Numerosity
Numerosity is the Rule 23 test that a class be so large that bringing every member into one lawsuit would be impractical.
Also called: numerosity requirement, Rule 23(a)(1), joinder impracticable
Numerosity is the Rule 23 test that a class be so large that bringing every member into one lawsuit would be impractical.
What it means
Numerosity is the first test a proposed class must pass under Rule 23(a). The rule says the class must be "so numerous that joinder of all members is impracticable." Joinder means making each person a party to the same case. In plain terms, there have to be so many people that suing together as individuals wouldn't be workable.
The rule sets no minimum number. Judges decide case by case, and a very large class usually meets the test on size alone. They can also weigh practical factors, such as how spread out the members are, whether small individual claims make separate suits unrealistic, and how easily members can be found. The question isn't whether joining everyone is impossible, only whether it is reasonably workable.
Don't confuse numerosity with the 100-member minimum in the Class Action Fairness Act. That is a separate test for when a large class action can be heard in federal court. In consumer cases, classes often run to thousands or millions of people, so numerosity is rarely the sticking point.
What it means for you
As a class member, you will rarely need to think about numerosity, because notices go out for classes of thousands or millions. It matters more as a reason some cases never become class actions. If only a handful of people were affected, a judge may say they can simply join the case or sue on their own. Before notice goes out in a settlement, the judge has to find it likely that the class can be certified, which includes meeting this test. Your job is to check whether you fit the class definition and file before the deadline.
Example
Say a bank charged an improper fee to 400,000 account holders in 30 states. Suing together as individual plaintiffs would be unworkable, so numerosity is easily met. Now say a landlord overcharged four tenants in one building. Those four could simply join one lawsuit, so a judge would likely find that a class is not needed.
FAQ
Rule 23 sets no minimum. The class has to be large enough that joining every member in one case would be impracticable, and judges decide that case by case. A separate federal law, the Class Action Fairness Act, requires at least 100 class members for many class actions to be heard in federal court.
Related terms
- Fed. R. Civ. P. 23, Class Actions (Cornell Legal Information Institute)
- A.B. v. Hawaii State Department of Education, No. 20-15570 (9th Cir. Apr. 4, 2022)
- 28 U.S.C. § 1332(d), class action jurisdiction (Cornell Legal Information Institute)
Updated Oct 18, 2026. Plain-English definitions for general information — not legal or tax advice.