Municipality Kozje, which is part of the Savinjska statistical region, measures 90 km2; this ranks it 80th among Slovene municipalities.
Statistical data for 2011 reveal the following:
In the middle of 2011 about 3,250 people (about 1,600 men and 1,650 women) were living in the municipality, which ranks Kozje 146th among Slovene municipalities. The population density was 36 people per square kilometre, which was lower than the national average of 101 people per square kilometre.
The number of live births was lower than the number of deaths, which means that natural increase per 1,000 population in the municipality was negative. It was -3.4 (in Slovenia 1.6). The number of people who moved from the municipality was higher than the number of people who moved into the municipality, so net migration per 1,000 population in the municipality was negative. It was -5.6. The sum of natural increase and net migration per 1,000 population in the municipality was negative. It was -9.0 (in Slovenia 2.6).
The mean age of people in Kozje was 42.5 years, which was higher than the national average (41.8).
As in most Slovene municipalities, in Kozje the number of old people was higher than the number of young people: there were 129 people, aged 65 or more, per 100 people aged 0–14. This ratio shows that the value of the ageing index was higher than the national average of 117. It also shows that the mean age of people in Kozje is growing more rapidly than in Slovenia as a whole. Data by sex show that the ageing index for women was in all Slovene municipalities higher than the ageing index for men. In Kozje – as in most Slovene municipalities – more women were 65+ years old than 14 or less; for men the situation was the same.
Kozje had 1 kindergarten. It was attended by 74 children or 66% of all children aged 1–5, which was less than the national average of 76%. In the school year 2011/2012 basic schools in Kozje were attended by about 250 pupils, while various upper secondary schools were attended by about 140 pupils. There were 60 students and 11 graduates per 1,000 people in the municipality; in Slovenia the averages were 51 per 1,000 and 10 per 1,000, respectively.
Among people aged 15–64 (i.e. working age population) about 59% were persons in employment (i.e. persons in paid employment or self-employed persons), which is more than the national average of 58%.
The registered unemployment rate was 13.8%, which is more than the national average (11.8%). As in most Slovene municipalities, more women than men were unemployed.
In Kozje, average monthly gross earnings per person employed by legal persons were about 13% lower than the annual average of monthly earnings for Slovenia; and net earnings about 11% lower.
Every 21st person in the municipality was receiving at least one financial social assistance. In Slovenia as a whole every 24th person received at least one financial social assistance.
In 2011, there were 462 dwellings per 1,000 population. About 62% of dwellings had at least three rooms. Average size of dwellings in the municipality was 81 m2.
About every other person in Kozje had a car (52 passenger cars per 100 population); cars in this municipality were on average 10 years old.
In 2011, 195 kg of municipal waste per person was collected in Kozje, which is 157 kg less than on average in Slovenia.