Iffwil has a population (as of December 2018) of 443.
As of 2010, 1.9% of the population are resident foreign nationals.
Over the last 10 years (2000–2010) the population has changed at a rate of −2.1%.
Migration accounted for 3.1%, while births and deaths accounted for 4.7%.

Most of the population (as of 2000) speaks German (405 or 98.1%) as their first language, French is the second most common (2 or 0.5%) and Spanish is the third (1 or 0.2%).

As of 2008, the population was 50.6% male and 49.4% female.
The population was made up of 206 Swiss men (50.1% of the population) and 2 (0.5%) non-Swiss men.
There were 197 Swiss women (47.9%) and 6 (1.5%) non-Swiss women.
Of the population in the municipality, 120 or about 29.1% were born in Iffwil and lived there in 2000.
There were 202 or 48.9% who were born in the same canton, while 61 or 14.8% were born somewhere else in Switzerland, and 13 or 3.1% were born outside of Switzerland.

As of 2010, children and teenagers (0–19 years old) make up 21.4% of the population, while adults (20–64 years old) make up 64% and seniors (over 64 years old) make up 14.6%.

As of 2000, there were 165 people who were single and never married in the municipality.
There were 213 married individuals, 19 widows or widowers and 16 individuals who are divorced.

As of 2000, there were 28 households that consist of only one person and 12 households with five or more people.
In 2000, a total of 150 apartments (89.3% of the total) were permanently occupied, while 12 apartments (7.1%) were seasonally occupied and 6 apartments (3.6%) were empty.
The vacancy rate for the municipality, in 2011, was 0.57%.

The historical population is given in the following chart:
Iffwil has a population (as of December 2018) of 443. As of 2010, 1.9% of the population are resident foreign nationals. Over the last 10 years (2000–2010) the population has changed at a rate of −2.1%. Migration accounted for 3.1%, while births and deaths accounted for 4.7%.
Most of the population (as of 2000) speaks German (405 or 98.1%) as their first language, French is the second most common (2 or 0.5%) and Spanish is the third (1 or 0.2%).
As of 2008, the population was 50.6% male and 49.4% female. The population was made up of 206 Swiss men (50.1% of the population) and 2 (0.5%) non-Swiss men. There were 197 Swiss women (47.9%) and 6 (1.5%) non-Swiss women. Of the population in the municipality, 120 or about 29.1% were born in Iffwil and lived there in 2000. There were 202 or 48.9% who were born in the same canton, while 61 or 14.8% were born somewhere else in Switzerland, and 13 or 3.1% were born outside of Switzerland.
As of 2010, children and teenagers (0–19 years old) make up 21.4% of the population, while adults (20–64 years old) make up 64% and seniors (over 64 years old) make up 14.6%.
As of 2000, there were 165 people who were single and never married in the municipality. There were 213 married individuals, 19 widows or widowers and 16 individuals who are divorced.
As of 2000, there were 28 households that consist of only one person and 12 households with five or more people. In 2000, a total of 150 apartments (89.3% of the total) were permanently occupied, while 12 apartments (7.1%) were seasonally occupied and 6 apartments (3.6%) were empty. The vacancy rate for the municipality, in 2011, was 0.57%.
The historical population is given in the following chart:
Iffwil