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What Are Middles & When to Bet Them: Part I

What are middles when it comes to sports betting?

Whether you’re familiar with the term, or just hearing it for the first time, it’s important to at least have a basic understanding of what they are and when they should be utilized.

In Part 1 of this two-part series:

  • What is a middle? I’ll tell you
  • I’ll also run you through some basic calculations to help you determine if a particular middle has any value

In Part 2:

  • I’ll answer one of the most common questions we get — when should I middle?
  • We’ll also tell you when not to middle and how to protect yourself from books who might frown on such behavior

Let’s start with the basics.

What Are Middles?

A middle is when you place two bets on opposite sides of the same market with a chance for both bets to win, but only one bet to lose.

For this to be possible, there must be a difference between the lines being offered. This opportunity arises when two sportsbooks have different lines for the same market, or when you place a bet and then the line moves to a different number.


DraftKings offers James Paxton strikeouts O/U 7.5 -110

FanDuel offers James Paxton strikeouts O/U 6.5 -110

If you bet $110 on Under 7.5 Ks at DraftKings and $110 on Over 6.5 Ks at FanDuel, you’ll win both bets for a $200 profit if James Paxton throws exactly 7 strikeouts. But you will only lose one of your bets for a $10 total loss if he throws anything other than 7.

This is an actual middle I hit in the 2019 season, though I changed the odds here a bit to make it easier to follow. The actual bets were O6.5 -134, U7.5 +111, and here’s what that looks like.

Here is a middling calculator if you want to play around with your own numbers.

How Do I Know if a Middle Has Value?

Determine The Odds

I see this question all the time, and while there are several components to the answer, we start by calculating the actual odds of our middle.

Just like a straight bet, the odds for a middle are described by a simple formula: return/risk

If you see odds of +100, you know that you need to risk $100 for a return of $100, and that you need that bet to hit 50% of the time to break even. For a middle, we can get a similar number by aggregating the risk and the return from both bets.

For a middle, only one bet can lose. So in the Paxton example above, our risk is just $10. But what if, as is more often the case, the two values are not identical? Then, simply average the risk on both sides to determine the value.

You can also bet to win different amounts to give yourself equal risk on both sides. Our potential return if we win both bets is $200. Using the simple formula of return divided by risk, we calculate the chances of our middle hitting as such:

200 (return) /10 (risk) = 20:1 = +2000 = 21.0 = 4.8%

We’ve simplified our middle down to its core component, and now we can think of the bet as:

James Paxton will have 7 Ks, +2000

Now Determine True Probability

The basic idea behind being a profitable sports bettor is simple: you need to place bets where the true odds of an event happening are greater than the implied odds of the line being offered. While it’s impossible to ever really know the true probability of a future event, we can make some reasonable estimates.

Let’s try doing so with our Paxton example.

The implied probability of +2000 odds is 4.8%. We’d need Paxton to strike out exactly seven batters >4.8% of the time, or more than once every 21 games, for this to be a bet with positive EV (expected value).

So what is the true probability of Paxton striking out seven batters in this game? That’s a complicated question, and figuring out the answer is part art, part science.

We can look at how often Paxton has struck out exactly seven batters in the past; in this case it was four times in 19 games up to that point, good for a rate of about 20%. However his O/U line in those games may have been different as he was facing completely different lineups, so this isn’t a perfect proxy, but it’s still promising.

If we have historical line data, we could look at how often all MLB pitchers finished within 0.5 of their closing line, and use that as a proxy for the true probability of Paxton doing the same thing.

I don’t have that dataset, but do track every bet I make, so I can check that instead. Going back through my 2019 database of K Prop bets, I find that 117 of 294 bets ended within 0.5 K of the actual line, for a hit rate of about 40%. Even if we cut that rate in half, since sometimes that 0.5 K will be in the opposite direction of what we’re hoping for, 20% is still much bigger than our 4.8% implied probability.

Playing with different assumptions is part of the art of it all. But in this relatively simple example, the odds of the event happening are roughly 20%, four times greater than what the price of +2000 / 4.8% implies, making this middle a no-brainer value.

Other Factors to Consider

They won’t all be layups like this though, and here are some other factors to consider:

  • The bigger the number, the more of a gap you need between the numbers. One strikeout is a massive 14% difference in this case. But take a PRA (Points+Rebounds+Assists) bet in the NBA, where a line might be set around 35. One PRA would only be about a 3% difference, and that is a much smaller window that we need to hit.
  • Some stats are “noisier” than others. Pitchers tend to face a similar number of batters each game, throw a similar number of pitches, and thus the volatility for their strikeouts is fairly low. In team sports where a player’s opportunities can be impacted by a lot of other external variables though (opposing defense, game script, game plan, etc.) there can be much higher volatility. Think receivers in football.
  • For football, the fact that points are mostly scored in increments of 3 and 7 causes some results to be much more common. A middle that covers a spread of -2.5 and +3.5 where you win both bets on a difference of 3 points is much more valuable than a middle that runs through 2.

In Part 2…

Hopefully, this has left you with a stronger understanding of middles and how to approach them.

In Part 2 next week, I’ll tell you when to middle, when not to middle, and how to protect yourself from being banned or limited by the books when you do it.

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