| Population Delaware, Ohio | ||
| Census | City | County |
| 1810 | 150 | 2000 |
| 1820 | 250 | 7639 |
| 1830 | 532 | 11504 |
| 1840 | 898 | 22060 |
| 1850 | 2074 | 21817 |
| 1860 | 3889 | 23902 |
| 1870 | 6000 | 25175 |
| 1880 | 6968 | 27381 |
| 1890 | 27189 | |
| 1900 | 7960 | 26401 |
| 1910 | 9227 | 27182 |
| 1920 | 9057 | 26013 |
| 1930 | 8722 | 26016 |
| 1940 | 8944 | 26780 |
| 1950 | 11785 | 30278 |
| 1960 | 13500 | 36107 |
| 1970 | 15008 | 42908 |
| 1980 | 18780 | 53840 |
| 1990 | 20030 | 66929 |
| 2000 | 25243 | 109989 |
| 2004 est. | 30012 | 142503 |