Comparing sizes

The diagram shows the size of a selection of organisms, different cells and other structures.

The diagram shows the size of three organisms, different cells and other structures.

We can compare sizes using a straightforward calculation.

For instance, the length of the leaf cell above is ten times the diameter of a red blood cell.

The calculation would be:

 \frac{70~μm}{7~μm} = 10

When comparing the size of a bacterium with the Human Immunodeficiency Virus (HIV) different units have to be converted to be consistent.

The length of the bacterium = 70 μm = 70 000 nm

The diameter of the HIV = 100 μm = 100 000 nm

Therefore the length of the HIV is  \frac{100~000}{70~000} = 1.4 times that of the bacterium. This means they differ by one order of magnitude.

Question

What is the width of a cheek cell compared with a Salmonella bacterium?

140 times the size.

Calculation:

 \frac {70~μm}{0.5~μm}

Order of magnitude

Differences in size are often described as differences in order of magnitude. That's the difference calculated in factors of 10.

curriculum-key-fact
The notation used to write numbers in multiples and sub-multiples of 10:
  • 1000 = 10 × 10 × 10 = 103
  • 100 = 10 × 10 = 102
  • 10 = 10 = 101
  • 1 = 1 = 100
Any number to the zeroth power is one.