Scale Explorer

Microscopic

How big is a DNA double helix?

DNA double helix · B-form DNA, the common form in cells

Diameter
≈ 2 nm 2 nm approx.
Diameter (fibre model)
2.04 nm 2.04 nm
Length of one helical turn
≈ 3.4 nm 3.4 nm approx.
Spacing between base pairs
≈ 0.34 nm 340 pm approx.
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DNA double helix, Red blood cell, Human hair (width), Human drawn to the same scaleDNA double helix: diameter ≈ 2 nm (too small to see at this scale). Red blood cell: diameter 7.5–8.7 µm (too small to see at this scale). Human hair (width): average diameter ≈ 70 µm (too small to see at this scale). Human: height 1.61–1.75 m.DNA double helix: ≈ 2 nm (too small to see at this scale)Red blood cell: 7.5–8.7 µm (too small to see at this scale)Human hair (width): ≈ 70 µm (too small to see at this scale)Human: 1.61–1.75 mHuman1.61–1.75 mDNA double helix≈ 2 nm · too small to seeRed blood cell7.5–8.7 µm · too small to seeHuman hair (width)≈ 70 µm · too small to see

DNA double helix, Red blood cell and Human hair (width) are too small to see at this scale and are marked with ▲.

Everything is drawn to the same scale, standing on the same ground line. Silhouettes are simplified outlines scaled to the measured dimension. A faded outline shows the top of a size range.

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Dimensions

MeasurementMetricImperialNotesSource
Diameter ≈ 2 nm ≈ 2 nm Width of the double helix, stated as 20 Å (2 nm). Approximate [1]
Diameter (fibre model) 2.04 nm 2.04 nm 20.4 Å for B-DNA from optimised fibre-diffraction parameters (Arnott and Hukins 1972), as listed by BioNumbers; converted from Å [2]
Length of one helical turn ≈ 3.4 nm ≈ 3.4 nm One complete turn holds about 10 base pairs; stated as 34 Å. Approximate [1]
Spacing between base pairs ≈ 0.34 nm ≈ 340 pm Stacked base pairs are 3.4 Å apart along the helix axis. Approximate [1]

About the DNA double helix

DNA carries genetic information as two strands wound around each other in a right-handed double helix. The paired bases are stacked in the middle like the steps of a spiral staircase, with the sugar-phosphate backbones on the outside. In the common B form the helix is about 2 nm across, and it makes one full turn roughly every 3.4 nm along its length.

  • Each complete turn of the B-DNA helix contains about 10 base pairs. [1]
  • In solution, water bound to DNA makes its apparent (hydrodynamic) diameter larger, about 22 to 26 Å. [2]

How many fit?

  • ≈ 840 million × DNA double helix · diameter lined up end to end match the Human · height
  • ≈ 4,000 × DNA double helix · diameter lined up end to end match the Red blood cell · diameter

Counts divide one sourced figure by another. "End to end" uses each object's main measurement; weight and volume rows appear only when both figures are sourced.

How it compares

  • 1/4,000 A red blood cell is about 4,000 times as across as a DNA double helix is wide. See side by side
  • 1/35,000 The width of a human hair is about 35,000 times as wide as a DNA double helix. See side by side
  • 1/31,000–1 million A grain of sand is about 31,000–1 million times as wide as a DNA double helix. See side by side
  • 1/1.7 million–2.5 million A black garden ant is about 1.7 million–2.5 million times as long as a DNA double helix is wide. See side by side
  • 1/3.5 million A grain of rice is about 3.5 million times as long as a DNA double helix is wide. See side by side
  • 1/840 million An average adult is about 840 million times as tall as a DNA double helix is wide. See side by side

Factors compare the diameter of a DNA double helix with each object’s own main measurement (height for a human). Where a size varies, the factor is given as a range.

Notes on these figures

  • Both sources give the width in angstroms (20 Å and 20.4 Å), a unit not in the site's list; the Guelph text itself also states 2 nm, while 2.04 nm is an exact conversion of 20.4 Å. Turn length and base spacing are likewise converted from 34 Å and 3.4 Å.
  • Only the helix width is shown; a DNA molecule's length depends on how many base pairs it has, so no single length is given.

Sources

  1. University of Guelph (eCampusOntario Pressbooks), The DNA Double Helix - BIOC*2580: Introduction to Biochemistry (John Dawson). Accessed 25 September 2026.
  2. BioNumbers, Harvard Medical School, Diameter of DNA double helix - BNID 105243. Accessed 25 September 2026.

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