Choosing a needle length
Length matters more than gauge, and 6mm is enough on most people. The big pharmaceutical pens now use needles of 4mm to 6mm for the same reason.
What the length is doing
Subcutaneous means into the fat layer under the skin, not into muscle. On the abdomen, thigh and upper arm that layer starts a couple of millimetres down. A 6mm needle inserted straight in lands in it on most people. If you are lean, use a light pinch.
The received wisdom about 8mm and 12mm needles comes from older, thicker steel and from techniques designed around pinching a fold of skin. With modern thin-wall needles the shorter length is both sufficient and easier to get right.
The pinch question
With 6mm, most people need no pinch: insert at ninety degrees and you are in the right place. If you are very lean, lift a light fold. With 8mm, a pinched fold is more often sensible, because there is a real chance of going deeper than intended on a lean site.
The mistake to avoid is squeezing hard. A pinch is meant to lift the fat away from the muscle, not to compress it flat. Gentle finger and thumb, not a grip.
| Length | Pinch needed | Suits |
|---|---|---|
| 6mm | Usually none, light pinch if lean | Abdomen, thigh, upper arm, most builds |
| 8mm | Often, if lean | Larger builds, or anyone who prefers a longer needle |
Gauge, briefly
Higher gauge is thinner. 32G is thinner than 31G by two hundredths of a millimetre, which almost nobody can feel reliably. The one practical difference is that thinner needles flex slightly more, which can make placement feel less definite.
Questions
- Is 6mm too short if I carry extra weight?
- No. The fat layer starts a couple of millimetres under the skin whatever your build. Extra weight adds depth below that point, and a 6mm needle only needs to reach the top of it.
- Can I use insulin syringe needles in a pen?
- No. Pen needles screw onto the pen's thread; syringe needles do not. Use pen needles with a pen.
Pen Needles 31G × 6mm, 10 pack
Ten 31G × 6mm pen needles.