How I Built a Chrome Extension for Smarter Betting: The Value Bet Calculator

The Idea

As a sports fan and data geek, I often wondered:

Is this bet actually worth it?

That’s how the Value Bet Calculator extension was born — a simple browser tool that helps you instantly calculate the expected value (EV) of a bet using the formula:

Value = (Estimated Probability × Bookmaker Odds) - 1

How It Works

You can use it in two modes:

Manual Mode – Just input your odds and estimated probability
Auto Mode – Select a match, and it pulls real-time data from the odds API

Here’s what the manual calculator looks like:

<input type="number" placeholder="Bookmaker Odds" />
<input type="number" placeholder="Estimated Probability (%)" />
<button>Calculate</button>

And here’s the core logic:

function calculateValue(probability, odds) {
  const value = (probability / 100) * odds - 1;
  return Math.round(value * 1000) / 10; // round to 1 decimal
}