How I Built a Real-Time Whale Tracker for Polymarket in a Weekend

Prediction markets just hit $3.6B in volume. I wanted to know what the biggest traders were betting on — in real time. So I built WhaleTrack.

Here's how it works under the hood.

The Problem

Polymarket has a public leaderboard. But it only shows P&L totals — not what whales are currently betting on, not their recent activity, not their win rate. If you want to follow smart money, you're flying blind.

I wanted something that answered: what are the top traders doing right now?

The Stack

Vanilla JS frontend (no framework, keeps it fast)

Vercel serverless function as a backend proxy (avoids CORS issues hitting Polymarket's API directly)

Polymarket's public data API — no auth required

Step 1: Finding the Whales

Polymarket exposes a leaderboard endpoint:

This returns their recent trades — market name, size in USDC, timestamp. I parse this into a live activity feed that refreshes every 60 seconds.

Step 3: Calculating Win Rate (the tricky part)

This one took some digging. The positions endpoint returns each position a whale holds, but figuring out won vs lost isn't obvious.

The key is the redeemable flag:

const won = positions.filter(p => p.redeemable === true).length;

const lost = positions.filter(p =>

p.currentValue === 0 && p.redeemable === false

).length;

const winRate = Math.round((won / (won + lost)) * 100);

redeemable: true = market resolved in their favour (shares worth $1)

currentValue: 0 + redeemable: false = market resolved against them

Simple once you know it — but it took a few wrong attempts with cashPnl (always negative, not useful).

Step 4: The Whale Alert Banner

The feature people love most. Every 60 seconds when activity refreshes, I check for trades over $5,000 placed in the last 10 minutes:

function checkBigTrades(activity) {

const big = activity

.filter(t => t.usdcSize >= 5000)

.sort((a, b) => b.usdcSize - a.usdcSize)[0];

if (!big) return;

const age = Math.floor(Date.now() / 1000) - big.timestamp;

if (age > 600) return; // older than 10 min, skip

showWhaleAlertBanner(big);

}

When it fires, a green banner slides down with the whale name, market, and amount. Auto-dismisses after 12 seconds.

First time I saw it fire live with a $28K bet — genuinely exciting.

Results

129+ users in the first few days

Zero ad spend

Traffic from Twitter, Reddit, Quora

What's Next

More whale wallets (suggestions welcome)

Click-through to open the same market on Polymarket directly

Email/push alerts for big trades

Check it out: whaletrack.app

All feedback welcome — especially if you spot a whale I'm missing.