EV Calculator for Polymarket & Kalshi
Enter the market price and your probability estimate. Get edge %, signal, and interpretation — instantly.
FOUNDATIONAL
Expected Value — Is this bet actually good?
What is this?
EV = Expected Value. It answers one question: "If I made this exact bet 100 times, would I make money or lose it?"
A positive EV bet makes money over time even if you lose today. A negative EV bet loses money over time even if you win today. Every casino game has negative EV for you. The goal is to only take positive EV positions — this tool tells you which side of that line you're on.
Real-World Example
→ The Scenario
Polymarket says there's a 55% chance Spain wins Group H. You buy YES at 55¢. But your research says Spain's real probability of winning their group is 72% (easier draw than people think).
EV = (0.72 × 45¢ profit) – (0.28 × 55¢ loss) = 32.4¢ – 15.4¢ = +17¢ EV per dollar
✅Action: +EV means bet it. +17¢ on a dollar is a strong edge. Size it proportionally.
Bottom line: If the EV number is negative, walk away. Doesn't matter how good it feels.
Full guide →If you're asking how to calculate EV on a Kalshi or Polymarket trade: it's the gap between the market price and your own probability estimate, normalized as a percentage of the market price. A contract priced at 28¢ when you believe fair value is 40¢ gives you a +43% edge — BUY territory. The ±5% threshold filters noise: BUY above +5%, SELL below −5%, SKIP between.
Caveat: the edge is only as good as your probability input. A sloppy estimate produces a confident-looking but worthless signal — sharpen the probability before trusting the signal.
Use the EV Calculator inside your AI agent
This tool is also live on our MCP server as calculate_ev. Connect Claude, ChatGPT, Cursor, or Copilot and call it directly — six core calculators are free.
Related Tools
What is Expected Value in Prediction Markets?
Expected Value is the single most important concept in prediction market trading. It answers the question: is the market price wrong enough that I should take a position? If the market says a recession has a 28% chance and your model says 40%, that gap is your edge. But the size of the gap relative to the market price is what actually matters — a 12-point gap at 28¢ is a much larger edge than a 12-point gap at 70¢.
The formula normalizes the gap to the market price, giving you a clean edge percentage. A +25% edgemeans the market is underpricing the event by 25% relative to what you believe is fair value. That's the number that tells you whether to trade — and how confident to be when you do.
When to BUY, SELL, or SKIP
The ±5% threshold filters out noise. Markets are rarely perfectly efficient, but small gaps often close before you can act on them. A BUY signal at +5% means you have meaningful edge. A BUY signal at +20% means the market is significantly mispriced — size accordingly with the Kelly Criterion calculator. To surface those large gaps without checking every contract by hand, run the Mispricing Scanner, which ranks the widest market-versus-model divergences across Kalshi and Polymarket in one pass, then bring the strongest candidate back here to confirm the edge before you take a position.
A SELL signal means the market is overpricing the YES outcome relative to your estimate. If you already hold YES contracts, this is a signal to exit. If you don't hold a position, you can consider buying NO — which on Kalshi is simply the inverse of the YES price.
EV vs. the market price
This is different from asking “is the market price low?” A market at 10¢ might be a terrible trade if you think it should be at 8¢. A market at 80¢ might be the trade of the week if you think it should be at 95¢. EV is always relative to your probability estimate — which is why the estimate matters more than anything else. Sharpen your model, and the calculator tells you exactly when to act.
Frequently Asked Questions
How do I calculate EV on a Polymarket trade?
Polymarket prices run 0 to $1.00, so multiply by 100 before entering them — a $0.28 contract is 28 in the EV Calculator. From there the math is identical to Kalshi: edge equals your probability estimate minus the market price, divided by the market price. A 28-cent contract you value at 40 cents is a +43% edge, squarely in BUY territory. The same ±5% threshold applies: BUY above +5%, SELL below −5%, SKIP between.
What does EV mean on a prediction market?
EV stands for Expected Value — not electric vehicles. On a binary prediction-market contract (Kalshi or Polymarket), Expected Value is the average profit per dollar risked given the contract price and your own probability estimate of the outcome. A contract priced at 28¢ that you believe is worth 40¢ carries positive expected value: the market is underpricing the event relative to your estimate, so over many such trades you expect to profit. This calculator turns that gap into a single edge percentage and a BUY / SELL / SKIP signal.
What is the quick answer on calculating EV for a Kalshi or Polymarket trade?
Expected value on a prediction market contract is the gap between the market price and your own probability estimate, normalized as a percentage of the market price. A contract priced at 28¢ when you believe fair value is 40¢ gives you a +43% edge — BUY territory. The ±5% threshold filters noise: BUY above +5%, SELL below −5%, SKIP between. Caveat: the edge is only as good as your probability input. A sloppy estimate produces a confident-looking but worthless signal, so sharpen the probability before trusting the signal.
What is expected value in prediction markets?
Expected value (EV) is the average profit per dollar risked given your probability estimate and the market price. A positive EV means the market is underpricing the event relative to your estimate — a signal to buy. On Kalshi, EV is calculated as: (your probability × payout) minus (1 - your probability) × cost.
How do I use the EV Calculator for Kalshi trades?
Enter the Kalshi market price in cents (e.g. 45 for a 45¢ contract) and your probability estimate as a percentage. The calculator returns your edge percentage and a BUY, SELL, or SKIP signal. A BUY signal means you have positive edge; SELL means the market is overpriced relative to your estimate.
When should I use the EV Calculator vs the Kelly Criterion Calculator?
Use the EV Calculator first to confirm you have a positive edge. If the signal is BUY or SELL, then use the Kelly Criterion Calculator to determine how much of your bankroll to risk. EV tells you whether to trade; Kelly tells you how much.
Is a 10% edge enough to trade a prediction market?
A +10% edge clears the calculator’s ±5% noise threshold, so it registers as a real BUY signal rather than SKIP — but the quality of that number depends entirely on your probability estimate. Treat +5% to +15% as thin edges that demand a calibrated input and a small position; +20% and up is where the market is meaningfully mispriced. Size every signal through the Kelly calculator, not by feel.
Does the EV Calculator work for buying NO contracts?
Yes — a NO contract costs 100 cents minus the YES price, so a 45-cent YES contract means NO trades at 55 cents. To evaluate the NO side, enter 55 as the market price and your probability that the event does not happen. A SELL signal on YES is mathematically the same call as a BUY on NO, since the two are mirror images of a single position.
Get The 7 Oracles' daily edge — subscribe free
No spam. Unsubscribe anytime.