There are a few things that can get lost in the general maelstrom of content when you revise your science.
One of those things is unit conversions.
In GCSE science, you need to make sure that you have all of your units in SI units for the maths questions.
Kilograms for mass
Amps for current
metres for distance
seconds for time
Joules for energy
If the units you have been given are different, you need to know how to convert them.
To convert them, you need to know the SI prefixes.
You can find a good table of the SI prefixes and what you need to multiply/divide by to get them here.
However, you don't need to know all of those, so I have made a handy Powerpoint with the prefixes you do need and how to interconvert them.
You can find the unit conversions Powerpoint here.