betting-calculators

Free, dependency-free JavaScript utilities for common sports-betting math — odds conversion, parlays/accumulators, hedging, market hold and expected value.

No frameworks, no build step, ~3 KB. Works in the browser and in Node.

Live demo & full tool suite: https://correctbetting.com/betting-calculators/

What's included

| Function | What it does | |----------|--------------| | americanToDecimal / fractionalToDecimal | Convert odds to decimal | | decimalToAmerican | Convert decimal odds back to American | | toDecimal(value, format) | Convert any supported format to decimal | | impliedProbability | Decimal odds → implied win probability | | singleBet | Payout & profit for one selection | | parlay | Combined odds, payout & profit for an accumulator | | hedge | Stake needed to lock in a guaranteed result | | hold | Bookmaker margin (overround) of a market | | value | Expected value & edge of a bet |

Install

Just drop the file in:

<script src="src/betting-calculators.js"></script>

Or in Node:

const BettingCalc = require("./src/betting-calculators.js");

Usage

// Convert American odds to decimal
BettingCalc.americanToDecimal(150);      // 2.5
BettingCalc.americanToDecimal(-200);     // 1.5

// 3-leg parlay on a $10 stake
BettingCalc.parlay(10, [2.0, 1.8, 2.5]);
// { combinedOdds: 9, payout: 90, profit: 80, impliedProbability: 0.111... }

// Bookmaker hold on a two-way market priced 1.91 / 1.91
BettingCalc.hold([1.91, 1.91]).toFixed(2); // "4.71" (percent)

// Is +100 a value bet if you think it's a 55% shot?
BettingCalc.value(0.55, 2.0, 10);
// { expectedValue: 1, edgePercent: 10, fairOdds: 1.81..., isValueBet: true }

Demo

Open demo/index.html in any browser for a working parlay calculator built on top of the library. A polished version of every tool (parlay, odds converter, hedge, hold and value) is hosted at correctbetting.com/betting-calculators.

Odds formats

• Decimal — 2.50
• American — +150, -200
• Fractional — 3/2

License

MIT — free to use, modify and ship.