“Why do the boundaries of my state legislative district change?” It’s a common question asked about the process of redrawing district lines after a federal Census. A new interactive map created by legislative staff provides easy answers.

First, some background on legislative districts and redistricting. Montana elects state legislators from State House and State Senate districts. Each legislator represents one district. According to the U.S. and Montana constitutions, those districts must be composed of approximately equal numbers of people. This is known as the “one person, one vote” rule.

Because population numbers change as people move in, move out, are born and die, periodic adjustments of district boundaries are necessary. Redistricting is the process of adjusting district lines to ensure each legislator represents about the same number of people. In redistricting, “about the same number of people” is called the “ideal population” of a district.

Sound complicated? Don’t overthink it. An ideal population is calculated by taking the total population of an area and dividing it by the total number of districts into which you’re dividing it. To get the ideal population for a Montana State House district, divide the population of Montana by the number of seats in the House of Representatives (100).  In 2010, the ideal population for a state House district was 9,894 (989,415 / 100).  In 2020, the ideal population for a state House district is 10,842 (1,084,225 / 100).*

Two House districts are combined (or nested) to build the 50 State Senate districts. The same equal population principle applies to those districts.

In Montana, five citizens are required by the Montana Constitution to redraw congressional and state legislative district boundaries every 10 years after receiving the Census data. In 2021, the commissioners adopted criteria to guide their work, including that each legislative district drawn after the 2020 Census be within 1% of the ideal population unless a slightly larger deviation from the ideal is necessary to comply with the federal Voting Rights Act.

Ten years before, the commissioners used a +/-3% deviation. The interactive map compares the approximately equal-population district created in the 2010 round of redistricting with the same district boundaries and their current population as determined in the 2020 Census.

After the 2010 round of redistricting, 70% of House Districts were within the target of 1% of ideal population.  The remaining 30% varied slightly more than 1%, but no district was more than 3% away from ideal population.

Based on 2020 Census data, only 10 House Districts are still within the 1% target of the ideal equal population. The remaining 90 districts are either significantly underpopulated or overpopulated. In fact, the majority (53) of districts are now significantly unbalanced, deviating at least 5% or more from ideal population.

The five commissioners must now rebalance the populations by adjusting boundaries. In essence, a district that needs more people to get close to the ideal population must expand its boundaries. A district that has too many people must shrink its boundaries to lower the population in it.

And the 10 districts that are still “ideal” in size? These boundaries will change, too. Because they are likely surrounded by districts that are too large or too small in population, even an “ideal district” might be changed to accommodate the needs of other districts. In redistricting, this is known as the “ripple effect”. A change in the geographic boundaries of one district can “ripple” across the state and affect other districts.

Want the 2020 district deviation data in table form? Download the table here.

Want to learn more about redistricting in Montana? Visit mtredistricting.gov, the website of the Montana Districting and Apportionment Commission.

*This ideal district calculation is prior to any adjustments to the Census data to move prisoners in institutions to a last known residential address, also known as prisoner reallocation.

Daniel Kayser and Rachel Weiss are joint authors of this post.