Cross-venue methodology
Exchange APIs report different native units. Prediction Market News does not add those raw values together. We normalize both venues to a shared economic definition before ranking markets or calculating a cross-venue total.
Open interest
Open YES market value
Volume
Premium traded in USD
Aggregation
Unique conditions by event
The problem
Both exchanges use the words volume and open interest, but their APIs do not expose the same economic quantity. Raw addition would favor the venue whose reporting unit produces the larger-looking number.
| Metric | Polymarket API | Kalshi API | Displayed here |
|---|---|---|---|
| Open interest | Collateral locked per condition from /oi, conventionally valued at the full $1 payout. | Outstanding contracts in open_interest_fp, with a $1 maximum payout per contract. | Open units × current YES midpoint, summed once per condition. |
| Volume | Traded premium in dollars from Gamma, including event-level lifetime and rolling-window totals. | Native API headline fields are contract counts, not necessarily dollars traded. | Premium traded in dollars on both venues. |
Open interest
This is an analytical normalization, not a replacement for either exchange's native risk or collateral figures. It is chosen because the same formula can be applied to both venues without making cheap longshots look artificially enormous.
Canonical event formula
normalized OI = Σ (open YES unitsi × midpointi)
The sum runs over unique binary conditions or Kalshi child markets inside the event. A two-cent longshot therefore contributes two cents per open unit—not its full one-dollar maximum payout.
condition collateral × YES midpoint
outstanding contracts × YES midpoint
Volume
Contract counts alone are not dollar volume. We value the outcome token actually taken in each fill, matching Polymarket's premium-traded convention.
reported traded USD
count × taker outcome price
Why normalization matters
Old gross-notional OI
$132M
Comparable OI now
$2.45M
Comparable total volume
$14.4M
The old Kalshi conversion multiplied every open golfer contract by its $1 maximum payout. On a board containing more than 150 mostly low-priced outcomes, that made penny contracts look like dollars of deployed capital. Price-valuing each open YES position removes the mechanical 50× inflation of a two-cent contract while preserving its actual current exposure.
Quality controls
Comparable does not mean every source has identical coverage. We expose gaps rather than silently substituting contract counts or maximum payout.
The ranking cohort receives per-condition /oi values and current midpoints before event aggregation.
Rows outside that cohort use event collateral × mean outcome midpoint. This preserves units but is less precise than condition-level data.
Top-50 OI events and strict cross-venue matches page through fills and value each trade at its taker-side price.
After the trade-page cap, daily candles estimate lifetime premium as contracts × mean YES price. Because candles omit taker side, this is an approximation.
Kalshi events outside the enrichment cohort show N/A. We do not relabel native contract counts as dollar volume.
Every snapshot carries generated and price-freshness timestamps. Static publishes remain frozen until the next verified refresh.
Sports · snapshot 281h ago
$69.1M normalized OI · $4.31B premium volume
Politics · snapshot 281h ago
$12.8M normalized OI · $665M premium volume
Politics · snapshot 281h ago
$13.5M normalized OI · $74.8M premium volume
Politics · snapshot 281h ago
$4.76M normalized OI · $1.26B premium volume
Sports · snapshot 281h ago
$9.56M normalized OI · $45M premium volume
No. The site converts both venues to the same analytical definition: the current market value of open YES positions. Native Polymarket collateral and native Kalshi contract counts remain useful venue-specific metrics, but they are not directly comparable.
For priority markets, each condition's collateral value from Polymarket's Data API is multiplied by that condition's current midpoint, then unique conditions are summed at event level. Tail rows estimate the same quantity from event collateral and the mean outcome midpoint.
Each child market's open_interest_fp contract count is multiplied by its current YES midpoint, then child markets are summed at event level. This avoids treating a two-cent longshot contract as one full dollar of capital.
Both venues use traded premium in dollars. Polymarket supplies traded USD through Gamma. Kalshi fills are reconstructed as contract count multiplied by the taker outcome's YES or NO price, so a 98-cent NO purchase counts as 98 cents rather than the complementary two-cent YES price.
Yes. Polymarket tail open interest uses an event-level estimate when exact condition data is outside the priority cohort. Deep Kalshi trade histories may use daily candlesticks, and Kalshi long-tail volume remains unavailable rather than being presented as exact.
Politics · snapshot 281h ago
$4.62M normalized OI · $679M premium volume