What is the approximate size of the human genome in base pairs?

The approximate size of the human genome is 3 billion base pairs. A base pair is formed when two complementary DNA bases pair across the DNA double helix, using the letters A, T, C, and G. The roughly three-billion figure refers to one copy of the human genome, arranged across 23 chromosomes.

The Human Genome Project, launched in 1990, aimed to determine the order of these DNA bases. An essentially complete sequence was announced in 2003, covering about 99% of the genome's gene-containing regions. Later projects filled remaining gaps, producing a fully gapless sequence in 2022.

A common mix-up is confusing base pairs with individual DNA bases. Because human cells usually contain two chromosome sets, a diploid cell has roughly 6 billion DNA bases, while the reference genome is commonly described as about 3 billion base pairs. The genome also contains far more DNA than the approximately 20,000 protein-coding genes it includes.