Municipality Sežana, which is part of the Obalno-kraška statistical region, measures 217 km2; this ranks it 25th among Slovene municipalities.

Statistical data for 2016 reveal the following:

In the middle of 2016 about 13,180 people (about 6,630 men and 6,550 women) were living in the municipality, which ranks Sežana 41th among Slovene municipalities. The population density was 61 people per square kilometre, which was lower than the national average 102 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 -3.6 (in Slovenia 0.3). 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  9.4. The sum of natural increase and net migration per 1,000 population in the municipality was positive. It was 5.8 (in Slovenia 0.8).

The mean age of people in Sežana was 44.7 years, which was higher than the national average (42.9).

As in most Slovene municipalities, in Sežana the number of old people was higher than the number of young people: there were 146 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 125. 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 Slovene municipalities 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.

In the school year 2016/2017 basic schools in Sežana were attended by about 1,010 pupils, while various upper secondary schools were attended by about 370 pupils. There were 33 students and 12 graduates per 1,000 people in the municipality; in Slovenia the averages were 39 per 1,000 and 15 per 1,000, respectively.

Among people aged 15–64 (i.e. working age population) about 60% were persons in employment (i.e. persons in paid employment or self-employed persons), which is the same as national average.

The registered unemployment rate was 9.3%, which is less than the national average (11.2%). As in most Slovene municipalities, more women than men were unemployed.

In Sežana, average monthly gross earnings per person employed by legal persons were about 9% lower than the annual average of monthly earnings for Slovenia; and net earnings about 7% lower.

In 2015, there were 446 dwellings per 1,000 population. About 69% of dwellings had at least three rooms. Average size of dwellings in the municipality was 88 m2.

Every other person in Sežana had a car (61 passenger cars per 100 population); cars in this municipality were on average 11 years old.

In 2016, 286 kg of municipal waste per person was collected in Sežana, which is 61 kg less than on average in Slovenia.