| | Belvidere, IL | Kirkland, WA | United States |
| Population | 25,179 | 91,656 | 329,725,481 |
| Female Population | 52.3% | 50.0% | 50.5% |
| Male Population | 47.7% | 50.0% | 49.5% |
| Median Age | 36.9 | 37.8 | 38.4 |
| space3 | | | |
| Population - 2010 | 25,598 | 80,869 | 309,327,143 |
| Population - 2000 | 20,820 | 45,054 | 285,036,114 |
| Population - 1990 | 15,958 | 40,052 | 251,960,433 |
| Pop. 2000 to Now | 18.8% | 111.8% | 15.6% |
| Pop. 1990 to Now | 55.0% | 138.3% | 30.8% |
| space4 | | | |
| Population Density | 2,085.9 | 5,146.9 | 93.4 |
| Land Area | 12.1 | 17.8 | 3,531,905.4 |
| Water Area | 0.2 | 2.8 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Belvidere, IL | Kirkland, WA | United States |
| Age 0 to 4 | 5.9% | 6.0% | 5.9% |
| Age 5 to 9 | 7.1% | 5.6% | 6.1% |
| Age 10 to 14 | 7.2% | 5.7% | 6.6% |
| Age 15 to 17 | 3.6% | 3.0% | 3.9% |
| Age 18 to 20 | 4.5% | 2.7% | 4.0% |
| Age 21 to 24 | 4.5% | 4.7% | 5.2% |
| Age 25 to 34 | 13.4% | 17.0% | 13.8% |
| Age 35 to 44 | 11.9% | 17.0% | 12.9% |
| Age 45 to 54 | 13.5% | 12.6% | 12.6% |
| Age 55 to 59 | 7.5% | 6.3% | 6.7% |
| Age 60 to 64 | 4.8% | 6.1% | 6.3% |
| Age 65 to 74 | 10.1% | 8.6% | 9.6% |
| Age 75 to 84 | 4.4% | 3.6% | 4.5% |
| Age 85 and over | 1.4% | 1.4% | 1.9% |
| space1 | | | |
| Race | | | |
| | Belvidere, IL | Kirkland, WA | United States |
| White | 60.1% | 67.9% | 59.4% |
| Black | 1.3% | 1.6% | 12.2% |
| Asian | 0.9% | 16.0% | 5.6% |
| Native American | 0.0% | 0.2% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.2% | 0.6% | 0.4% |
| Two or More Races | 1.1% | 6.2% | 3.2% |
| Hispanic | 36.4% | 7.5% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Belvidere, IL | Kirkland, WA | United States |
| Married Population | 42.0% | 55.3% | 49.8% |
| Currently Married | 39.9% | 54.1% | 48.0% |
| Married but Separated | 2.1% | 1.2% | 1.8% |
| Single Population | 58.0% | 44.7% | 50.2% |
| Never Married | 36.6% | 30.0% | 33.8% |
| Divorced | 14.5% | 10.9% | 10.8% |
| Widowed | 6.9% | 3.8% | 5.6% |
| space5 | | | |
| Household Size | 2.71 | 2.43 | 2.60 |
| Households | 9,193 | 37,156 | 124,010,992 |
| Family Households | 5,829 | 23,196 | 80,755,759 |
| Married couple, w/children | 26.5% | 38.3% | 30.5% |
| Married couple, no children | 37.6% | 45.0% | 42.9% |
| Non Family Households | 3,364 | 13,960 | 43,255,233 |
| Single householder, w/children | 23.0% | 8.4% | 15.8% |
| Single householder, no children | 12.9% | 8.3% | 10.8% |