Prediction Odds Helper

Appends sportsbook-style odds next to prediction market prices on Polymarket and Kalshi. Shows Decimal, American, and Fractional. Optional fee adjustment (in percentage points) to model effective probability before conversion.

Install (Chrome / Brave / Edge)

1. chrome://extensions → toggle Developer mode (top-right).
2. Load unpacked → select this folder.
3. Visit Polymarket or Kalshi; you’ll see small badges next to prices. Use the Odds pill (bottom-right) to change formats or fee adj.

What it does

• Finds text like 55% or $0.55 and computes:
  • Decimal: 1 / p
  • American: if p >= 0.5 → -(p/(1-p))*100, else ((1-p)/p)*100
  • Fractional: (1 - p) / p as a simplified fraction (approx.)
• Works on SPA updates using a MutationObserver.
• Avoids duplicate badges via a data-attribute.