At the end of 2008 about 8,300 people (about 4,100 men and 4,200 women) were living in the municipality, which ranks Vojnik 63th among Slovene municipalities. The population density was 110 people per square kilometre, which was higher than the national average of 100 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 -1.3 (in Slovenia 1.7). The number of people who moved from the municipality was lower than the number of people who moved into the municipality, so net migration per 1,000 population in the municipality was positive. It was 5.8. The sum of natural increase and net migration per 1,000 population in the municipality was positive. It was 4.4 (in Slovenia 10.9).
The mean age of people in Vojnik was 40.8 years, which was lower than the national average (41.3 years).
Vojnik had 6 kindergartens. They were attended by 367 children. Of all children aged 1–5, 81% were included in kindergartens, which was more than the national average of 70%. In the school year 2008/09 elementary schools in Vojnik were attended by about 810 pupils. About 440 pupils from the municipality attended various secondary schools. There were 60 students and 7 graduates per 1,000 people in the municipality; in Slovenia the averages were 55 per 1,000 and 8 per 1,000, respectively.
The registered unemployment rate was 6.2%, which is less than the national average (6.7%). As in most Slovene municipalities, more women than men were unemployed.
The value of gross investment in new fixed assets in the municipality (EUR 973 per capita) was lower than the national average (EUR 3,165 EUR per capita).
In 2008, there were 377 dwellings per 1,000 population. About 55% of dwellings had at least three rooms. Average size of dwellings in the municipality was 74 m2.
Every other person in Vojnik had a car (51 passenger cars per 100 population); cars in this municipality were on average 8 years old.
In 2008, 410 kg of municipal waste per person was collected in Vojnik, which is 9 kg less than on average in Slovenia.