betting-odds-converter

Convert betting odds between decimal, fractional, American and implied probability formats. Includes payout and bookmaker margin (overround) calculators. Zero dependencies, ~2 KB.

🖥 Try it in the browser: Betting Odds Converter (free tool, no signup)

Install

npm install betting-odds-converter

Usage

import {
  toDecimal,
  fromDecimal,
  payout,
  bookmakerMargin,
} from 'betting-odds-converter';

// Parse any format to decimal odds
toDecimal('+150', 'american');     // 2.5
toDecimal('3/2', 'fractional');    // 2.5
toDecimal('40%', 'probability');   // 2.5
toDecimal('2,50', 'decimal');      // 2.5 (comma tolerated)

// Convert decimal odds to every format
fromDecimal(2.5);
// { decimal: 2.5, fractional: '3/2', american: 150, impliedProbability: 40 }

// Payout for a stake
payout(100, 2.5);
// { stake: 100, totalReturn: 250, profit: 150 }

// Bookmaker margin (overround) of a market
bookmakerMargin([1.9, 2.0]);
// { totalImpliedProbability: 102.63…, marginPercent: 2.63…, fairOdds: [1.95, 2.05…] }

API

| Function | Description | |---|---| | toDecimal(value, format) | Parse "decimal", "fractional", "american" or "probability" input to decimal odds. Returns NaN on invalid input. | | decimalToFractional(d) | 2.5 → "3/2" (simplified fraction) | | decimalToAmerican(d) | 2.5 → 150, 1.5 → -200 | | impliedProbability(d) | 2.5 → 40 (percent) | | fromDecimal(d) | All formats at once | | payout(stake, d) | Stake, total return and profit | | bookmakerMargin(oddsArray) | Total implied probability, margin % and fair odds of a market |

Formulas

implied probability = 1 / decimal × 100
fractional          = (decimal - 1) as a simplified fraction
american            = decimal >= 2 ? (decimal - 1) × 100 : -100 / (decimal - 1)
margin (overround)  = (Σ 1/odds - 1) × 100

Related tools

Free, open-source, browser-based tools by the same maintainers:

• Betting Odds Converter — this library, as a web tool
• Provably Fair Verifier — verify crypto casino game outcomes
• House Edge Calculator — true cost of casino games

License

MIT — see LICENSE.