• The route of administration directs the choice of needle length. Refer to the discussion related to each particular route for specific guidelines regarding needle length.
• Body size is also taken into consideration in relation to needle length. An obese person requires a longer needle to reach muscle tissue than does a thin person. A thin person or an older adult with decreased muscle mass or subcutaneous tissue requires a shorter needle.
• When looking at a needle package, the first number is the needle gauge or diameter of the needle (e.g., 18, 20) and the second number is the length in inches (e.g., 1, 1.5).
• As the gauge number becomes larger, the size of the needle becomes smaller; for instance, a 24-gauge needle is smaller than an 18-gauge needle.
• When giving an injection, the viscosity of the medication directs the choice of gauge (diameter). A thicker medication is given through a needle with a larger gauge, and a thinner-consistency medication is given through a needle with a smaller gauge, such as a 24-gauge.
• The amount of medication to be administered directs the choice of the size of the syringe. Smaller syringes should be used as needed for precise dosing because they provide smaller increments of measurement—never estimate a dose. For example, a 1-mL syringe provides increments of 0.01 mL, but a 5-mL syringe allows for precise measures only down to 0.2 mL.
• The type of medication also influences the choice of syringe. There are special syringes for certain uses. An example is the insulin syringe used only to inject insulin.

Needle package showing first number (gauge or diameter of the needle) and second number (length of the needle in inches).

Different needle sizes.

An 18-gauge needle (top) and a 24-gauge needle (bottom).