Balsthal has a population (as of December 2018) of 6,121.
As of 2008, 28.9% of the population are resident foreign nationals.
Over the last 10 years (1999–2009) the population has changed at a rate of 1.7%.

Most of the population (as of 2000) speaks German (4,728 or 84.8%), with Albanian being second most common (163 or 2.9%) and Italian being third (153 or 2.7%).
There are 40 people who speak French and 2 people who speak Romansh.

As of 2008, the gender distribution of the population was 51.0% male and 49.0% female.
The population was made up of 1,993 Swiss men (34.4% of the population) and 962 (16.6%) non-Swiss men.
There were 2,032 Swiss women (35.1%) and 802 (13.9%) non-Swiss women.
Of the population in the municipality 2,040 or about 36.6% were born in Balsthal and lived there in 2000.
There were 1,179 or 21.2% who were born in the same canton, while 1,006 or 18.0% were born somewhere else in Switzerland, and 1,156 or 20.7% were born outside of Switzerland.

In 2008 there were 29 live births to Swiss citizens and 30 births to non-Swiss citizens, and in same time span there were 39 deaths of Swiss citizens and 6 non-Swiss citizen deaths.
Ignoring immigration and emigration, the population of Swiss citizens decreased by 10 while the foreign population increased by 24.
There were 15 Swiss men and 9 Swiss women who immigrated back to Switzerland.
At the same time, there were 56 non-Swiss men and 25 non-Swiss women who immigrated from another country to Switzerland.
The total Swiss population change in 2008 (from all sources, including moves across municipal borders) was a decrease of 34 and the non-Swiss population increased by 87 people.
This represents a population growth rate of 0.9%.

The age distribution, as of 2000, in Balsthal is; 401 children or 7.2% of the population are between 0 and 6 years old and 976 teenagers or 17.5% are between 7 and 19.
Of the adult population, 300 people or 5.4% of the population are between 20 and 24 years old.
1,681 people or 30.2% are between 25 and 44, and 1,295 people or 23.2% are between 45 and 64.
The senior population distribution is 651 people or 11.7% of the population are between 65 and 79 years old and there are 270 people or 4.8% who are over 80.

As of 2000, there were 2,237 people who were single and never married in the municipality.
There were 2,693 married individuals, 381 widows or widowers and 263 individuals who are divorced.

As of 2000, there were 2,232 private households in the municipality, and an average of 2.4 persons per household.
There were 677 households that consist of only one person and 165 households with five or more people.
Out of a total of 2,282 households that answered this question, 29.7% were households made up of just one person and there were 16 adults who lived with their parents.
Of the rest of the households, there are 655 married couples without children, 744 married couples with children There were 120 single parents with a child or children.
There were 20 households that were made up of unrelated people and 50 households that were made up of some sort of institution or another collective housing.

In 2000 there were 962 single family homes (or 67.9% of the total) out of a total of 1,417 inhabited buildings.
There were 256 multi-family buildings (18.1%), along with 113 multi-purpose buildings that were mostly used for housing (8.0%) and 86 other use buildings (commercial or industrial) that also had some housing (6.1%).
Of the single family homes 85 were built before 1919, while 92 were built between 1990 and 2000.
The greatest number of single family homes (288) were built between 1919 and 1945.

In 2000 there were 2,454 apartments in the municipality.
The most common apartment size was 4 rooms of which there were 731.
There were 108 single room apartments and 835 apartments with five or more rooms.
Of these apartments, a total of 2,179 apartments (88.8% of the total) were permanently occupied, while 126 apartments (5.1%) were seasonally occupied and 149 apartments (6.1%) were empty.
As of 2009, the construction rate of new housing units was
Balsthal has a population (as of December 2018) of 6,121. As of 2008, 28.9% of the population are resident foreign nationals. Over the last 10 years (1999–2009) the population has changed at a rate of 1.7%.
Most of the population (as of 2000) speaks German (4,728 or 84.8%), with Albanian being second most common (163 or 2.9%) and Italian being third (153 or 2.7%). There are 40 people who speak French and 2 people who speak Romansh.
As of 2008, the gender distribution of the population was 51.0% male and 49.0% female. The population was made up of 1,993 Swiss men (34.4% of the population) and 962 (16.6%) non-Swiss men. There were 2,032 Swiss women (35.1%) and 802 (13.9%) non-Swiss women. Of the population in the municipality 2,040 or about 36.6% were born in Balsthal and lived there in 2000. There were 1,179 or 21.2% who were born in the same canton, while 1,006 or 18.0% were born somewhere else in Switzerland, and 1,156 or 20.7% were born outside of Switzerland.
In 2008 there were 29 live births to Swiss citizens and 30 births to non-Swiss citizens, and in same time span there were 39 deaths of Swiss citizens and 6 non-Swiss citizen deaths. Ignoring immigration and emigration, the population of Swiss citizens decreased by 10 while the foreign population increased by 24. There were 15 Swiss men and 9 Swiss women who immigrated back to Switzerland. At the same time, there were 56 non-Swiss men and 25 non-Swiss women who immigrated from another country to Switzerland. The total Swiss population change in 2008 (from all sources, including moves across municipal borders) was a decrease of 34 and the non-Swiss population increased by 87 people. This represents a population growth rate of 0.9%.
The age distribution, as of 2000, in Balsthal is; 401 children or 7.2% of the population are between 0 and 6 years old and 976 teenagers or 17.5% are between 7 and 19. Of the adult population, 300 people or 5.4% of the population are between 20 and 24 years old. 1,681 people or 30.2% are between 25 and 44, and 1,295 people or 23.2% are between 45 and 64. The senior population distribution is 651 people or 11.7% of the population are between 65 and 79 years old and there are 270 people or 4.8% who are over 80.
As of 2000, there were 2,237 people who were single and never married in the municipality. There were 2,693 married individuals, 381 widows or widowers and 263 individuals who are divorced.
As of 2000, there were 2,232 private households in the municipality, and an average of 2.4 persons per household. There were 677 households that consist of only one person and 165 households with five or more people. Out of a total of 2,282 households that answered this question, 29.7% were households made up of just one person and there were 16 adults who lived with their parents. Of the rest of the households, there are 655 married couples without children, 744 married couples with children There were 120 single parents with a child or children. There were 20 households that were made up of unrelated people and 50 households that were made up of some sort of institution or another collective housing.
In 2000 there were 962 single family homes (or 67.9% of the total) out of a total of 1,417 inhabited buildings. There were 256 multi-family buildings (18.1%), along with 113 multi-purpose buildings that were mostly used for housing (8.0%) and 86 other use buildings (commercial or industrial) that also had some housing (6.1%). Of the single family homes 85 were built before 1919, while 92 were built between 1990 and 2000. The greatest number of single family homes (288) were built between 1919 and 1945.
In 2000 there were 2,454 apartments in the municipality. The most common apartment size was 4 rooms of which there were 731. There were 108 single room apartments and 835 apartments with five or more rooms. Of these apartments, a total of 2,179 apartments (88.8% of the total) were permanently occupied, while 126 apartments (5.1%) were seasonally occupied and 149 apartments (6.1%) were empty. As of 2009, the construction rate of new housing units was 
Balsthal