Weight and mass are often used to mean the same thing. But they are different.
The mass of an object is a measure of the amount of matter it contains.
The weight of an object is a measure of the force exerted by gravity on the object. The weight of the object will vary according to its distance from the centre of the earth.
The basic unit for mass in the metric system of measurement is the kilogram (kg).
The commonly used units are milligrams, grams, kilograms and tonnes.
Unit 
Symbol

Examples 
milligrams 
mg

A feather weighs 25 mg 
grams 
g

A kitten has a weight of 240 g 
kilograms 
kg

My weight is 85 kg 
tonnes 
t

A car weighs 1.1 tonnes. 
Converting between Units
To convert between the units shown in bold in the first column and the units in the second column, multiply or divide by the factors indicated in the third column.
Unit

Convert to

Action 
milligrams (mg) 
g

÷ 1000 
kg

÷ 1000 000  
t

÷ 1 000 000 000  
grams (g) 
mg

× 1000 
kg

÷ 1000  
t

÷ 1 000 000  
kilograms (kg) 
mg

× 1 000 000 
g

× 1000  
t

÷ 1000  
tonnes (t) 
mg

× 1 000 000 000 
g

× 1 000 000  
kg

× 1000 