The magnitude of a measurement is the size of that quantity. Have a look at the following table of physical constants that you may be familiar with. It has been split up to clearly show the difference between the magnitude and the units in each measurement.
|Acceleration at the Earth's surface due to gravity||9.8||m / s^2|
|Absolute Zero||-273.15||degrees Celsius|
|Freezing point of Silver||960.8||degrees Celsius|
|Speed of light||299792458||m / s|
|Density of Mercury at room temperature||13.6||g / ml|
All of these quantites are measured in units that permit an arbitrary degree of precision. For example. just as we can measure one metre, we can measure 0.1 metres, 0.01 metres and so on, being as accurate as we would wish.
It may seem that this is true of all quantities, but this is not the case. Sometimes we can "measure" something exactly --- as on the next page.