Municipality Kozje, which is part of the Savinjska statistical region, measures 90 km2; this ranks it 80th among Slovene municipalities.
Statistical data for 2021 reveal the following:
In the middle of 2021 about 3,040 people (about 1,530 men and 1,510 women) were living in the municipality, which ranks Kozje 155th among Slovene municipalities. The population density was 34 people per square kilometre, which was lower than the national average of 104 people per square kilometre.
The number of live births was the same as the number of deaths, which means that natural increase per 1,000 population in the municipality was zero (in Slovenia -2.0). 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 -1.3. The sum of natural increase and net migration per 1,000 population in the municipality was negative. It was -1.3 (in Slovenia -0.9).
The mean age of people in Kozje was 45.2 years, which was lower than the national average (43.8).
As in most Slovene municipalities, in Kozje the number of old people was higher than the number of young people: there were 156 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 139. 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 municipalities except in five (Črna na Koroškem, Dobrovnik/Dobronak, Jezersko, Loški Potok in Mislinja) 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, which was attended by 90 children. Of all children in the municipality aged 1–5, 82% were included in kindergartens, which was more than the national average of 81%. In the school year 2021/22 elementary schools in Kozje were attended by about 220 pupils, while various upper secondary schools were attended by about 120 pupils. There were 37 students and 8 graduates per 1,000 people in the municipality; in Slovenia the averages were 39 per 1,000 and 8 per 1,000, respectively.
Among people aged 15–64 (i.e. working age population) about 65% were persons in employment (i.e. persons in paid employment or self-employed persons), which is less than the national average (67%).
In Kozje, average monthly gross earnings per person employed by legal persons were about 21% lower than the annual average of monthly earnings for Slovenia; net earnings were about 19% lower.
Among 1,000 inhabitants in the municipality, 610 had a passenger car. The car was on average 12 years old.
In 2021, 249 kg of municipal waste per person was collected in Kozje, which is 116 kg less than on average in Slovenia.