Mensa International is a high IQ society for those who score in the top two percent in terms of intelligence. That is, scores at the 98th percentile or higher on a standardized, supervised IQ test are required for admission into Mensa.
Mensa is both the oldest and largest high IQ society in the world. Mensa meetings are perhaps most popular in the United States and England. In fact, Mensa was founded in England right after World War II. Mensa from the outset has operated as a true meritocracy that doesn't discriminate based on political leanings, religious orientation or race.
Because Mensa has many different IQ tests that can be used for admission, there technically isn't a minimum IQ requirement. Instead, to make it even across the board, you must score in the 98th percentile or above on any one of Mensa's qualifying admission tests.
An IQ test usually has a mean, or arithmetic average, of 100. This means the average person scoring at the 50th percentile will achieve a score of 100 on popular IQ tests like the Stanford Binet and Wechsler Adult Intelligence Scale (WAIS).
The difference for getting into Mensa is that the early Stanford Binet has a standard deviation of 16 points and the WAIS has a standard deviation of 15 points. For practical purposes, this means that one would need to score a 132 or higher to gain admission into Mensa based on your Stanford Binet test score. A score of 130 or higher on the WAIS would be required to achieve admission.
Those trying to gain admission to Mensa International could also test their mettle by taking Mensa's proprietary test. This test is designed and proctored by Mensa itself. The results aren't as nuanced as the Stanford Binet or WAIS since the Mensa test simply tells test takers whether they scored in the 98th percentile or higher. In essence, the Mensa test separates the wheat from the chaff and decides who's smart enough to join Mensa.
If you wish, you can take the Mensa home test for a small fee. This test will give you an indication of whether or not you're in the ballpark for Mensa qualifying. The Mensa home test can also clue you into the types of questions that are asked on Mensa's official test that many applicants wrestle to gain admission to Mensa International.
Mensa Home Test
Many standardized tests can be used for proof of aptitude and admission into Mensa, provided the test was taken prior to 1989. For instance, the ACT can be used as a Mensa qualifying score granted you scored a 29 or higher and the test came before August, 1989.
Similarly, if you scored a 1250 or higher on the SAT that was administered from 1974 to 1994, then you may inform Mensa of that score to gain admission. A GRE score of 1250 or higher on the GRE is good enough for admission, granted you achieved that score in 1994 or earlier. The best possible score on both the early SAT and GRE is 1600.
In closing, you will need to show that you can score in the 98th percentile or higher on a standardized, proctored test of cognitive ability to join Mensa International.