Every live number on Prediction Tape comes from the public data feeds that Polymarket and Kalshi publish for developers. This page explains what we collect, how often, how each figure is calculated, and where the numbers can mislead. It describes the system as it ran in October 2026.
Where the data comes from
| Feed | What we take | How often |
|---|---|---|
| Polymarket Gamma API | Active events, up to the 800 with the most 24-hour volume, with their markets and prices | Every 5 minutes |
| Kalshi Trade API | Open events with their markets; we keep the 500 with the most 24-hour volume | Every 5 minutes |
| Polymarket Data API | The public leaderboard: top 50 accounts by profit or volume, by period and category | Cached for 10 minutes |
A scheduled job on our server fetches the feeds; pages never call Polymarket or Kalshi when you load them. Open pages then check our server for new prices every 60 seconds. Board rows keep each event’s eight leading outcomes; event tables fetch the full list of outcomes and cache it for 10 minutes. If a feed fails, we keep showing the last good data for that venue rather than an empty board, so prices can be older than the refresh time suggests.
Prices and implied chances
- Polymarket: the outcome price reported by Polymarket’s market API.
- Kalshi: the last traded price. Where an outcome hasn’t traded, we use the midpoint between the best bid and the best ask.
- Chance: a share pays $1 if the outcome happens, so a price of 62¢ reads as a 62% chance. That’s the market’s view, not ours, and it includes no adjustment for fees or for the spread between buyers and sellers.
- Resolved markets: events whose leading outcome is priced below 2% are left off the boards, because that usually means the market has effectively been decided.
Change, volume and ranking
- 24h change: Polymarket’s reported one-day price change; for Kalshi, the current price minus the price Kalshi reports for 24 hours earlier. Shown in percentage points.
- Volume: as each venue reports it, US dollars for Polymarket and contracts for Kalshi. Each Kalshi contract settles at $1.
- Ranking: to compare the two on one board, we rank by approximate dollars traded in 24 hours. For Kalshi that’s contracts multiplied by the leading price, with the price held between 5% and 95% so very cheap or very expensive contracts don’t distort it.
- Biggest moves: outcomes whose price moved at least 2 points (and no more than 40) in 24 hours, in markets with at least $100,000 traded, priced between 5% and 95%. Single games in play, weather, tweet counts and other same-day markets are excluded.
Polymarket vs Kalshi price gaps
- Matching: we compare only events we’ve checked individually and confirmed are the same question on both venues. Outcomes are matched by name (candidate, team or party); a yes/no market is matched to its yes/no counterpart.
- Gap: the absolute difference between the two prices, in percentage points.
- Exclusions: outcomes whose average price across the two venues is below 4% or above 96% are left out, because gaps there are mostly noise from thin order books.
- Weekly figures: every hour we record the ten largest gaps. The weekly gaps page shows the largest gap seen for each outcome over the last seven days; the log keeps 35 days.
- Caveat: a gap is not a guaranteed profit. The two markets can settle under different rules, the price shown may not be the price you can buy at, and fees apply on both sides.
The Polymarket leaderboard
Our leaderboard shows Polymarket’s own public leaderboard: the top 50 accounts by profit or by volume, for the day, week, month or all time, overall or in politics, sports or crypto. Profit includes unrealised gains and losses on open positions, as Polymarket reports it. Account names are what each account has chosen. Once a day we save the all-time profit top 50, which powers the leaderboard movers page.
Odds and calculators
- Decimal odds = 1 ÷ price. American odds: for a price of 50% or more, −100 × price ÷ (1 − price); below 50%, +100 × (1 − price) ÷ price. Fractional odds are the decimal odds minus one, as a fraction.
- The no-vig calculator converts each outcome’s odds to a chance and scales them so they add up to 100%, which removes the bookmaker’s margin.
Dated snapshots
Some pages are built from snapshots we take ourselves: the hourly gap log, the daily leaderboard save, and a one-off record of every House, Senate and governor market at 4 p.m. ET on November 3, 2026, used for our midterms accuracy review. Each data page states when its snapshot was taken and how it was produced.
Known limits
- Thin markets can show prices set by a single small trade.
- Feeds can be delayed, incomplete or briefly wrong, and we may not notice straight away.
- Matching outcomes by name can fail when venues word them differently; where it does, we show the venues in separate tables rather than force a match.
- We track only the larger events on each venue, so smaller markets may be missing.
- The venue’s own page is always the final word on a market’s price and on how it resolves.
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