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How big is the human genome?
Dr. Matic Broz Computational chemist
Table of contents
The human genome is small enough to fit on an ordinary laptop, but large enough that the unit matters. One copy is about 3.1 billion DNA base pairs. As plain text, those letters take about 3 GB before compression; as a compressed reference file, the common hg38 FASTA download is under 1 GB.
That is the clean answer. The exact number changes depending on whether you mean a haploid reference, a diploid person, a complete assembly, a FASTA file, or raw sequencing data.
How big is the human genome?
One copy of the human genome is about 3 billion base pairs. In the GRCh38.p14 reference assembly, the total length across all scaffolds is 3,099,734,149 bp; across placed scaffolds, it is 3,088,269,832 bp.[1][2]
Most human cells are diploid, meaning they carry two chromosome sets: one inherited from each parent. That puts the nuclear DNA sequence in a typical cell at roughly 6.2 billion base pairs before considering small additions such as mitochondrial DNA.
The newer Telomere-to-Telomere CHM13 assembly gives a related but not identical answer. The 2022 T2T paper reported a complete 3.055-billion-base-pair sequence for chromosomes 1-22 and X, closing many repetitive regions that earlier human genome sequencing references left unresolved.[3]
For most public uses, "about 3.1 billion base pairs per copy" is the right answer. For technical work, name the assembly and whether you are counting all scaffolds, only placed chromosomes, gaps, or non-gap bases.
What is the human genome size in KB, MB, and GB?
The GRCh38.p14 human genome is about 3,099,734 kb, 3,100 Mb, or 3.10 Gb in genomics units. If the same bases are stored as one plain-text character per base, they take about 3.10 GB, or 2.89 GiB, before FASTA headers and line breaks.
| Unit | Human genome size | What the unit means |
|---|---|---|
| bp | 3,099,734,149 bp | bp in GRCh38.p14 across all scaffolds |
| kb | 3,099,734 kb | Kilobases; 1 kb = 1,000 bp |
| Mb | 3,100 Mb | Megabases; 1 Mb = 1,000,000 bp |
| Gb | 3.10 Gb | Gigabases; 1 Gb = 1,000,000,000 bp |
| GB | About 3.10 GB | Gigabytes if one base is stored as one byte |
The bp, kb, Mb, and Gb rows are direct conversions from the Genome Reference Consortium's GRCh38.p14 all-scaffold length. The GB row is a storage calculation: one ASCII DNA letter takes one byte, so 3.099 billion letters take about 3.099 billion bytes before file-format overhead.[2]
The capitalization is easy to miss. In genomics, Gb usually means gigabases. In computing, GB means gigabytes. A human genome is about 3.1 Gb as a biological sequence and about 3.1 GB as uncompressed one-letter-per-base text.
How large is a human genome file?
A human reference genome file is usually under 1 GB when compressed, but a person's raw whole-genome sequencing data is much larger. UCSC lists the compressed hg38 FASTA file, hg38.fa.gz, at 938 MB and the compact hg38.2bit file at 797 MB.[4]
| File or data type | Typical size | What it contains |
|---|---|---|
hg38.2bit | 797 MB | Compact two-bit encoded hg38 reference |
hg38.fa.gz | 938 MB | Gzip-compressed hg38 FASTA reference |
| Plain sequence text | About 3.1 GB | One character per base, before headers and line breaks |
| Two-bit minimum | About 0.78 GB | Four DNA letters can be represented with two bits per base |
FASTA is larger before compression because it is a text format: it includes a definition line for each sequence and then lines of nucleotide letters. NCBI recommends keeping nucleotide FASTA sequence lines no longer than 80 characters, which adds line breaks to the file.[5]
Raw whole-genome sequencing data answers a different question. A 30x run reads the genome many times over; Illumina's NovaSeq X specifications assume more than 120 Gb of sequence data per sample to achieve 30x coverage. FASTQ files also store read names and quality scores, so the files produced by sequencing are much larger than a single reference genome file.[6]
Why do different sources give different human genome sizes?
Human genome size estimates differ because they count different things: a rounded educational value, a specific reference assembly, only placed chromosomes, all scaffolds, gap characters, non-gap bases, one chromosome set, two chromosome sets, or sequencing reads.
GRCh38.p14 illustrates the difference. Its all-scaffold total length is 3,099,734,149 bp, but its all-scaffold ungapped length is 2,948,611,470 bp because the ungapped count excludes N bases used for unresolved sequence. The placed-scaffold ungapped length is smaller again, at 2,937,639,396 bp.[2]
This is why the rounded answer and the exact answer can both be correct. "About 3 billion base pairs" is the right scale for a general explanation. "3,099,734,149 bp in GRCh38.p14 all scaffolds" is the better answer when the reference assembly and counting rule matter.
Genome size is also separate from physical DNA length and gene count. The same 3.1-billion-base reference can be discussed as a file, a molecule, or an annotation system.
Sources▼
- Base Pair National Human Genome Research Institute · July 1, 2026. https://www.genome.gov/genetics-glossary/Base-Pair
- Human Genome Assembly GRCh38.p14 Genome Reference Consortium · July 1, 2026. https://www.ncbi.nlm.nih.gov/grc/human/data
- The complete sequence of a human genome Science · 2022. https://pubmed.ncbi.nlm.nih.gov/35357919/
- Index of /goldenPath/hg38/bigZips UCSC Genome Browser · July 1, 2026. https://hgdownload.soe.ucsc.edu/goldenPath/hg38/bigZips/
- FASTA Format for Nucleotide Sequences NCBI GenBank · July 1, 2026. https://www.ncbi.nlm.nih.gov/genbank/fastaformat
- NovaSeq X Specifications Illumina · July 1, 2026. https://www.illumina.com/systems/sequencing-platforms/novaseq-x-plus/specifications.html
