Hypodermic needle gauge to mm
Find the actual outer diameter of a needle from the gauge printed on its pack.
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How to use it
- Enter needle gauge.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Each gauge has a fixed nominal outer diameter in inches in the needle gauge chart (Birmingham); diameter (mm) = inches × 25.4. ISO 9626 allows a tolerance range around each value.
Example
Inputs: Needle gauge = 21G. Result: Nominal outer diameter: 0.8191 mm; Diameter in inches: 0.03225 in.
Limits and caveats
- Inner diameter depends on wall type (regular or thin wall) and is not calculated here.
- This is for information only; choosing a medical needle is a decision for a clinician.
Questions about this tool
What is the diameter of a 21G needle?
Its nominal outer diameter is 0.819 mm, often printed on packs as 0.8 mm.
Is a higher gauge a thicker needle?
No. A higher gauge is thinner, so 30G is much finer than 18G.
Are hub colors the same everywhere?
Hypodermic hub colors follow ISO 6009, but IV cannula colors are a different system.