NFL Odds Conversion for UK Punters: American, Decimal and Fractional Side-by-Side

Three prices, one bet, and why the formats argue

Three screens, three prices, one bet. The American sheet on my laptop has the Bills moneyline at -190. The bet365 tab open next to it shows 10/19 on the same team. And the no-vig calculator I run in the background says the fair price is 1.526 in decimal. Same wager. Same team. Same kickoff. Three different ways of writing what the bookmaker thinks Buffalo’s chance of winning actually is.

This is the part of NFL betting that loses British punters more money than any single tactical mistake. Not bad reads on the line — bad arithmetic on the price. You back a team you fancy, you take the first odds you see, and you have no quick way of telling whether the 10/11 you just accepted is the same line as the 4/5 the shop down the road would have given you. Across a season, that gap is the difference between profit and a slow leak.

Eight years on, I can tell you the maths is genuinely trivial — three formulas, none of them harder than primary-school division. What is not trivial is building the muscle memory to do them in the two seconds before you click confirm. That is what this guide is for. Formulas first, worked examples next, then a quick-look table you can keep open on your phone, and a list of the shortcuts that turn the conversion into something you do without thinking. No line-reading, no key numbers — just the price columns, decoded.

The three formats lined up next to each other

The fastest way to internalise the three formats is to look at the same bet rendered three ways and notice which question each format is answering. Each format is built around a different assumption about what you, the punter, already know — and that assumption is what makes the format feel natural in some places and clumsy in others.

Take a Bills moneyline of -190. The American format is telling you the stake required to win 100 units. So you must stake £190 to win £100, returning £290 in total. The minus sign is doing the work — it flags this as a favourite, and the absolute value is the stake-to-100 ratio. American format is built around a fixed payout of £100 and asks how much you need to risk to get there.

The decimal equivalent is 1.526. Decimal is the simplest format on the page because it bakes the stake back into the payout. Multiply your stake by 1.526 and you get your total return — stake plus profit, rolled into one number. £100 staked at 1.526 returns £152.60. The arithmetic is one multiplication and you are done. Decimal format is built around the total return and asks how much one unit of stake comes back.

The fractional equivalent — the format every UKGC-licensed shop will quote you by default — is 10/19. Read that as profit-to-stake. Stake £19 to make £10 in profit, on top of getting your £19 back. £100 staked at 10/19 returns roughly £152.63, the tiny rounding gap reflecting the way fractional odds are rounded to neat denominators. Fractional format is built around the profit and asks how much you stand to win on top of the money you put down.

Three formats, three perspectives, identical implied probability. The reason all three survive is historical — American books used the hundred-unit convention because it suited fixed-stake telephone betting, European books moved to decimal because it suited automated tote calculation, and British books stuck with fractional because that is what the on-course markets had used for two centuries. None of the formats is more accurate than the others. They are just three ways of expressing the same fraction.

From American to decimal in one short step

If you only ever learn one conversion, learn this one — American to decimal. Decimal is the bridge format. Once you have a decimal number you can get to fractional with one extra step, and you can compute implied probability with one division. American to decimal is the gateway.

The formula splits at zero. Plus odds and minus odds need different handling because of the way American notation works around the hundred-unit hinge. Memorise both halves and you have the entire conversion.

For plus odds — the underdog side — the formula is simple division and an addition. Take the American number, divide by one hundred, then add one. So plus 150 becomes 150 divided by 100, which is 1.5, plus 1, giving 2.50 in decimal. A £50 bet at plus 150 returns £50 times 2.50 — that is £125, which is your £50 stake back plus £75 profit. Plus 200 becomes 2.00 plus 1, or 3.00 decimal. Plus 100 is exactly evens, decimal 2.00. The pattern is consistent and you can do it in your head once you have run the formula five or six times.

For minus odds — the favourite side — the formula inverts. Take one hundred, divide by the absolute value of the American number, then add one. So minus 190 becomes 100 divided by 190, which is roughly 0.526, plus 1, giving 1.526 decimal. Minus 110 becomes 100 divided by 110, or roughly 0.909, plus 1, giving 1.909 decimal — the standard spread price every American sheet defaults to. Minus 150 is 0.667 plus 1, or 1.667. Minus 200 is 0.50 plus 1, or 1.50.

Two worked examples to make this concrete. The Eagles open as plus 130 underdogs on a sheet you printed Tuesday morning. Decimal conversion is 130 divided by 100, plus 1, equalling 2.30. A £25 stake returns £25 times 2.30, which is £57.50 — that is £25 back plus £32.50 profit. Same week, the Chiefs are minus 240 favourites against a struggling AFC West rival. Decimal is 100 divided by 240, plus 1, equalling 1.417. A £100 stake on Kansas City returns £141.70, of which £41.70 is profit.

Practice the formula on a printed sheet for one week and it stops feeling like maths. By the second weekend you will be reading the American number and the decimal will appear in your head without conscious arithmetic.

From decimal to the fractional your bookmaker actually shows

Decimal to fractional is the conversion that British punters need most often and that I see done wrong most often. The arithmetic is straightforward — but the way UK shops round their fractional displays means a strict-formula answer and the actual price on the slip can disagree by a sliver. Understanding why that gap exists is half the battle.

The pure formula is one subtraction and a fraction. Take the decimal price, subtract one, and you have the profit-to-stake ratio as a decimal. Express that ratio as a vulgar fraction and you have your fractional odds. So decimal 1.526 becomes 0.526, which is roughly 526 over 1,000 — simplify and you get something close to 10 over 19. Decimal 2.50 becomes 1.50, which is 3 over 2. Decimal 1.909 becomes 0.909, which is 10 over 11. Decimal 3.00 becomes 2.00, which is 2 over 1, displayed as 2/1.

The complication is that UK shops do not display every theoretically valid fraction. They round to a fixed set of “practical” odds — denominators like 2, 4, 5, 6, 8, 10, 11, 12, 13, 15, 16, 20 — that customers are used to seeing. So a decimal price of 1.909 will show as 10/11, even though the strict fraction is closer to 0.909/1. A decimal of 1.526 will show as either 5/9 or 10/19 depending on which rounding tier the shop uses. A decimal of 1.833 will show as 5/6 across almost every UK book.

Here is the punter-friendly shortcut. For prices the bookmaker quotes regularly — anything from 1/4 up to about 5/1 — there is a standard rounded fractional equivalent, and you learn the dozen or so common ones by repetition. Beyond 5/1, the rounding tier widens and you see jumps from 9/2 to 11/2 to 13/2, skipping over the awkward middle.

One worked example. You read minus 120 on an American sheet. Decimal conversion gives 1.833. Strict fractional is 0.833/1, which simplifies to 5/6. That is exactly what bet365 will show you for a -120 favourite — 5/6, not minus 120, not 1.833, just 5/6. Same price, different format, no edge gained or lost in the translation.

When you place the bet, you are betting on the displayed fractional, not the calculator-perfect decimal. If your shop shows 4/5 and a competitor shows 5/6 on the same line, those are different prices — 4/5 is decimal 1.80, while 5/6 is decimal 1.833. The third decimal place matters in volume.

Implied probability — the number every conversion is really asking about

Implied probability is the single most useful number in betting, and the easiest one to compute. Once you can take any price on the sheet and convert it to a percentage in under three seconds, you can answer the only question that actually matters before placing a bet — does the bookmaker think this is more likely than I do.

The formula is one division. Take one, divide by the decimal price, multiply by one hundred. That is it. A decimal of 1.909 — the standard minus 110 spread price — gives 1 divided by 1.909, which is 0.5238, or 52.38 percent. A decimal of 2.50 gives 0.40, or 40 percent. A decimal of 1.50 gives 0.667, or 66.7 percent. A decimal of 3.00 gives 0.333, or 33.3 percent.

The 52.38 percent number deserves attention because it is the break-even threshold every NFL punter who bets standard spread prices is fighting against. To turn a profit on minus 110 odds you need to win 52.38 percent of your bets. At minus 115, the bar lifts to 53.49 percent. The bookmaker’s margin is hiding inside that gap between fifty percent — the theoretical fair price for a coin flip — and 52.38 percent, the actual price you are charged for a coin-flip outcome. Every percentage point you can shave off the price closes that gap.

The reason this matters in practice is that implied probability turns betting into a comparison problem instead of an instinct problem. You read a Bills moneyline at minus 190, you compute 1 divided by 1.526, you get 65.5 percent. Now you have one question to ask — do I think Buffalo win this game more than 65.5 percent of the time? If the answer is yes, the bet has positive expected value at this price. If no, you walk away or look for a better line. There is no agonising over feel or hunch. There is the number you computed and the number you believe, and you bet on the gap.

The frequency-of-genuine-edges question is uncomfortable. The advisory consensus says sharp bettors only wager on twenty to thirty percent of available games, focusing on spots where their model disagrees with the implied probability by enough margin to overcome the vig. A 55 percent win rate at minus 110 odds is considered excellent and profitable. The reason for those tight thresholds is precisely the 52.38 break-even — anything below it across volume is a slow bleed.

Overround, the margin, and what the bookmaker pockets either way

Add up the implied probabilities on a two-way NFL market — say the spread on Bills versus Dolphins, both quoted at minus 110 — and you get a number greater than one hundred percent. That excess over one hundred is the overround, also called the vig or juice in American shorthand. It is the bookmaker’s margin baked into the price, and on standard spread markets it sits at about 4.76 percent across both sides combined.

Worked out in numbers, the picture is clear. Minus 110 on the Bills spread is 52.38 percent implied probability. Minus 110 on the Dolphins spread is also 52.38 percent. Add them and you get 104.76 percent. The four-point-seven-six over one hundred is the operator’s margin — what they take from the market regardless of which side wins, given balanced action on both sides.

The reason this matters more than punters realise is that the overround tells you which markets are sharp and which are loose. A two-way market with 4.76 percent overround is a tight, competitive market — the operator is running it on thin margin and pricing it carefully. A two-way market with eight percent overround is a market where the operator is taking less action and pricing it lazily. The first market is where line shopping pays off because the lines themselves are close to fair. The second is where you find soft prices but also where you find the bookmaker happy to limit you fast if you are winning.

On a three-way market the maths is the same but the overround is usually larger. NFL moneylines are sometimes offered as three-way — home win, away win, tie — and the tie line is so unlikely on an American football scoreline that operators price it at long odds and lift the overround on the other two sides. If you want to strip the margin out and see what the bookmaker thinks the fair price actually is, the calculation runs through a simple normalisation — divide each side’s implied probability by the total overround percentage, and you get the de-vigged fair probability. From there you can work back to a fair decimal price and compare your bookmaker’s number against it. The full method, including the two-way and three-way variants and how to handle pushes on totals, is worth a separate read — the no-vig fair price calculation is the cleanest way I have seen punters benchmark a sportsbook’s pricing across a full slate.

One last note on the practical scale. UK gross gambling yield from remote real-event betting hit £596 million in the fourth quarter of the 2024-25 financial year, a five percent rise year on year. A meaningful chunk of that figure is overround on retail-grade markets where most punters never compute the implied probability at all. The arithmetic above is one of the cheapest edges you can give yourself.

A quick-look table from minus a thousand to plus a thousand

Print this section. Stick it inside the back cover of your notebook. The table below is the cheat sheet I have used for six years and the one I hand to anyone who asks me where to start. Fifty rows from heavy favourite to heavy underdog, with the three formats and the implied probability stacked side by side so you can sanity-check any price you see by eye.

I will not pretend this is exhaustive. Decimal odds in the 1.50 to 3.00 range have a denser grid of common values than the extremes, and the table below picks the prices you will see most often on regular-season NFL markets — the favourite-side prices from minus 1000 to minus 105, evens, and the underdog side from plus 105 to plus 1000. Within those bands, every meaningful price tier is covered.

The implied probability column is the one to glance at first. Once you have anchored a few prices in your head — minus 110 is 52.38, evens is 50, plus 200 is 33.3 — the rest interpolates naturally. The remote UK betting market overall pulled in £2.6 billion in gross gambling yield in the year to March 2025, with online operators growing seven percent year on year through the same window. A meaningful fraction of that figure comes from punters who never check the implied probability column. Knowing it gives you a structural edge over the room.

Five anchor prices every British NFL punter should know cold

Nobody computes formulas at the betting counter. The reason every experienced punter looks like they are reading three formats at once is that they have memorised five or six anchor prices and they interpolate the rest by feel. The list below is exactly the anchors I learned in my first year and have leaned on every weekend since.

Minus 110 is 10/11. This is the most important anchor in the entire NFL betting world because it is the default spread price on almost every American sheet. Decimal 1.909. Implied probability 52.38 percent. The break-even bar. If you only memorise one anchor, this is it.

Minus 120 is 5/6. Decimal 1.833. A common short-favourite price on UK shops, including bet365 and Sky Bet, and the price you see most often when a moneyline favourite is firm but not heavy. Once you see 5/6 quoted, you should instantly read minus 120 — no calculator needed.

Plus 100 is evens. Decimal 2.00. Implied probability 50 percent. This is the only price where all three formats look obvious. Fractional shops sometimes write it as 1/1 instead of evens, which means exactly the same thing.

Plus 150 is 6/4. Decimal 2.50. Implied probability 40 percent. This is the standard moderate-underdog price and the one you will see most often on slate dogs that the public is not enthusiastic about. The fractional shorthand 6/4 reads cleanly as “back four to win six”.

Plus 200 is 2/1. Decimal 3.00. Implied probability 33.3 percent. Heavy underdog territory, the price at which a £100 stake doubles your money on top of your stake return. Useful as an anchor for sniffing the moneyline edge on a road dog you fancy.

Five anchors. Memorise them, use them as reference points either side of which everything else interpolates, and within a season you will be reading three formats fluently without ever opening a calculator app.

The mistakes I see in my inbox every week

The errors I see in reader emails repeat themselves with a depressing regularity. Almost none of them are about the formulas themselves — the maths is forgiving. They are about the small habits that creep in once you start betting under time pressure on a Sunday evening. Three patterns account for most of them.

The first is forgetting that decimal prices include the stake. New punters look at a decimal of 2.50 and read it as “I win two and a half times my stake”, which leads them to expect £250 profit on a £100 bet. The actual profit is £150, because 2.50 is the total return including the original £100. The fractional equivalent — 6/4 — is profit-only, which is why it reads as a smaller number. Always remind yourself which side of that fence you are on when you compute payout.

The second is treating American odds at face value when the favourite and underdog have asymmetric prices. A spread market with minus 110 on both sides has a 4.76 percent overround. A market with minus 120 favourite and plus 100 dog also looks tight, but in implied probability terms — 54.55 plus 50 — it has 4.55 percent overround and prices the favourite differently. Asymmetric American prices are not always a sharper line. Sometimes they are a softer line on one side and a harsher one on the other. Convert both, sum the implied probabilities, and read the overround before you decide which side has value.

The third is rounding fractional odds in your head and accepting the rounded number as the true price. Bookmakers do not round when they settle bets. They settle on the exact fractional displayed, and the difference between 10/11 and 5/6 — both of which look like “near evens favourite” to a casual eye — is meaningful over volume. 10/11 returns £190.91 on a £100 bet. 5/6 returns £183.33 on the same £100. That £7.58 gap times a hundred bets a season is real money. Read the fraction as written. Do not round.

A fourth one that comes up around the playoffs. Some shops post NFL futures in fractional and others in decimal — Super Bowl winner markets especially. Comparing a 10/1 from one book to a 12.00 from another seems trivial, but 10/1 is decimal 11.00, not 12.00, so the second book is offering a meaningfully better price. The conversion takes three seconds and it is the cheapest edge on the page.

Conversion questions that come up after the formulas land

Conversion questions are the most predictable category in my inbox, so I will close the article with the four that account for almost every one I receive in a typical season. If you have asked one of these of yourself in the last week, you are not alone — these are universal.