I want to clear up right away is this issue of the number “36” and the space utilization formula.

I keep seeing from CPS Central Office that “using 36 students per homeroom” as their maximum in the space utilization efficiency range “is not true.”  They are correct in one respect, it is totally true that CPS does not document the number 36 anywhere in their space utilization guidelines.  Instead they use “30 plus 20%” as the maximum efficient capacity per total homerooms for the school.  This is just semantics.  Whether CPS uses:

  • 36 per homeroom and keep 23% of the  total classrooms as ancillary rooms or
  • 30 per homeroom and lose ancillary rooms in order to accommodate more students

…it is the same thing.  CPS puts 20% more students in the school than 77% of its total rooms can accommodate at the maximum class size, period, and says “that’s fine.”

That also creates the situation where a school with less than 24 students in each homeroom can be considered underutilized. Which is fascinating considering that the average (midpoint) homeroom size in the State of Illinois last year was between 21-23 students per homeroom. So there were fewer students per homeroom in the average State of Illinois school than in a CPS District school which Central Office would consider underutilized.

That’s the short explanation.  If you want the longer diatribe, grab your coffee and read on. I’ve made pictures for you below!

Before I get into an explanation of the math, let me re-state that  the Apples to Apples project began in August 2012 to accomplish one thing: To understand the difference between what CPS says is going on in schools and what CPS parents (of which I am one) are experiencing in CPS schools.

That gap (and there is a gap between what we are told and what we experience) explains a lot about the mistrust that many CPS parents have of Central Office and District Leadership.  And I’ll explain later about how that trust affects the District, from the revenue it secures to fund schools all the way to the kitchen table of a CPS student. For now, let’s just say that I’m interested in how the CPS message doesn’t often match CPS reality and why.

Trying to understand this mismatch led to my examination of many things, including the space utilization formula.  The original question I was trying to answer was this one:

  • Why do schools that parents perceive as overcrowded get rated by CPS as “efficient,” and why do schools that parents perceive as pretty full/functional get rated by CPS as “underutilized”?


In regards to the formula, there are facts that cannot be disputed.  For example:

  • The District has said it has contractual maximum limits for K-12 classrooms of 28 (Grades K-3) to 31 (Grades 4-12) students per homeroom.
  • The District confirmed that it used these contractual classroom limits for each grade to determine a K-12 homeroom average of 30 students each for the space utilization formula.
  • The Webster’s Dictionary definition of “limit” is “the utmost extent” and “a prescribed maximum or minimum amount.”
  • The District has chosen to use a range method of determining space utilization, establishing a range of efficient use with a minimum number, an Ideal number to anchor the range midpoint, and a maximum.


  • The width of the range is +/- 20% of the ideal number (in this range, the ideal is at the midpoint). More than 20% of the ideal number midpoint is considered to be “overcrowded” and less than 20% of the ideal number midpoint is considered to be “underutilized”.
  • The District has determined that it will set aside 76.9% of total classrooms in a school to serve as homerooms, and the remainder as ancillary rooms (rooms with uses that are supplementary to homerooms, such as art. music, special education/bilingual pull-out rooms, etc.)
  • Thus, the CPS formula looks like this:

(Total # of Classrooms x 76.9%) x Average maximum of students per homeroom 

The problem comes in when CPS wants to use the average maximum of students per room as the ideal number to anchor its formula range.

So instead of using the contractual maximum of students in the classroom as the maximum in the range for the school, like so:


…they are using the average contractual maximum class size limits as a midpoint (30 students), and using 20% OVER the contractual maximum class size limit as the maximum end of their efficiency range (30 students * 120%).  In schools, it ends up looking like this:


I can understand that Central Office would like to keep classrooms as close to the 28-31 contractual maximums as possible because they are trying to save money.  Their assumption is that they will save money in the long run by using more students per teacher, while getting the same educational outcomes that they are hoping to have.

But a limit is a limit.  For example, for safety’s sake, I hope that drivers obey the actual posted speed limit in school zones.


This formula creates all sorts of problems in real life that CPS parents frequently experience.  For the most part, only the parents of students at neighborhood schools experience the problems created by this formula.

Why is that?

Schools that can control their enrollments (number of students, grades filled, timing of admission) such as selective enrollment schools and lottery enrollment schools (charters, magnets) have fewer problems with this formula. I call them “restrictive enrollment schools” because they have restrictions on the type, timing and number of students who can enroll.  If restrictive enrollment schools want to take 31 students each for their fourth grade classrooms, no problem. They only accept the number of students that they want and then they stop accepting students.  They control the flood gates as to when and how many (and, in the case of selective schools, which specific students according to assessment) get in.


Neighborhood schools don’t have such controls.  They have to take whoever shows up, whenever they show up. Period.

Okay, so who cares?

This creates a number of hurdles that a neighborhood school has to work even harder to overcome in order to teach the students and run the school effectively that non-neighborhood schools don’t have.  For neighborhood schools:

  • it doesn’t matter if 20 students or 45 students show up for first grade at the beginning of the year, they have to work with that.  If 30% of those students have Individualized Education Plans (IEPs) or if 5% do, they work with that.
  • If they have three resident families that show up as refugees from a war torn country in February, they work with that.
  • If they have 24 students per grade show up, or 30% of their student population has an IEP (and legally requires smaller class sizes) or if 4 of their total classrooms are smaller than 600 sq ft, they have to work with that.

The reality of the unpredictability of neighborhood school enrollments (in number of students per grade, needs of students per grade, and timing of enrollments) punishes neighborhood schools either because they are not acknowledged as being overcrowded (when they are) or are being accused of being more underutilized than they are.  Not acknowledging the mis-alignment of the formula creates a utilization dilemma for principals of many neighborhood schools, as explained here.

Add this to the fact that the CPS Space Utilization formula does not recognize the different sizes of classrooms in each school (especially older schools), or the physical layout of a school (where a close bathroom might need to be accessible to lower grades, for example) or the reduced class sizes legally required for special needs populations, and the fact that the Space Utilization numbers that CPS releases don’t match the reality in many schools is no surprise at all.

5 thoughts on “Space Utilization: Does Central Office Use 36 as the Average Maximum Class Size or Not?

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