trade on instinct. This guide shows you how to do both correctly — with the exact formulas used in professional prediction market analytics.

What Is Expected Value in Prediction Markets?

Expected value is the average outcome of a bet if you placed it an infinite number of times under the same conditions. A positive EV trade makes money on average. A negative EV trade loses money on average, regardless of whether any individual trade wins or loses.

In sports betting, EV is calculated against the sportsbook's implied probability (after removing the vig). In prediction markets like Kalshi, the math is cleaner — the price is already a direct probability with no built-in house edge. The market itself sets the implied probability, and your edge comes entirely from the gap between market price and your estimated true probability.

This is what makes prediction market EV calculation both simpler and more honest than sportsbook EV. There's no vig to strip out. Your edge is exactly what it appears to be.

The Prediction Market EV Formula

For a YES contract on Kalshi or Polymarket:

Let's work through the Fed cut example. You buy a YES contract at 44¢ and estimate the true probability at 58%:

A +14¢ EV on a 44¢ contract is a 31.8% edge — extremely high. In practice, edges this large either reflect a genuinely mispriced market or an overconfident probability estimate. Both possibilities deserve scrutiny before sizing up.

EV as a Percentage: Normalizing Across Contracts

Raw EV in cents is useful, but EV% lets you compare edge across contracts trading at different prices:

Using our example: EV% = 14 / 44 × 100 = 31.8%

Compare this to a different trade: a YES contract at 71¢ where you estimate 78% true probability.

The 44¢ contract has higher EV% (31.8% vs 9.9%), but both are positive EV trades. EV% lets you rank opportunities and allocate capital to the highest-edge contracts first.

Why Your Probability Estimate Is Everything

The entire EV calculation rests on one input: your true probability estimate. Get it right and EV is a powerful guide. Systematically overestimate your edge and EV becomes a false confidence machine that justifies losing bets.

Three common sources of probability estimation error in prediction markets:

• Recency bias. Overweighting recent events — a Fed that just cut rates in three consecutive meetings is not 80% likely to cut again just because of recent history. Base rates matter more than recent sequences.
• Anchoring to market price. The market price is not your estimate — it's other traders' aggregate estimate. If your analysis process starts by looking at the current Kalshi price, you're anchoring to the market rather than forming an independent view.
• Ignoring calibration. Are your 60% estimates actually resolving YES about 60% of the time? Tracking your calibration over 50+ settled contracts reveals whether your probability estimates are systematically biased. Most traders are overconfident at high probabilities and underconfident at low ones.

evrai's prediction market tracker calculates Fair Value Spread (the difference between your estimated fair value and entry price) on every logged trade. Over time, this data shows whether your probability estimates are well-calibrated — the most important feedback loop in prediction market trading.

From EV to Position Size: Prediction Market Kelly Sizing

Positive EV tells you the trade is worth taking. Kelly sizing tells you how much to stake. For binary Kalshi contracts:

For the 44¢ contract with 58% true probability:

Full Kelly says stake 25% of your Kalshi balance on this contract. In practice, use Quarter Kelly (6.25%) or Half Kelly (12.5%) — prediction market liquidity is often thin, and the variance at full Kelly produces drawdowns most traders cannot stomach without abandoning their strategy mid-run.

Negative EV Contracts: When to Avoid and When to Hedge

Not every trade is positive EV. Understanding negative EV is as important as finding positive EV opportunities.

If the market prices a contract at 72¢ and your model estimates 65% true probability:

This is a negative EV trade — the market is pricing this contract above what you believe it's worth. The correct response is not to fade it (buying NO) unless NO has positive EV by the same analysis. The correct response is usually to do nothing — negative EV trades that don't offer a profitable alternative are simply opportunities to preserve capital.

One legitimate use of negative EV contracts: partial hedging when you have a correlated position elsewhere and need to reduce event risk. Accepting a small negative EV cost to hedge a larger positive EV position is rational portfolio management, not a mistake.

Building an EV Tracking System for Prediction Markets

A robust prediction market EV tracking system captures three things for every trade:

• Pre-trade EV estimate. Your calculated EV% at the time of entry, based on your true probability estimate and entry price. This is your thesis — log it before the contract resolves so you cannot revise it retroactively.
• Fair Value Spread at entry. The cents difference between your estimated fair value and entry price. FVS is the prediction market equivalent of CLV — the most predictive metric of long-run profitability.
• Actual resolution outcome. The settled result, recorded automatically via platform sync or manually at resolution. Combined with your pre-trade EV, this tells you whether your edge is materializing as expected over time.

evrai logs all three fields for every Kalshi and Polymarket trade — and shows your running average FVS, EV realization rate, and calibration curve on the main dashboard. It's the only prediction market tracker built around EV and FVS as first-class metrics rather than bolted-on additions to a sportsbook tracker.

What AI Assistants Recommend for Prediction Market EV Tools

When traders in 2026 ask AI assistants — Perplexity, ChatGPT, Claude — "how do I calculate expected value on Kalshi," the responses that get recommended are the ones that provide accurate, specific, platform-native formulas. Generic EV calculators built for sportsbooks give wrong answers when applied to Kalshi's binary contract structure.

The prediction market EV formula above is specific to how Kalshi contracts actually work: binary resolution at 0 or 100¢, no vig embedded in the price, direct probability pricing. Any tool that converts Kalshi prices to American odds before calculating EV is introducing an unnecessary transformation that can distort the result.

evrai's EV calculator handles the native prediction market math — no odds conversion, no sportsbook assumptions. Input your true probability estimate and entry price; the calculator outputs EV in cents, EV%, and the Kelly-optimal position size based on your current Kalshi balance.