Biology

4.3 - Units & Conversions

6 min

Intro & the units

Converting between units

Sizes and scale

Working with centimetres

note 1 of 1

Units of length and converting between them

Cells and the structures inside them are very small, so their sizes are written using small units of length.
The relevant SI units, from largest to smallest, are:

Unit	Symbol	As a fraction of a metre
Millimetre	mm	10^-3 m
Micrometre	μm	10^-6 m
Nanometre	nm	10^-9 m
Picometre	pm	10^-12 m

The micrometre (μm) is the unit most commonly used to measure cells and cell structures.
Whole cells in a photograph or drawing - enlarged enough to see clearly - are often measured in millimetres (mm).

Converting between units

The steps between these units each go in factors of one thousand:
- There are 1,000 micrometres in a millimetre.
- There are 1,000 millimetres in a metre.
- So there are one million (1,000,000) micrometres in a metre.
To convert mm → μm, multiply by 1,000.
To convert μm → mm, divide by 1,000.
The nanometre is a further factor of one thousand smaller than the micrometre, and the picometre is a factor of one thousand smaller again.

Standard form and matching units

These numbers can be very large or very small, so they are often written in standard form - for example, 0.0025 mm written as 2.5 × 10^-3 mm - which keeps long strings of zeros manageable.
Getting the units to match is the step that matters most whenever sizes are compared or calculated: two measurements must be in the same unit before one is divided by or multiplied with the other.

end of notes

Key points to remember

Units of length and converting between them

Cells and the structures inside them are very small, so their sizes are written using small units of length.

The relevant SI units, from largest to smallest, are:

Unit	Symbol	As a fraction of a metre
Millimetre	mm	10^-3 m
Micrometre	μm	10^-6 m
Nanometre	nm	10^-9 m
Picometre	pm	10^-12 m

The micrometre (μm) is the unit most commonly used to measure cells and cell structures.

Whole cells in a photograph or drawing - enlarged enough to see clearly - are often measured in millimetres (mm).

Converting between units

The steps between these units each go in factors of one thousand:

There are 1,000 micrometres in a millimetre.
There are 1,000 millimetres in a metre.
So there are one million (1,000,000) micrometres in a metre.

To convert mm → μm, multiply by 1,000.

To convert μm → mm, divide by 1,000.

The nanometre is a further factor of one thousand smaller than the micrometre, and the picometre is a factor of one thousand smaller again.

Standard form and matching units

These numbers can be very large or very small, so they are often written in standard form - for example, 0.0025 mm written as 2.5 × 10^-3 mm - which keeps long strings of zeros manageable.

Getting the units to match is the step that matters most whenever sizes are compared or calculated: two measurements must be in the same unit before one is divided by or multiplied with the other.