Back to Basics: What is Ultimate Zone Rating?

Ultimate Zone Rating (UZR) is a defensive statistic that measures fielding ability. It attempts to calculate how many runs a defender saved or lost for his team. The value is represented in relation to the positional average, meaning the average UZR differs based on position. For example, the UZR for a leftfielder is different than the average UZR for a second basemen.

What qualities do you like in a position player? Personally, I like when fielders are fast because they can reach more balls. I also like when they make good throws to get outs, and commit few errors. In the most basic sense, UZR attempts to consider these qualities that make up good defensive players and combines them into one statistic.

What is a good UZR value?

Zero is considered the league average UZR value. This is because after the UZR for a player is determined, it is then adjusted (see below for more details) to a league average 0 value for the position. Anything greater than zero indicates that a fielder is above average skill level and makes plays that average fielders may not be able to. Anything less than zero indicates that a fielder may have a tendency to miss plays, even average ones from time to time.

In the most basic sense, the higher the better.

What goes into a player’s Ultimate Zone Rating?

In simpler terms, there are three main values needed to calculate UZR:

• Double Play Ability (for infielders only)
• Errors
• Outfield Arm (for outfielders only)
• Range

A little more complicated, the actual terminology and definitions of the metrics considered in the UZR are:

• Double-Play Runs (DPR): For infielders only, DPR is the ability to make double plays. Amount of runs prevented that a fielder saves by participating in double plays in relation to an average value.
• Error Runs (ErrR): Ability to avoid errors. Amount of runs a fielder accumulated or lost in relation to an average value
• Outfield Arm Runs (ARM): For outfielders only, ARM is the ability to hold runners or get runners out based on the fielder’s arm. The amount of runs that a fielder saves by using his arm to prevent base runners from advancing in relation to an average value.
• Range Runs (RngR): Measures a fielder’s range and ability to cutoff batted balls. How many runs a fielder accumulated or lost in relation to an average value

How do you Calculate UZR?

Calculating UZR is tough, but let’s try to break it down.

When a fielder makes a play, the play is a certain level of difficulty. For example, a routine fly ball is easier to catch than a fly ball that the fielder needs to run for. The level of difficulty is related to a percentage for how likely it is that the play is made. Let’s say that the fly ball play has a successful completion rate by average fielders of 30%. That means there is a 30% chance that the fielder makes that play, and a 70% chance that he doesn’t.

That completion percentage is attached to a run value. For this example, let’s say that the fly ball play with a 30% chance of being made is attached to an average worth of 0.8 runs. (In general, plays with higher completion percentages correlate to less runs).

The contribution to the fielder’s UZR is then calculated as follows:

Step 1: 100%-30%=70 %

Essentially this step gives the fielder bonus points (in this case, 0.7) based on the difficulty of the play.

Step 2: 0.7*0.8 runs=0.56

Meaning that 0.56 is added to the fielder’s UZR.  

If that play is not made, the fielder is penalized and the calculation is as follows:

0.3* 0.8 runs = 0.24

Meaning that 0.24 is subtracted from the fielder’s UZR. The easier the play, the more he is penalized.

But we aren’t done yet…

The UZR we just calculated is considered the unadjusted UZR.

After this value is calculated, adjustments are made based on the conditions of the play. Adjustments include:

• The Ballpark
• Whether the pitcher is a righty or a lefty
• The number of outs there were before the play
• The number of runners were on base
• Where the runners on base were before the play
• The speed of the batted ball

After this process, we are left with an adjusted UZR. Adjustments are necessary to determine a more precise value and to consider the conditions of the play that may have impacted its result.

It’s Called Ultimate Zone Rating…Where does the “Zone” come in?

In the most basic sense, the zone helps determine the fielder’s position and whether or not he was responsible for the play. UZR is based on positional average, so the zone helps divide the field and identify who should gain or lose points for a play.

The field is split into 78 zones. 64 of the zones are relevant in determining UZR (the remaining ones are in the territory of the pitcher and catcher). In each zone, the out rate and the percentage of balls that result in outs are calculated. This determines the probability that a play will be made in a certain zone. Going back to the fly ball example from earlier, using the zone and making those calculations is how that 30% completion rate is determined.

Back to Basics: Ultimate Zone Rating

What does all this mean?

This stat can be really easy to understand if we take a step back and conceptualize. Think about your favorite centerfielder and go through the following questions for him:

• Does he typically (more than half the time) reach balls that he has to run for?
• Does he have a good arm? (Able to hold runners/ Throw runners out)
• Do we rarely see him commit errors?

If the answer to all three of these questions is yes, chances are he has an above average UZR. If the answer is no to some or all, then he probably has a UZR closer to or below UZR.  

For example, I know that Kevin Kiermaier makes great running catches all the time. I know that he throws runners out at the plate from centerfield, and that runners don’t advance when he has the ball. I don’t remember him making a ton of errors, at least not more than an average centerfielder. So Kiermaier is likely to have an above average UZR. Checking Fangraphs, I see that his UZR was 6.9 in 2019, well above average.

By watching your team day in and day out, you can tell who is more talented defensively than others. So you should already have an idea for whose UZR is higher than average.

How do I use UZR?

The higher the value the better the player is defensively. But be careful and make sure you understand the sample size.

If a player has one year or less in the majors, you should not be basing his defensive skill off his UZR. All stats need time to adjust to the true value for the player. I would use UZR for players who have played at least 3 full seasons in the majors before using that as a judgement of their defensive skill.

And even though UZR has adjustments and is a reliable defensive stat, stats aren’t the end all be all of talent. They certainly aren’t perfect. So I recommend using the stats more as a supplement to watching a player and figuring out his ability for yourself.

If you have any questions or want to learn more about UZR, let me know in the comments below.

If you want to read a more in depth article about UZR, check out this Fangraphs piece.

What stat do you want to learn about next?