Gauge (G) is the standard scale for needle outer diameter, and it runs inversely: the higher the number, the thinner the needle. A 31G needle is roughly 0.26 mm across, 29G is about 0.34 mm, 27G about 0.41 mm and 18G about 1.27 mm. Wall thickness varies too, so a thin-wall needle of a given gauge has a wider bore and faster flow than a regular-wall one.
Why gauge matters in peptide research
Flow resistance scales with the fourth power of internal radius, so small gauge changes have large effects on how hard a solution is to push. That is the main handling consideration: viscous or concentrated solutions move slowly through fine needles, and forcing them generates shear that can contribute to aggregation of sensitive sequences. Conversely, wide-bore needles used repeatedly on a vial stopper are more likely to core it, shedding rubber fragments into the solution.
Common practice separates the two jobs. A wider gauge draws diluent and transfers between vials during reconstitution or when preparing an aliquot; a fine gauge is used for measured withdrawal on a U-100 syringe. See 31G syringes, luer-lock syringes and the gauge and volume guide.