what size needle is used for iv injections ：

Subcutaneous injection 1ml 4.5～5, intramuscular injection 2.5ml 5.5～6, intravenous injection, intravenous blood collection 2.5ml 10ml 6.5～7,

Note: The thickness of the needle is expressed by the number of gauges, generally 4.5, 5, 6, 7, etc., the 7 is the needle diameter of 0.7 mm (may be the outer diameter), generally children use 4.5 or 5, adults use Number 7.