Multiples and Submultiples of Units

Multiplies of Units

In physics, the magnitude of a quantity being measured may be very large. Image how far it is to the nearest star, alpha centauri. It’s a long way!

Actually, it is about forty-one thousand three hundred and twenty million, million meters from Earth. That’s 41,320,000,000,000,000 m, and that is the nearest star!

When dealing with large numbers, with lots of zeros, it can quickly get confusing. To make it simpler to present these large numbers we can use multiples of SI units.

Multiples and Submultiples of Units, figure 1

A good example of this is reporting distances.

If you want to measure the length of a room you would give the result in the base SI unit of the meter. If you wanted to tell someone the distance from London to Paris you wouldn’t tell them its about 375,000 m, you would say its 375 kilometers (km).

The ‘k’ in front of the meter means 1000. So 1 km is equal to 1000 m.

We can also use M for mega (1 million) and G for giga (1 billion).

Just as we do in computer storage of a Gigabyte or a Megabyte.

So Alpha century is 41,320,000 Gm away, it is still a big number but with fewer zeros to report the chances of making a mistake are reduced.

Multiples and Submultiples of Units, figure 2


The standard atmospheric pressure at sea level is 101,325 Pa or 101.3 kPa.

Sub-multiple of Units

Physicists often has to measure very small objects or amounts. When scientists do this they often use a prefix to the SI unit to indicate the fraction of the unit.


The length of your mobile phone might be 0.145 m, but we would normally write as 14.5 cm. The ‘c’ in this example stands for centi. As in, centimeters and means 1 /100 of a meter. We could also write it as 145mm, where the first ‘m’ means milli, as in millimeters. The term milli means 1/1000th of a unit.

Multiples and Submultiples of Units, figure 1


The width of a carbon atom = 0.3 nm (0.00000000003 m)

The wavelength of green light is between 520 to 560 nm

If a device has a power rating of 14.7 MW. how many watts is this equal to?
Your answer should include: 14 / 700 / 000 / 14700000 / 14700000W
Explain why it is an advantage to use multiples of SI units when communicating large values.
Your answer should include: accurate / reliable communication / easier to understand