How is the size of a religion determined for the purposes of this list?

When referring to the "size" of a religion, what is usually meant is its number of adherents. Other measurements, such as how many churches or meeting places a faith group owns or how many congregations/meeting groups there are, can also be instructive, but are usually not used as a measure of overall size. Measures of religiosity and the degree to which a religious tradition has a meaningful impact on its adherents may be more important than raw adherent counts, but such measures are not as readily available nor are they easily comparable between groups. A detailed description of what an adherent is, and the different types/levels of adherents can be found on the FAQ page.

How are adherents counted?

There are five main methods for determining the number of adherents in a faith group:

1. Organizational reporting: Religious bodies (such as churches or denominations) are asked how many adherents or members they have. This is the simplest and least expensive method, but it can be highly unreliable. Different faith groups measure membership differently. Some count as members only those who are actively attending services or who have passed through a lengthy initiation process. Others groups count all who have been baptized as infants and are thus on the church records, even though some of those people may have joined other faith groups as adults. Some groups over-report membership and others under-report membership. When asked what religion they consider themselves to be a part of, many may name a religion that does not have them on their rolls. In the United States, for instance, three times as many people claim to be Unitarian Universalists than are actually on church records.

2. Census records: Many countries periodically conduct a comprehensive household-by-household census. Religious preference is often a question included in these census counts. This is a highly reliable method for determining the religious self-identification of a given population. But censuses are usually conducted infrequently. The latest census may be too old to indicate recent trends in religious membership. Also, many countries either have no accurate census data, or do not include questions regarding religious affiliation. It has been over fifty years since the United States included such a question in its national census, but Canada, India, New Zealand, Australia and other countries have very thorough, recent census data on the topic.

3. Polls and Surveys: Statistical sampling using surveys and polls are used to determine affiliation based on religious self-identification. The accuracy of these surveys depends largely on the quality of the study and especially the size of the sample population. Rarely are statistical surveys of religious affiliation done with large enough sample sizes to accurately count the adherents of small minority religious groups.

4. Estimates based on indirect data: Many adherent counts are only obtained by estimates based on indirect data rather than direct questioning or directly from membership roles. Wiccan groups have traditionally been secretive and often their numbers can only be estimated based on magazine circulations, attendance at conferences, etc. The counts of many ethnic-based faith groups such as tribal religions are generally based on the size of associated ethnic groups. Adherents of some tribal religions (such as Yoruba) are sometimes counted simply by counting the members of the tribe and assuming everybody in it is an adherent of the religion. Counts of Eastern Orthodox religious bodies are often done the same way. Such estimates may be highly unreliable.

5. Field work: To count some small groups, or to count the number of adherents a larger group has within a specific geographical area, researchers sometimes do "field work" to count adherents. This is often the only way to count members of small tribal groups or semi-secretive, publicity-shy sects. Field work may involve contacting leaders of individual congregations, temples, etc., conducting interviews with adherents, counting living within enclaves of the group, or counting those participating in key activities. There is substantial overlap between "estimates" and "field work."

For the purposes of this list of major religions, we have used adherent counts or estimates based on self-identification. We have also favored inclusive rather than exclusive adherent counts (meaning all people who are part of a religious community, children as well as adults, rather than "full communicants"). It should be remembered, however, that self-identification is not the only legitimate measure of a religious group's size. In collecting census or survey data based on self-identification statisticians find that nearly everybody claims to belong to a religion. Some people claiming membership in a certain denomination may actually attend religious services regularly, contribute resources to the group, and be influenced by its teachings. Other people may name the denomination, but choose it as their religion only because they recall its name as the church their grandfather attended as boy. Detailed analysis of the size of individual groups requires a knowledge of both self-identification data as well as data based on organizational reporting. Finally, let me make it clear that these definitions are simply working definitions for the purposes of making this list. They should not be taken as definitive outside of this context. Many of our reasons for defining the parameters as we have done have to do with the availability of data. Other definitions and parameters may be more meaningful or useful in other situations.