The largest county in Idaho
There are two right answers. By land area it is Idaho County; by population it is Ada County. Which one you want depends on what you meant by largest.
Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025. Retrieved 30 August 2026.
By land area
| Rank | Name | Land area | Population |
|---|---|---|---|
| 1 | Idaho County | 8,477 sq mi | 17,874 |
| 2 | Owyhee County | 7,668 sq mi | 12,661 |
| 3 | Custer County | 4,922 sq mi | 4,636 |
| 4 | Lemhi County | 4,564 sq mi | 8,474 |
| 5 | Valley County | 3,665 sq mi | 12,831 |
| 6 | Elmore County | 3,075 sq mi | 29,465 |
| 7 | Blaine County | 2,638 sq mi | 25,517 |
| 8 | Shoshone County | 2,637 sq mi | 14,130 |
| 9 | Cassia County | 2,566 sq mi | 26,397 |
| 10 | Clearwater County | 2,457 sq mi | 9,118 |
By population
| Rank | Name | Population | Density |
|---|---|---|---|
| 1 | Ada County | 546,141 | 519 per sq mi |
| 2 | Canyon County | 275,123 | 469 per sq mi |
| 3 | Kootenai County | 191,864 | 155 per sq mi |
| 4 | Bonneville County | 135,771 | 72.8 per sq mi |
| 5 | Twin Falls County | 97,539 | 50.8 per sq mi |
| 6 | Bannock County | 91,591 | 82.3 per sq mi |
| 7 | Madison County | 55,172 | 118 per sq mi |
| 8 | Bonner County | 54,420 | 31.4 per sq mi |
| 9 | Bingham County | 51,153 | 24.4 per sq mi |
| 10 | Nez Perce County | 42,905 | 50.6 per sq mi |
And the smallest
More on Idaho
Rebuilt from source data 30 August 2026. How these figures are produced.