Cell Size and Scale
Learn the units of the microscopic world and why most cells stay small, with a little geometry.
Quick answer: Most cells are only 1 to 100 micrometres across because a cell takes in nutrients and gets rid of waste through its surface, and as a cell grows its volume rises faster than its surface area. A small cell has a higher surface-area-to-volume ratio, and diffusion across it is quick.
What you will learn
- Convert between millimetres, micrometres, and nanometres.
- Place a virus, a bacterium, an animal cell, and a human egg on a size scale.
- Explain why a small cell has a higher surface-area-to-volume ratio.
- Describe two ways cells overcome the limits of size.
Units for the very small
Everyday units such as the metre or centimetre are far too large to talk about cells. Biologists use three smaller ones:
| Unit | Symbol | Size | Example |
|---|---|---|---|
| Millimetre | mm | one thousandth of a metre | A grain of sand is about 1 mm |
| MicrometreMicrometer (µm)One millionth of a metre, or one thousandth of a millimetre. A typical animal cell is about 10 to 30 µm across and a typical bacterium about 1 to 5 µm. | µm | one thousandth of a millimetre | A red blood cell is about 7 to 8 µm |
| Nanometre | nm | one thousandth of a micrometre | A ribosome is about 25 nm |
Each step down is a factor of 1,000. So 1 mm = 1,000 µm, and 1 µm = 1,000 nm.
A map of the microscopic world
Here are some typical sizes, from largest to smallest. The values are rounded, because real cells vary.
- Human egg cell: about 100 µm. This is just visible to the naked eye as a speck.
- Typical animal cell: 10 to 30 µm.
- Red blood cell: 7 to 8 µm.
- Mitochondrion: 1 to 10 µm long.
- Typical bacterium: 1 to 5 µm.
- Virus: roughly 20 to 300 nm, much smaller than most cells.
- Ribosome: about 25 nm.
- DNA double helix: about 2 nm wide.
A useful anchor: you could line up about ten average animal cells across the width of a human hair, which is around 70 to 100 µm thick.
Some cells are giants
A few cells break the rules by being long, not wide.
- A single neuron can have an axonAxonThe long, thin extension of a neuron that carries electrical signals away from the cell body toward other cells. that reaches over a metre in length, running from the base of the spine to the foot. The cell body is still only tens of micrometres wide.
- A skeletal muscle fiber can grow to many centimetres long. It manages this by fusing together many cells, so one fiber contains hundreds of nuclei.
Notice what these cells have in common. They are long and thin, so every part of the cell stays close to a surface. That brings us to the key reason cells are small.
Why most cells stay small
A cell takes in food and oxygen, and gets rid of waste, through its surface. The amount of material it needs depends on its volume, the amount of living stuff inside. As a cell grows, its volume grows faster than its surface.
To see this, imagine a cell as a cube:
| Side length | Surface area (6 × side²) | Volume (side³) | Surface-area-to-volume ratio |
|---|---|---|---|
| 1 µm | 6 µm² | 1 µm³ | 6 |
| 2 µm | 24 µm² | 8 µm³ | 3 |
| 10 µm | 600 µm² | 1,000 µm³ | 0.6 |
Doubling the side makes the surface-area-to-volume ratioSurface-area-to-volume ratioThe amount of surface a cell has for each unit of its volume. As a cell grows, volume increases faster than surface area, so the ratio falls. fall by half. A large cell has proportionally less membrane to feed each part of its interior.
There is a second problem. Molecules spread inside a cell by diffusionDiffusionThe spreading of particles from a region of higher concentration to a region of lower concentration through random motion. It needs no energy input from the cell., and diffusion is quick over short distances but slow over long ones. The time it takes grows with the square of the distance. If you make a cell ten times wider, diffusion across it takes about a hundred times longer.
A cube-shaped cell doubles its side length. What happens to its surface-area-to-volume ratio?
How cells beat the limit
Cells that need a lot of surface have found clever tricks.
- Folding the surface. Cells lining the small intestine have thousands of finger-like microvilliMicrovilliTiny finger-like folds on the surface of a cell that greatly increase its surface area for absorption, as in the lining of the intestine. that multiply the area available for absorbing nutrients.
- Becoming long and thin. Neurons and muscle fibers keep a high ratio by stretching out in one direction.
- Internal membranes. Organelles such as the endoplasmic reticulum and mitochondria pack huge membrane area into small spaces.
What is the main benefit of microvilli on intestinal cells?
How many micrometres are there in 1 millimetre?
Key takeaways
- 1 millimetre is 1,000 micrometres, and 1 micrometre is 1,000 nanometres.
- Typical bacteria are 1 to 5 µm across and typical eukaryotic cells 10 to 100 µm.
- Doubling a cell's side length cuts its surface-area-to-volume ratio in half.
- Cells beat the size limit by folding their surface, becoming long and thin, or using internal membranes.
Sources
- Biology 2e, Chapter 4: Cell Structure (OpenStax) Concepts adapted and rewritten; no text reproduced verbatim.
- BioNumbers, the database of useful biological numbers
- NIH 3D: Neuron model
Last reviewed October 1, 2026