The union density or union membership rate conveys the number of trade union members who are employees as a percentage of the total number of employees in a given industry or country.^[1] This is normally lower than collective agreement coverage rate, which refers to all people whose terms of work are collectively negotiated. Trade unions bargain with employers to improve pay, conditions, and decision-making in workplaces; higher rates of union density within an industry or country will generally indicate higher levels of trade union bargaining power, lower rates of density will indicate less bargaining power.^[1]

The causes of higher or lower union membership are widely debated. Common causes are often identified as including the following:

• whether a jurisdiction encourages sectoral collective bargaining (higher coverage) or enterprise bargaining (lower coverage)
• whether collective agreements to create a closed shop or allow automatic enrollment in union membership are lawful
• whether the government, for instance through a Ministry or Department of Labour, actively promotes collective agreement coverage with a power to impose terms if employers refuse to bargain with the workforce
• whether a country enables collective agreements to be extended by government regulations to all workers when the coverage rate reaches a majority in a sector, or similar level
• whether laws on collective bargaining and strikes are more or less favourable

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	Now	Older
	8%[2]
	68%
	14.3%
	13.7%[3]

In the United States in 2015 there were 14.8m union members, and 16.4m people covered by collective bargaining or union representation. Union membership was 7.4% in private sector, but 39% in the public sector. In the five largest states, California has 15.9% union membership, Texas 4.5%, Florida 6.8%, New York 24.7% (the highest in the country), and Illinois had 15.2%.^[4]

In December 2021, 14.3% of the Australian workforce were union members; this was a decline of more than 5 percentage points since 2010 and nearly 10 percentage points since 2005.^[5]

In Sweden union density was 68% in 2019.^{[6] [7]} In all the Nordic countries with a Ghent system—Sweden, Denmark and Finland—union density is almost 70%. In all these countries union density has declined.^[8][9]

In France while the overall union density is 8%, in companies over 50 employees this level reaches 43%.

US Bureau of Labor in 2010 notes a difference of median income of 200 dollars between union-members (917), and non-union members (717) without indicating if higher salaries link to more unionisation, or the reverse or in mutuality.^[10]

This section needs expansion. You can help by adding to it. (August 2016)

• Australian labour law
• European labour law
• US labor law
• UK labour law

• Dickens, William T.; Leonard, Jonathan S. (April 1985). "Accounting for the decline union membership, 1950–1980" (PDF). Industrial and Labor Relations Review. 38 (3). SAGE: 323–334. doi:10.1177/001979398503800301. JSTOR 2523761. S2CID 155005060.
• Visser, Jelle (September 2002). "Why fewer workers join unions in Europe: a social custom explanation of membership trends". British Journal of Industrial Relations. 40 (3). Wiley: 403–430. doi:10.1111/1467-8543.00241. S2CID 154877898.
• Schnabel, Claus; Wagner, Joachim (January 2005). "Determinants of trade union membership in West Germany: evidence from micro data, 1980–2000". Socio-Economic Review. 3 (1). Oxford Journals: 1–24. doi:10.1093/SER/mwh011. S2CID 55433670.
• Schnabel, Claus (September 2013). "Union membership and density: Some (not so) stylized facts and challenges" (PDF). European Journal of Industrial Relations. 19 (3). Sage: 255–272. doi:10.1177/0959680113493373. S2CID 14168949.

• European Trade Union Institute
• ILO Trade Union Statistics
• OECD Statistics on Trade Union Density, since 1960 for some countries