Scientists use the kilogram and the gram for the units of mass.
Most scientists use the International System of Units (Système International d'Unités), abbreviated as SI.
In SI, the base unit of mass is the kilogram.
The kilogram is defined as being equal to the mass of the International Prototype of the Kilogram (IPK).
A 1 L bottle of water has a mass of 1 kg, plus a little bit extra for the plastic.
The kilogram is a rather large unit for most measurements in the laboratory, so many measurements are made using the unit "gram".
The gram is defined as
A paperclip has a mass of about 1 g.