What size needle for peptides?

Gauge: 29–31, and why thinner is the norm

Needle gauge runs backwards — a higher number is a thinner needle, so 31 gauge is finer than 29 gauge. The 29–31 gauge range is standard for subcutaneous peptide work, and 30 gauge is the most common factory default on pre-packaged insulin syringes. Thinner needles are favored because low-viscosity peptide solutions flow easily through them and they cause less pain, bruising, and tissue trauma than thicker needles.

Length: 5/16 inch (8 mm) for subcutaneous

The standard subcutaneous length is 5/16 inch (8 mm) — long enough to clear the dermis and reach the subcutaneous fat layer, short enough not to enter muscle on a pinched fold. A 1/2-inch (12.7 mm) length exists but is intended for intramuscular injection, which is rarely the right route for peptides; a small number of protocols specify it, so the individual peptide's profile is the place to check before assuming.

The syringe the needle comes on

For peptide research the needle almost always arrives fixed to a U-100 insulin syringe (100 units = 1 mL). Barrels come in 0.3 mL (30 units), 0.5 mL (50 units), and 1 mL (100 units); across all three the needle gauge and length are typically identical — barrel capacity is the only variable, chosen for the draw volume and tick precision, not the needle. Smaller barrels have finer tick spacing, which is why 0.3 mL and 0.5 mL are common for the small 2–10 unit draws typical of peptide doses.

• Gauge: 29–31 (higher number = thinner); 30 g is the usual default.
• Length: 5/16 in (8 mm) for subcutaneous; 1/2 in (12.7 mm) is for intramuscular.
• Syringe: U-100 insulin syringe; 0.3 or 0.5 mL barrel for typical microdoses, 1 mL for larger volumes.