How Big Is a Cell? Most Are a Hair's Width or Far Narrower

Bench card

Question

How big is a cell?

Answer range

most cells about 0.4–100 µm [10⁻⁷–10⁻⁴ m]; measured single cells run from about 0.2 µm (SAR11 bacteria) to about 8 cm (an ostrich egg); a human nerve cell's axon reaches 1 m or more, and so does Caulerpa if a cell with many nuclei counts as one cellSource: cell-sizes.csv on this page

Method

compiled measurements

Inputs

62 rows from 22 sources (55 as stated by the sources, 7 computed as spheres of a stated volume)Source: cell-sizes.csv on this page

Weakest input

what counts as one cell: Caulerpa and Valonia have many nuclei, a xenophyophore's 24 cm is the size of its test, not of the cell inside, and the 8 cm ostrich egg is a figure quoted in a 2012 essay from a 1934 book

Key source

Marshall WF et al. 2012, What determines cell size?, BMC Biology (essays by Young and Frankel)

how-big-is-a-cell-bench.csv (CSV) · where each value comes from, and its license (JSON)

Data: Marshall WF et al. 2012, What determines cell size?, BMC Biology 10:101, CC BY 2.0, https://doi.org/10.1186/1741-7007-10-101 (retrieved 2026-10-25); Gooday AJ, Durden JM, Smith CR 2020, Communicative & Integrative Biology 13(1):189-197, CC BY 4.0, https://doi.org/10.1080/19420889.2020.1843818 (retrieved 2026-10-25); row and source counts computed 2026-10-25 for this page from cell-sizes.csv

The downloadable table behind this page holds 51 sizes from 18 sources. The 23 that are plotted are listed under the chart. Each row names what was measured and where the figure comes from, and marks the sources whose abstract alone was available. A size computed for this page, such as the width of a sphere that holds a measured volume, says computed.

How big are cells, on one log scale?

Put every cell in the table on an ordinary ruler 1 m long, and anything under 1 mm crowds into the first 0.1% of it. A log scale gives each factor of ten the same room, so a 0.2 µm bacterium and a metre-long alga fit on one line. For sizes spread this wide, a logarithmic axis is the only honest one.

On that axis the cells bunch up. Kevin Young, writing in a 2012 BMC Biology forum on cell size, puts most known bacteria at 0.4–2 µm in diameter and 0.5–5 µm long. The human cells counted by Sender, Fuchs and Milo in 2016 include red blood cells of about 100 µm³, among the smallest human cell types, and fat and muscle cells usually over 10,000 µm³. Spheres of those volumes would be about 6 µm and about 27 µm or more across, computed for this page.

Slide the diagram sideways to read its labels, or open it full size.

What is the smallest cell?

The smallest cells in these sources are bacteria. In 2015 Luef and colleagues reported cells from groundwater that had passed through a filter of about 0.2 µm, with volumes of 0.009 ± 0.002 µm³ (abstract). A sphere of 0.009 µm³ would be about 0.26 µm across, computed for this page.

Young calls the marine SAR11 bacteria "the tiniest 0.2 µm cells". Yet one measured SAR11 strain is larger by volume than the groundwater cells. Frozen-hydrated cells of Candidatus Pelagibacter ubique (Pelagibacter ubique in GBIF), strain HTCC1062, averaged 0.037 µm³, by Zhao and colleagues' 2017 account (abstract). That is about four times the groundwater cells' volume, and a sphere that size is about 0.41 µm across, both computed.

Zhao and colleagues describe SAR11 as bacterioplankton found throughout the oceans, part of the plankton. Prochlorococcus, 0.5–0.7 µm in diameter, was the smallest known photosynthetic organism as of a 1999 review by Partensky, Hess and Vaulot (abstract). Young describes Escherichia coli as a rod normally about 1 µm wide and 2 µm long.

Slide the diagram sideways to read its labels, or open it full size.

Cells this small sit at the edge of what a light microscope can show. Abbe's limit of 1873, as a 2021 methods review by Valli and colleagues gives it, is about the wavelength divided by twice the numerical aperture, a measure of the range of angles the lens accepts. For a standard fluorescence microscope detecting light of 470–700 nm, the review puts it at around 170–250 nm. Two points closer than that blur into one spot.

The first two sizes above were measured with electron microscopes. Luef's study used cryogenic transmission electron microscopy, and Zhao's used cryo-electron tomography. Viruses are not cells and are smaller still: the head of bacteriophage T4, a virus that infects bacteria, is 120 nm long and 86 nm wide, by Rao and Black's 2010 account.

How big is an ordinary cell?

Grown in culture, Escherichia coli cells averaged 1.0–1.5 µm wide and 1.6–3.9 µm long across 22 growth conditions (Volkmer and Heinemann 2011). How a cell that size moves through water is set by its Reynolds number, a ratio of inertia to stickiness that grows with both size and speed.

In a 70 kg reference man, Sender and colleagues count about 3.0×10¹³ human cells, 84% of them red blood cells, and about as many bacteria (How Many Cells in the Human Body?). Red cells have no nucleus, and so no nuclear DNA. Fat and muscle cells are about 0.2% of the count.

Across the whole body, Hatton and colleagues found in 2023 that cell masses span about seven orders of magnitude, from a red blood cell to the largest muscle fibres, each fibre a cell with thousands of nuclei. For cells of the same shape, a million times the mass means only a hundred times the width, computed as a cube root.

Slide the diagram sideways to read its labels, or open it full size.

Pors and colleagues measured 1,563 human egg cells in 2022, collected immature and cultured in the laboratory: 60 to 171 µm across, mean 115 µm, not counting the outer coat, the zona pellucida. Hooke reckoned the naked eye could not resolve finer than one minute of arc, a sixtieth of a degree, as quoted in de Jong's 2024 history of visual acuity testing. Seen from 25–40 cm, a viewing distance chosen for this page, that angle spans about 73–116 µm, computed for this page.

Paramecium caudatum cells measure 140–311 µm long across the studies compiled in the BOSO-Micro database by Velho Rodrigues and colleagues in 2021 (Paramecium). A male Dicopomorpha echmepterygis (eschmepterigis in the paper), the smallest known parasitoid insect, is smaller than that whole range: 139 µm in size by Polilov's 2015 account, citing earlier measurements (smallest insect). Budding yeast, Saccharomyces cerevisiae, also listed in GBIF as baker's yeast, commits to its next division only at a set size, as Swaffer, Wood and Nurse describe in the 2012 forum.

What is the largest single cell?

Bacteria hold their own records. Young puts some species of Thiomargarita and Epulopiscium at over 600 to 700 µm, visible to the naked eye. Spherical Thiomargarita cells are typically 100–300 µm and can reach 750 µm, as Gooday and colleagues note in 2018. In 2022 Volland and colleagues described Candidatus Thiomargarita magnifica (Thiomargarita magnifica in GBIF), with an average cell length greater than 9,000 µm (abstract). That is more than 4,000 times the roughly 2 µm they give for most bacterial species, computed for this page.

Several algae and protists reach centimetres as single cells. Joseph Frankel, in the same 2012 forum, lists the stalked alga Acetabularia at 3–5 cm and the deep-sea protist Gromia sphaerica at 3 cm. The bubble alga Valonia ventricosa is typically 1–5 cm across, by the account of Razavi and colleagues in 2022.

Past this point the answer depends on the definition. Frankel calls the green alga Caulerpa, with many nuclei and a stolon a metre or more long, the champion unicellular organism. Valonia itself, long treated as one giant cell, is built of many cytoplasmic domains with one nucleus each, by the account of Shepherd, Beilby and Bisson in 2004 (abstract), who use the synonym Ventricaria ventricosa.

Gerber and colleagues write that the plasmodium of the slime mold Physarum polycephalum, a single cell with many nuclei, can grow to cover square metres. A human neuron is one cell too, and its axon can run a metre or more, by Roy and Tedeschi's account.

The xenophyophores, giant foraminifera of the deep sea, build tests of foreign particles up to 24 cm in size, among the largest single-celled organisms known, by Gooday and colleagues' 2020 account. The cell itself takes up only a small part of that test. Rough earlier estimates for 12 Pacific specimens, recounted by Gooday's group in 2018, mostly put the cell's branching strands at 1% of the test volume or less; that group's own scans of three species found 1.7–5.1%.

Is an egg one cell?

Frankel writes that in animals the largest dividing cells are eggs. The most familiar frogs and salamanders lay eggs 1 to just over 2 mm across, and some amphibian eggs reach 10 mm. The egg of the frog Xenopus laevis, 1.2 mm across, is a single cell until it divides. After 12 divisions its cells are about 50 µm, John Wallingford notes in the same forum.

Illustration

Frankel's list of centimetre-scale cells starts with the ostrich egg, measured at 8 cm, a figure he credits to E. B. Wilson. In bird and reptile eggs, he adds, only a small portion of the mass cleaves. The essay does not say which part of the egg the 8 cm describes. The mean human egg cell above, 115 µm, is about 1/700 of that, computed for this page.

Why do most cells stay small?

Young gives one reason for bacteria. Their nutrients reach them by diffusion alone, so the ratio of surface to volume has to stay high: "to maximize this ratio, most bacteria produce cells in the 0.2 to 10 μm size range".

Geometry adds to that. A sphere's surface divided by its volume is 6 divided by its diameter: 6 per µm at 1 µm, 0.06 per µm at 100 µm, computed for this page. Diffusion time grows with the square of distance.

In water at 25 °C, oxygen diffuses about 1 µm in 0.24 ms, 100 µm in 2.4 s, 1 mm in 4 minutes and 1 cm in 6.6 hours. These times come from a table shared with the page on how insects breathe, built on Somerville and Proctor's diffusion coefficient and on t = x²/(2D) (Vestergaard and colleagues).

Slide the diagram sideways to read its labels, or open it full size.

Large cells, colonies and animals work around the limit. In Thiomargarita, a liquid-filled vacuole takes some 98% of the volume, leaving the active cytoplasm a thin outer layer, Gooday and colleagues note. Oxygen diffuses nearly 10,000 times more readily in air than in water, Somerville and Proctor write, and in many tissues an insect's finest air tubes may reach practically every cell, by Maina's 2012 account. Volvox, a spherical shell of many green cells, would outgrow what diffusion alone can feed; its beating flagella stir enough water to get past that limit, by Short and colleagues' account (abstract).

Every tenfold step in width multiplies the diffusion time a hundredfold. A cell 1,000 times wider than a 1 µm bacterium would wait a million times longer for oxygen to cross it by diffusion alone.

Sources

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Ants, Cells & Atoms. “How Big Is a Cell? Most Are a Hair's Width or Far Narrower.” Published 2026-10-25. https://antscellsandatoms.com/scale/how-big-is-a-cell/. Accessed (add the date you read it).

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