The public support test is a provision of IRS tax code that requires most public charities to meet to maintain their tax-exempt status. The test ensures that a nonprofit’s income comes from a diverse set of donors or payors for charitable services, rather than from a single source. If your public charity fails the public support test, the IRS could change your status to that of a private foundation. This effect is commonly known as “tipping.”

IRS Pub. 557: Tax-Exempt Status for Your Organization has a section on qualifying as publicly support that describes how to look at where your organization’s funding comes from and calculate your level of public support.

To calculate the public support of a nonprofit, divide the amount of public support by the total support received. If the nonprofit receives 33.33% or more of its total support from public support, then it qualifies as a public charity.

The Wonderful World of Foundation Classification (video)
IRS

18-minute animated film that explains foundation/public charity classification and public support. Also covers how to calculate public support test. Users can view outline, notes, and advance slides.

Public Support Tests: Does Your Public Charity Pass the Section 509(a)(1) Test?
Nonprofit Accounting Basics

An explanation of the 509(a)(1) public support test courtesy of the Greater Washington Society of CPAs.

Public Charity: Public Support Tests Part I: 509(a)(1) 
Nonprofit Law Blog

An explanation with examples of the two public supports tests, known as the One-Third Support Test and the Facts and Circumstances Test.

Public Charity: Public Support Tests Part II: 509(a)(2)
Nonprofit Law Blog

Another way to determine whether a nonprofit passes the public support test under a different section of the tax code.

Compliance Guide for 501(c)(3) Public Charities (PDF)
IRS

Covers recordkeeping; activities that may jeopardize a charity's exempt status; required filings; governance considerations; changes to be reported to the IRS; required public disclosures; and resources for public charities.

The Public Support Test for 501(c)(3) Organizations
Sharinglaw.net

Attorney Lisa Nachmias Davis provides step-by-step instructions for those wishing to calculate the test. Part of her guide, "Ten Steps for Starting a Nonprofit, Tax-Exempt Corporation in Connecticut".

Publication 557: Tax-Exempt Status for Your Organization (PDF)
IRS

Discusses rules and procedures for organizations seeking recognition of exemption from federal income tax under section 501(a) of the Internal Revenue Code (IRC). Also describes which types of organizations are eligible for tax-exempt status and how to calculate public support.