The NEA and the AFT, both teachers' unions, collectively represent over 4.5 million workers. Other large unions include AFSCME, which represents state and local government employees, UFCW, SEIU, and the Teamsters (IBT), which each represent workers in a diverse array of industries.

But even these large unions together represent barely half of all unionized workers. Millions more belong to industry-specific small international unions, like the 22,000-strong United Union of Roofers, Waterproofers and Allied Workers (RWAW).

And another 760,681 belong to independent, unaffiliated local unions.

For comparison, here are the largest public and private employers in the U.S., sized by the number of their employees: Walmart (2.2 million) and the Department of Defense (3.2 million).

For comparison, here's Walmart, the largest private employer in the U.S., with $79.6 billion in net assets.

That's around ten times the net assets of all the international unions combined. In a single company.

With corporations operating on this kind of scale, it's no surprise that the UFCW (net assets of $336 million) has struggled to unionize Walmart's retail employees.

The picture changes slightly when we sort unions by their spending per member. For example, both the IBEW and the UAW are in the upper half of unions by wealth. But while the UAW has around quadruple the net assets per member of the IBEW, the IBEW spends almost triple what the UAW does per member.

This split becomes even clearer when unions are ranked by how much they spend, relative to how much they're worth.

Unions like the AFT or the AFGE spent around three times their total assets in 2018, while the UAW and the CJA spent less than a quarter of their total assets.

Most union spending, after taxes and AFL-CIO dues, can be roughly broken up into four categories:

• Representational: contract negotiations, grievance handling, corporate research, litigation, and new worker organizing.
• Political: spending on political candidates (including through PACs) and lobbying.
• Benefit & grant: pensions, health and life insurance, charitable contributions, scholarship funds, and more.
• Overhead: membership meeting, conferences, supplies, rent, support staff, member education programs, and more.

Several important unions which prioritize these categories differently are highlighted below.

It's important to note that local unions also spend in these categories, and are not included in the graphics below.

Of the very large unions, the SEIU spends the largest portion of its budget on representation and organizing. The IUJAT, the oldest union in continuous existence in the U.S., and the AFA, which has been organizing aggressively in major airlines, also top the chart for representational spending.

Unsurprisingly, the NEA, AFT and other largely public-sector unions like AFSCME and the IAFF, are heavily involved in lobbying. The SEIU is also known for its political spending, like the $28 million it spent on Barack Obama's campaign in 2008.

Notably, the much smaller IUJAT and AFA, which both spend heavily on representation and organizing, each spend less than 2% of their budgets on lobbying and politics.

The NEA stands out for spending nearly half its budget on benefits, grants, and contributions. It donates heavily to nonprofit organizations across the country. The SEIU, in contrast, spends around 7% of its budget on benefits and grants.

There are few clear patterns in how much unions spend on administration and overhead. Some, like the IAFF, spend nearly 60%; others, like the IUJAT, spend 5%.

We can actually visualize all of this together, mapping unions onto a triangle, below, based on their spending priorities. A union which spends 100% of its budget on representation and organizing would be located at the top of the triangle, while a union that spent a third of its budget on representation, a third on grants, and a third on lobbying and overhead would be located in the middle.

While there are unions spread all over the chart, the striking pattern is just how many unions are clustered in the bottom right. Nearly all international unions spend a majority of their budgets on administration, overhead, and lobbying. There are good reasons for this, in many cases.

But some of the unions which have grown most aggressively—like the SEIU— or which are widely recognized as democratic and innovative—like UNITE HERE, the AFA, or UE—are closer to the middle. And many unions which have not seen membership growth—like IBEW, CJA, or IUOE—are in the bottom-right corner.

While there is large variation in how many staff unions hire, there is slightly less variation in what these staff are paid. Most major unions pay their staff a median salary between $70,000 and $130,000 a year, though some, like the IUEC, pay as much as $212,000 a year. Of the unions with at least a million members, the IBT pays its staff the least: just over $80,000 a year.

For comparison, the dashed line shows the median wage for union workers, before benefits.

Finally, international unions are just the tip of the iceberg. The majority of representational and organizing work happens at the local level. And while unfortunately there doesn't exist readily accessible complete data on all local unions in the country, across all sectors, we can look at one important aspect of local unions: their size.

Below, unions are arranged by the average size of their locals. Unions must strike a balance between the added efficiency of pooling resources across many workers and workplaces, and the challenges in representing large numbers of workers at once.

The SEIU is by far the most centralized union, with more than 18,000 members in its average local. On the other end of the spectrum, craft unions like the Insulators (HFIA) or city-based unions like the East coast Longshoremen (ILA) have smaller locals.