quick Needle gauge question-diameter

twarlik · Apr 22, 2004

i dont understand what it means when it says the smaller the needle gauge, the larger the diameter.
i know that a 21 gauge is larger than a 25. what i dont understand is, what do they mean by diameter? is diameter the length of the needle, such as 1.5 inch for a 22 gauge? or is the diameter referring to the internal hole of the needle.

A diameter is the length of a straight line across a circle. So, a bigger circle will have a larger diameter. With needles this simply means that if you have a small gauge it will have a larger diameter and thus, will be a thicker needle.

Length, is just...well...length of the needle. No trick there.

wonderbee, BSN, RN · Apr 22, 2004

i dont understand what it means when it says the smaller the needle gauge, the larger the diameter.
i know that a 21 gauge is larger than a 25. what i dont understand is, what do they mean by diameter? is diameter the length of the needle, such as 1.5 inch for a 22 gauge? or is the diameter referring to the internal hole of the needle.

I suppose you could say it that way. The diameter refers to the size of the hole in the needle. The 16 would be better for transfusing blood than the 21 to 23, which would be better for an IM injection. If you used the 21 to transfuse blood, a bag of blood would take forever to infuse.

Carolanne · Apr 22, 2004

I had trouble getting that concept too when we learned it. I try to think of spaghetti ... :chuckle I don't know if they still do it, but Ronzoni used to number their pastas - no. 4 or something was like the widest linguine and then the numbers went up, regular spaghetti was no. 8 and vermicelli was no. 16 or something. So the lower numbers were the broader pasta and the higher numbers were the finer and thinner size. That kind of helped me remember how they number the needle gauges. I'm just worried that some day I'll tell a patient that I have their fettucine shot for them! :chuckle

twarlik · Apr 22, 2004

I was thinking about this a little more last night. I did some research on gauge and discovered that it is measuring exactly the same thing as diameter. That is, it's measuring the distance across a circle, or in the case of needles, the distance across the hole at the end of the needle. I couldn't find out why a smaller gauge equals a larger diameter. I guess it's just a weird system of measurement.

At any rate, I hope all of this has helped you somehow and not made the issue more confusing.

SickleMoon · Apr 22, 2004

I was thinking about this a little more last night. I did some research on gauge and discovered that it is measuring exactly the same thing as diameter. That is, it's measuring the distance across a circle, or in the case of needles, the distance across the hole at the end of the needle. I couldn't find out why a smaller gauge equals a larger diameter. I guess it's just a weird system of measurement.
At any rate, I hope all of this has helped you somehow and not made the issue more confusing.

Twarlik, your comments stuck in my mind and I went on a hunt to try to find out why the smaller gauge equals a larger diameter. The best explanation I found said that it's because when they first started making round wire (as opposed to cutting it from sheets of metal in strips and then working it with tools to make it round), they forced the metal through round dies. They started with a large hole and gradually worked it down through the smaller holes--kind of like when you're making homemade pasta and put it through the rollers multiple times to get it really thin. The gauge number refers to the number of times the wire had to be put through successively smaller holes to get it to the proper size. Thus a 20 gauge wire had to be put through successively smaller dies 20 times, while a 10 gauge wire only had to be put through 10 times. ...clear as mud? At least it helps me remember it!

I shouldn't probably mention the site here, but it was an encyclopedia site that I got to via search engine--kind of a fun search and a nice break from the studying I'd been doing! :chuckle Thanks for the diversion!

orrnlori, RN · Apr 22, 2004

Speaking of wire, as in jaw wired shut, this is still correct, the higher the gage, the finer the wire. However, pins, like those fine pins used to pin fractures in fingers is the other way around, the higher the number the larger the pin. Catheters, as in foley and straight urinary catheters, 6 french foley's are for itty bitty babies, 22's (ouch) are quite large. IV catheters go the way of hypos, the lower the number, the larger the bore. This stuff drove me nuts in school. Kind of like the fact that every drug has two names. And many instruments in the OR can have up to three names for the same daggone instrument, as in Tenotomy scissor, Stevens scissor, or tissue scissor, they are all the same scissor. Don't you just love it?

twarlik · Apr 23, 2004

Twarlik, your comments stuck in my mind and I went on a hunt to try to find out why the smaller gauge equals a larger diameter. The best explanation I found said that it's because when they first started making round wire (as opposed to cutting it from sheets of metal in strips and then working it with tools to make it round), they forced the metal through round dies. They started with a large hole and gradually worked it down through the smaller holes--kind of like when you're making homemade pasta and put it through the rollers multiple times to get it really thin. The gauge number refers to the number of times the wire had to be put through successively smaller holes to get it to the proper size. Thus a 20 gauge wire had to be put through successively smaller dies 20 times, while a 10 gauge wire only had to be put through 10 times. ...clear as mud? At least it helps me remember it!

Cool! That's really interesting! Thanks for looking that up.

psychomachia · Apr 23, 2004

i dont understand what it means when it says the smaller the needle gauge, the larger the diameter.
i know that a 21 gauge is larger than a 25. what i dont understand is, what do they mean by diameter? is diameter the length of the needle, such as 1.5 inch for a 22 gauge? or is the diameter referring to the internal hole of the needle.

There are many different types of wire "gauge" standards, which determine the thickness (diameter) of a particular piece of wire, or needle size in the medical field. The Birmingham (aka Stub's Iron Wire) gauge is used when manufacturing hypodermic / medical needles. Each standard differs in the size of a particular gauge. As an example, a 22 gauge wire will have the following measurements:

American Wire (Brown & Sharpe) Gage 0.0253 inches

US Steel Wire Gage 0.0286 inches

British Standard (Imperial) Wire Gage 0.028 inches

Music (Piano) Wire Gage 0.049 inches

Birmingham (Stub's Iron) Wire Gage 0.028 inches

Stub's Steel Wire Gage 0.155 inches

So when you're talking about 22 gauge piano wire, it will not be the same size as a 22 gauge needle.

The production of wire as described in a previous post leads to the inverse relationship between size and gauge (small gauge - 14 = big needle, large gauge - 25 = small needle). A heated rod is pulled through a slightly smaller hole and can be repeated through smaller holes until the desired size. The first size of wire pulled through would be 1, then 2, then 3, which would correspond to the wire gauge. Therefore, the higher the number, the smaller the wire.

Confusion occurs when switching from gauge to "French" sizes. The Fr. size is based on millimeters - 1 Fr. = 1/3 mm. An 8 Fr cath would be equal to 2 and 2/3 mm (8/3 = 2 and 2/3).