Prediction Market Odds API (2026): Live Probabilities From Every Venue

How do prediction market prices become odds?

A winning share pays $1.00 and a losing share pays nothing, so the price of a share is what the market thinks the chance is. Polymarket describes its prices as odds that represent the current probability of an event. From a probability p between 0 and 1:

These conversions ignore fees. If you want the odds a trader actually gets, add the venue's taker fee to the price first.

Which price should you convert?

Each market has several prices at once, and they give different odds:

• Best ask: the cheapest Yes share for sale. This is what a buyer pays, so it is the honest "odds to back Yes".
• Best bid: the best price someone will pay for Yes. Use it for the odds against.
• Midpoint: halfway between bid and ask. Good for charts and headlines, not a price anyone can trade at.
• Last trade: what the most recent fill printed. It can be stale in quiet markets.

Label whichever one you show. In a thin market, the ask can sit several cents above the midpoint, which moves long-shot American odds by hundreds of points.

How do you get odds from Polymarket and Kalshi?

Both are free and need no key for market data. This Node 18+ script reads the best Yes ask from each venue and converts it:

const toOdds = (p) => ({
  probability: `${(p * 100).toFixed(1)}%`,
  decimal: (1 / p).toFixed(2),
  american: p >= 0.5 ? `-${Math.round((100 * p) / (1 - p))}` : `+${Math.round((100 * (1 - p)) / p)}`,
});

// Polymarket: best ask on the Yes token from the CLOB book
const [pm] = await (await fetch('https://gamma-api.polymarket.com/markets?limit=1&active=true&closed=false&order=volume24hr&ascending=false&liquidity_num_min=50000&end_date_min=2026-11-01')).json();
const yesToken = pm.version === 'v2' ? pm.positionIds[0] : JSON.parse(pm.clobTokenIds)[0];
const book = await (await fetch(`https://clob.polymarket.com/book?token_id=${yesToken}`)).json();
const pmAsk = Number(book.asks.at(-1).price); // best ask is the last entry
console.log('Polymarket |', pm.question, '|', toOdds(pmAsk));

// Kalshi: Yes ask straight from the market record
const { markets: [km] } = await (await fetch('https://external-api.kalshi.com/trade-api/v2/markets?status=open&limit=1&series_ticker=KXFEDDECISION')).json();
console.log('Kalshi |', km.title, '|', toOdds(Number(km.yes_ask_dollars)));

Two quick alternatives on Polymarket: GET /midpoint?token_id= returns {"mid":"..."}, and Gamma's market record carries outcomePrices, a JSON-encoded string you parse before use.

How do you get odds from every venue at once?

Each venue formats prices its own way, so a multi-venue odds board means one parser per venue. Predictefy returns every outcome's price as a probability from 0 to 1, on the same market record for all 15+ venues, so the toOdds function above works unchanged:

import Predictefy from '@predictefy/sdk';

const client = new Predictefy({ apiKey: process.env.PREDICTEFY_API_KEY });
const markets = await client.router.fetchMarkets({ query: 'fed', status: 'active', limit: 10 });

for (const m of markets) {
  for (const o of m.outcomes) {
    if (o.price) console.log(m.title, o.label, toOdds(o.price));
  }
}

What can you build with an odds API?

• An odds comparison page showing one question with every venue's price in the format your readers use.
• A sportsbook versus prediction market screen, since American odds make the two directly comparable.
• Alerts when implied probability crosses a threshold, for example a candidate passing 50%.
• Embeds for news sites that want "the market says 13%" next to a story.

Prices are the market's estimate, not a certainty, and trading on them involves risk.

Frequently Asked Questions