Municipality Kostanjevica na Krki, which is part of the Posavska statistical region, measures 58 km2; this ranks it 119th among Slovene municipalities.
Statistical data for 2020 reveal the following:
In the middle of 2020 about 2,440 people (about 1,220 men and 1,220 women) were living in the municipality, which ranks Kostanjevica na Krki 170th among Slovene municipalities. The population density was 42 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 -3.3 (in Slovenia -2.5). 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 9.0. The sum of natural increase and net migration per 1,000 population in the municipality was positive. It was 5.7 (in Slovenia 6.2).
The mean age of people in Kostanjevica na Krki was 44.0 years, which was higher than the national average (43.6).
As in most Slovene municipalities, in Kostanjevica na Krki the number of old people was higher than the number of young people: there were 131 people, aged 65 or more, per 100 people aged 0–14. This ratio shows that the value of the ageing index was lower than the national average of 136. It also shows that the mean age of people in Kostanjevica na Krki is growing more slowly than in Slovenia as a whole. Data by sex show that the ageing index for women was in all municipalities except in three (Dobrovnik/Dobronak, Jezersko in Loški Potok) higher than the ageing index for men. In Kostanjevica na Krki – as in most Slovene municipalities – more women were 65+ years old than 14 or less; for men the situation was the same.
Kostanjevica na Krki had 1 kindergarten, which was attended by 99 children. Of all children in the municipality aged 1–5, 81% were included in kindergartens, which was the same as national average. In the school year 2020/21 elementary school in Kostanjevica na Krki was attended by about 240 pupils, while various upper secondary schools were attended by about 100 pupils. There were 34 students and 7 graduates per 1,000 people in the municipality; in Slovenia the averages were 39 per 1,000 and 7 per 1,000, respectively.
Among people aged 15–64 (i.e. working age population) about 68% were persons in employment (i.e. persons in paid employment or self-employed persons), which is more than the national average (66%).
In Kostanjevica na Krki, average monthly gross earnings per person employed by legal persons were about 27% lower than the annual average of monthly earnings for Slovenia; net earnings were about 23% lower.
Among 1,000 inhabitants in the municipality, 627 had a passenger car. The car was on average 12 years old.
In 2020, 252 kg of municipal waste per person was collected in Kostanjevica na Krki, which is 103 kg less than on average in Slovenia.