Size Scales of Cell Biology

Understanding the Size Scales Relevant to Cell Biology

The width of a typical person’s thumb is approximately 20 millimeters (mm). If we zoom in further and looked closely at the fingerprint ridges of the thumb, the width of several of those ridges is about 2 mm. Inside the thumb, we see thousands or hundreds of cells. This hundred cluster of cells is approximately 0.2 mm. Notice how we are transitioning down by tens, from 20 to 2 to 0.2, in the same mm unit. This pattern will help you remember the general transition of size scales in cell biology.

Next, we will look more closely at a group of five cells. In the diagram below, retrieved from the university textbook Essential Cell Biology, a group of two to three cells has a width of 20 micrometers (µ m). 2 µ m is about the size of an organelle, such as the depicted mitochondria in the figure, and 0.2 µ m is the size of chains of ribosomes.

Size Scales of Cell Biology

Moving into the units of nanometers (nm), we observe that 1 ribosome is approximately 20 nm and molecules are approximately 2 nm. Finally, the lone atom is about 0.2 nm.


Summarized Biological Size Scales


  • 20 mm: Width of your thumb
  • 2 mm: Width of several thumb fingerprint ridges
  • 2 mm: Tissues of cells (About hundreds of cells)

Going into Micrometers:

  • 20 µ m: Cell size
  • 2 µ m: An organelle
  • 2 µ m: Chain of ribosomes

Going into Nanometers

  • 20 nm: A ribosome
  • 2 nm: Molecues
  • 2 nm: An atom

In general, the size of cells vary, depending on the cell type, its components, age, etc. Small bacteria are usually around 1 µ m (micrometer) – about the size of large organelle in an eukaryotic cell. A typical eukaryotic cell is about 10 µm – 20 µm. Organelles are typically about 1 µm – 5 µm.

Here is a summarized breakdown of general sizes for organelles, bacteria, and eukaryotes:

  • Bacteria ~1 µm
  • Typical Eukaryotic Cell ~10-20 µm
  • Organelle ~ 1-5 µm (Some are even bigger than bacteria)
  • Ribosomes ~25 nm (consistent with the diagram above)
  • Hydrogen atom ~0.1 nm


Source: Essential Cell Biology Third Edition