Implied probability calculator & odds converter

Implied probability calculator and odds converter: turn American, decimal or fractional odds into implied probability and back, with the formulas shown.

Turn American, decimal or fractional odds into implied probability, and convert between formats.

The calculator runs in your browser with JavaScript; the formulas it uses are explained below.

An implied probability calculator turns a betting price into the chance of winning that the price implies. Enter American odds such as -135 or +150, decimal odds such as 1.74, or fractional odds such as 20/27, and you get the implied probability along with the same price in the other two formats. It also works in reverse: enter a probability and get the matching odds. Below: the inputs and outputs, the formulas, two worked examples, common mistakes, and how to get implied probabilities for every book from the API.

How to use the implied probability calculator

The calculator takes one input: a price in any common format, or a probability. American odds carry a sign (-135, +150), decimal odds are a number above 1 (1.741, 2.40), fractional odds use a slash (20/27, 7/5), and a probability ends in a percent sign (57.45%).

It returns four values: the implied probability, the American price, the decimal price and the closest fractional price. That makes it an odds converter as much as a probability tool: US sportsbooks post American odds, European audiences expect decimal, and fractions still appear in UK racing.

A price only tells you what a book is offering. The implied probability includes the book's margin, so the probabilities of both sides of a market add up to more than 100%. To take the margin out, use the no vig calculator.

The formulas

Every conversion runs through decimal odds: the total return per unit staked, stake included.

American odds converter formulas

Negative American odds (the favorite), e.g. -135:
  implied probability = 135 / (135 + 100) = 0.5745
  decimal odds        = 1 + 100 / 135     = 1.741

Positive American odds (the underdog), e.g. +150:
  implied probability = 100 / (150 + 100) = 0.4000
  decimal odds        = 1 + 150 / 100     = 2.500

Going back from decimal to American: if the decimal price is 2.00 or more, American = (decimal − 1) × 100 with a plus sign. Below 2.00, American = −100 / (decimal − 1). Even money is +100, which means the same as -100, and no American price sits between the two.

Decimal odds to probability

Converting decimal odds to probability is a single division: implied probability = 1 / decimal. A price of 2.40 implies 1 / 2.40 = 41.67%, and the same division takes you back: 1 / 0.4167 = 2.40.

Fractional odds

Fractional odds a/b state the profit per unit staked, so decimal = 1 + a/b and implied probability = b / (a + b). 7/5 is decimal 2.40, and 5 / 12 = 41.67%.

Worked example 1: one price in every format

A book posts a favorite at -135.

  1. Implied probability: 135 / 235 = 57.45%.
  2. Decimal: 1 + 100 / 135 = 1.741. A $100 stake returns $174.07 in total, $74.07 of it profit.
  3. Fractional: 100 / 135 reduces to 20/27.

Typing 57.45% or 1.741 into the implied probability calculator gives the same row back, a quick way to test your own conversion code.

Worked example 2: both sides of a market

The same game has the underdog at +115: 100 / 215 = 46.51%, decimal 2.15, fractional 23/20.

Add the two sides: 57.45% + 46.51% = 103.96%. The 3.96% above 100 is the overround, the margin the book has built in. Expressed as hold, the share of the money the book expects to keep on a balanced market, it is 1 − 1 / 1.0396 = 3.81%.

Neither 57.45% nor 46.51% is the true chance of winning. They are the prices the book charges. The no-vig fair odds method divides each side by 1.0396, which gives fair probabilities of 55.26% and 44.74%.

Common mistakes

Getting implied probabilities from the API

The API does the same conversion as the implied probability calculator for every price on the board. Add format=probability to any odds endpoint and prices come back as implied probabilities, margin included. This request pulls NBA moneylines from three books:

curl "https://api.preciseapi.com/v1/odds?league=nba&markets=moneyline&books=pinnacle,draftkings,fanduel&format=probability" \
  -H "Authorization: Bearer $PRECISEAPI_KEY"

Each market in a book's markets array then carries prices between 0 and 1:

{
  "id": "mkt_1fhfizlqj3n",
  "key": "moneyline",
  "period": "full_game",
  "outcomes": [
    { "id": "sel_0tn021zwi2l", "name": "Boston Celtics", "side": "home", "price": 0.5745 },
    { "id": "sel_15ey8l4dfho", "name": "New York Knicks", "side": "away", "price": 0.4651 }
  ],
  "suspended": false,
  "updated_at": "2026-11-11T18:04:12.000Z"
}

format=decimal and the default format=american work the same way, so one call can feed a converter, a model or an odds screen. A /v1/odds call costs 2 credits; the odds reference lists every parameter and the formats guide shows the rounding. To get the best price per side across every book, the odds comparison API returns it in one request, and the no vig odds API page shows how to turn these probabilities into fair prices.

Frequently asked questions

What does implied probability mean?

It is the break-even win rate of a price: the probability at which a bet at those odds returns exactly its stake on average. It includes the book's margin, so it slightly overstates the true chance of each side.

How do I convert American odds to probability?

For a negative price, divide its absolute value by itself plus 100: -150 is 150 / 250 = 60%. For a positive price, divide 100 by the price plus 100: +150 is 100 / 250 = 40%.

Why do both sides of a market add up to more than 100%?

Because the book builds a margin into both prices. The excess is the overround: a -110 / -110 market adds up to 104.76%. A no-vig calculation removes it.

Can the API return implied probabilities directly?

Yes. Add format=probability to /v1/odds, /v1/props, /v1/futures, /v1/best-lines or /v1/history and prices come back as implied probabilities. API access starts on the Edge plan; see pricing.

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