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. 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.
Causes.
The causes of higher or lower union membership are widely debated. Common causes are often identified as including the following:
By country.
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%.
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.
In Sweden union density was 68% in 2019. 
 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.
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.
Notes.
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