DNA molecules have a consistant width of about 2.5 nanometers (a nanometer is one billionth of a meter; human hair has a thickness of roughly 20,000 nm to 180,000 nm), but the length of DNA molecules varies greatly. Scientists usually describe the length of DNA using a unit called kb or kbp. One kb is 1000 base pairs, the base pair being the basic repeating nucleotide unit of the DNA chain. Each base pair has a length of 0.33 nm. Plasmid DNA might have a length of 1-200 kb, or 0.33 nm to 66 nm. Bacterial chromosomal DNA length would be perhaps 3800 kb, or 1300 nm (1.3 microns).