Every betting price answers two questions. How much do I get back if I win? And how likely does the person setting the price think that is? The three common formats answer the first question in different ways and hide the second, which is why the same price looks so different written as -110, 1.91 and 10/11. This guide explains each notation, shows how to convert between them without a calculator, and then gets to the part that matters: reading the probability underneath.
The three formats, one price
Take a price you will meet on the first American sportsbook you open: -110. It is the standard line on point spreads and totals.
- In American odds it reads -110: risk 110 to win 100.
- In decimal odds it reads 1.91: every 1.00 staked returns 1.91, stake included.
- In fractional odds it reads 10/11: win 10 for every 11 staked.
All three describe the same bet. If you stake 110 and win, you get your 110 back plus 100 profit, a total of 210. Decimal odds say the same thing as 110 × 1.91 = 210.1 (the exact decimal is 1.9091, rounded to two places for display). Fractional odds say 110 × 10/11 = 100 profit. The odds converter shows all three side by side for any price you type.
American odds
American odds are the format used by US sportsbooks and the one that confuses newcomers most, because the number changes meaning depending on its sign.
A negative number is the stake needed to win 100. At -110 you risk 110 to win 100; at -200 you risk 200 to win 100; at -500 you risk 500. Negative prices are favourites: the larger the number, the shorter the price and the more likely the bookmaker thinks the outcome is.
A positive number is the profit on a stake of 100. At +150 a 100 stake wins 150; at +400 it wins 400. Positive prices are underdogs: the larger the number, the longer the price.
The two meet at even money, which can be written +100 or -100. Both mean risk 100 to win 100. Convention favours +100, and there is no such thing as -50 or +50: the format skips everything strictly between -100 and +100.
positive: decimal = 1 + american / 100 (+150 → 2.50) negative: decimal = 1 + 100 / |american|
(-110 → 1.9091)
Because the scale is not symmetric, moving from -110 to -105 is a smaller change in price than moving from +110 to +115, and comparing two American prices by subtracting them is unreliable. Convert to decimal or probability before comparing.
Decimal odds
Decimal odds are the default in Europe, Australia and on betting exchanges, and they are the easiest to calculate with. The number is the total return per unit staked, including the stake. Multiply your stake by the price and you have your total return; subtract the stake and you have the profit.
- 1.50 returns 150 on 100: profit 50.
- 2.00 returns 200 on 100: profit 100 (even money).
- 3.50 returns 350 on 100: profit 250.
There is no sign to interpret. Everything below 2.00 is odds-on (a favourite), everything above is odds-against (an underdog), and the price never dips to 1.00 because that would mean no profit at all. Decimal odds also make the implied probability a one-step calculation: one divided by the price.
decimal ≥ 2.00: american = (decimal − 1) × 100 (2.50 → +150)
decimal < 2.00: american = −100 / (decimal − 1) (1.50 → −200)
One thing to watch: the displayed decimal is rounded. 1.91 on a screen is usually the exact -110, which is 1.909090…; 1.91 exactly would be -109.89. Bookmakers display two places and settle on the displayed value, but conversions done from the display can drift by a fraction of a point.
Fractional odds
Fractional odds are the traditional format of British and Irish bookmakers. The fraction is profit over stake: 5/2 wins 5 for every 2 staked, 10/11 wins 10 for every 11, and 1/1, called "evens", wins 1 for every 1.
To get the decimal price, divide top by bottom and add one: 5/2 becomes 2.5 + 1 = 3.50. To get the implied probability, divide the bottom by the sum of top and bottom: 5/2 implies 2 / 7 = 28.57%.
Fractional bookmakers use a fixed ladder of traditional prices rather than every possible fraction, so you will see 6/4 rather than 3/2, 100/30 rather than 10/3, and 11/10 next to 6/5 with nothing in between. Prices step from rung to rung, which makes small moves invisible: a bookmaker who wants to shorten 5/2 slightly has to jump to 9/4, a change of about 1.7 percentage points in implied probability.
5/2 in every format
5/2 → decimal 1 + 5/2 = 3.50 → American (3.50 − 1) × 100 = +250 → implied probability 2 / (5 + 2) = 28.57% → a 100 stake wins 250 and returns 350.
The second question: implied probability
The payout is the easy half of a price. The other half is what the price says about likelihood. Every price implies a probability: the share of the time the outcome would need to happen for a bet at that price to break even.
p = 1 / decimal -110 → 1 / 1.9091 = 52.38% +150 → 1 / 2.50 = 40.00% 5/2 → 1 / 3.50 = 28.57%
This is the number to compare prices with, because it is on a linear scale. Two prices can be compared by subtracting their implied probabilities, and the answer means something: -110 versus -105 is 52.38% versus 51.22%, a 1.16-point difference; +110 versus +115 is 47.62% versus 46.51%, a 1.11-point difference. Those two moves, which look like "five points" in American odds, are nearly the same size.
Implied probability includes the bookmaker's margin. Both sides of a -110 / -110 spread imply 52.38%, and 52.38% plus 52.38% is 104.76%. The 4.76% above 100 is the overround, and it is why the implied probability of a single price slightly overstates the bookmaker's true opinion. The guide to vig and devigging shows how to remove it; for reading a single price, the implied probability is the right first approximation.
Prediction-market prices
Polymarket and similar markets skip odds altogether and quote the probability directly, in cents. A YES share that costs 64¢ pays $1.00 if the outcome happens, so the price is the implied probability: 64%. Converting to odds is the same division in reverse: 1 / 0.64 = 1.5625 decimal, or -178 American. The Polymarket guide covers the details, including why the two sides of a market add up to slightly more than a dollar.
Which format should you use?
Use whichever your bookmaker displays for placing bets, and convert to decimal or probability for thinking. Decimal odds make payout arithmetic trivial; implied probability makes comparison honest. American odds are worth learning to read fluently because so much of the market is quoted in them, but they are a poor format for arithmetic: the sign flip at even money and the non-linear scale invite mistakes.
A few habits that help:
- When you see a price, say the implied probability to yourself. -110 is "about 52%", +300 is "25%", 1.50 is "two-thirds".
- Compare prices across bookmakers in the same format, ideally decimal, so that a better price is simply a bigger number.
- Remember that the implied probability is the bookmaker's price, not the truth. The truth is a little lower on every outcome, by roughly the margin divided among them.
Worked conversions
From a moneyline to a fair probability
An NFL moneyline shows the favourite at -180 and the underdog at +155. Decimal: 1 + 100/180 = 1.556 and 1 + 155/100 = 2.55. Implied probabilities: 64.3% and 39.2%, summing to 103.5%. The overround is 3.5%. Dividing each probability by 1.035 gives fair estimates of 62.1% and 37.9%, which correspond to fair prices of 1.61 (-164) and 2.64 (+164). Try it in the
devig calculator
.
From a fraction to a stake
You want to win exactly 100 at 4/6. The stake is 100 × 6/4 = 150. Decimal check: 4/6 is 1.667, and 150 × 1.667 = 250, which is 150 back plus 100 profit.
From decimal to American across even money
1.95 is below 2.00, so it is negative: −100 / 0.95 = −105.3, displayed as -105. 2.05 is above, so (2.05 − 1) × 100 = +105. Two prices that differ by 0.10 in decimal sit at -105 and +105 in American odds, ten "points" apart on either side of a gap that does not exist in decimal.
Other formats you may meet
Three regional formats appear on Asian-facing sites. Hong Kong odds are decimal odds minus one: profit per unit staked. 0.91 is 1.91. Malay odds run from −1 to 1: positive values are the profit per unit like Hong Kong odds for underdogs, negative values are the stake needed to win one unit for favourites, so −0.91 is the same price as 1.91. Indonesian odds are American odds divided by 100: −1.10 is −110. All convert through the same implied probability; the converter on this site handles the three main formats plus prediction-market prices, and the regional ones are planned for a later release.
Common mistakes
- Reading -110 as "110% likely" or "10% juice". The implied probability is 52.38%; the juice on the whole market is 4.76%.
- Adding stake to profit twice. Decimal odds already include the stake. Fractional and positive American odds do not.
- Comparing American prices by subtraction. Convert to probability first.
- Treating the displayed decimal as exact. 1.91 is usually 1.9091.
- Forgetting that both sides carry margin. A -110 favourite and a -110 underdog cannot both be 52.38% likely.
Once the two questions become automatic — what does it pay, what does it imply — every price on every site reads the same way, and the format stops mattering.