Municipality Sežana, which is part of the Obalno-kraška statistical region, measures 217 km2; this ranks it 25th among Slovene municipalities.
Statistical data for 2021 reveal the following:
In the middle of 2021 about 13,820 people (about 6,990 men and 6,830 women) were living in the municipality, which ranks Sežana 39th among Slovene municipalities. The population density was 64 people per square kilometre, which was lower than the national average of 104 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 -3.8 (in Slovenia -2.0). 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 7.0. The sum of natural increase and net migration per 1,000 population in the municipality was positive. It was 3.3 (in Slovenia -0.9).
The mean age of people in Sežana was 45.5 years, which was lower than the national average (43.8).
As in most Slovene municipalities, in Sežana the number of old people was higher than the number of young people: there were 157 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 Sežana 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 Sežana – as in most Slovene municipalities – more women were 65+ years old than 14 or less; for men the situation was the same.
Sežana had 6 kindergartens, which were attended by 500 children. Of all children in the municipality aged 1–5, 73% were included in kindergartens, which was less than the national average of 81%. In the school year 2021/22 elementary schools in Sežana were attended by about 1,160 pupils, while various upper secondary schools were attended by about 400 pupils. There were 30 students and 5 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 Sežana, average monthly gross earnings per person employed by legal persons were about 12% lower than the annual average of monthly earnings for Slovenia; net earnings were about 10% lower.
Among 1,000 inhabitants in the municipality, 628 had a passenger car. The car was on average 12 years old.
In 2021, 310 kg of municipal waste per person was collected in Sežana, which is 55 kg less than on average in Slovenia.