# Physically determine unknown wire's gauge

Is there any way to easily and accurately determine the gauge of stranded wire by measuring it?

I know you can pretty easily determine the gauge of solid-core wire with just a pair of calipers by measuring the conductor diameter, but with stranded wire, wouldn't there be variance in packing-efficiency of the strands? I also had little luck finding a wire-gauge to stranded-wire-diameter table.

I have a bunch of spools of wire that are incorrectly labeled. It appears that someone has respooled a number of the reels, as there are (at least) multiple reels all labeled "22 gauge" with different diameter wire on them.

• Would you be able to compare to a (small) sample of stranded wires with similar gauges that are known? I was thinking just use calipers on the outside, but insulation thickness varies greatly with different wire. Commented May 23, 2013 at 4:42
• Measure the diameter of the individual strands, and count the strands? Then work out what AWG stranded wire is typically made with that combination. Although I can imagine it takes some careful caliper work to tell 32 awg from 34 awg. Commented May 23, 2013 at 4:54
• @ThePhoton - Yeah, that is an option, but it means I would need a micrometer. Commented May 23, 2013 at 6:07
• Well, I wound up needing to use a 0-1" micrometer. Commented May 26, 2013 at 11:23
• @ThePhoton I just bought a mechanical caliper that can easily tell 32 AWG (.202 mm) from 34 (.160 mm). It really is a huge difference; tick marks on the dial are spaced 0.02 mm. Just stick the wire between the jaws and close the gap. The needle should stop sharply at .16 for one, and .20 for the other.
– Kaz
Commented May 27, 2013 at 5:32

http://www.zierick.com/pdf/wire.pdf

Image is high-resolution to maintain legibility of the small text.

This shows the stranded diameter and sub wire gauge+qty.

• Oh cool! That is excellent. Commented May 23, 2013 at 6:01
• Don't quite get this. Isn't an cross section area of a circle pi*r^2 instead of (2r)^2 ??
– KMC
Commented Jun 8, 2017 at 1:33
• I don't get this either. This has got to be the most poorly illustrated yet most referenced table on the internet. Given their own equation this table doesn't add up. Ie take the last row in the left-hand column for #20. Number of strands = 7, dia. = 0.012, Area circ. mills = 1025. But given their equation, 12 * 12 * 7 = 1008. Can someone please explain how they're getting 1025? Or the last column, total dia = 0.036? How in the world does this chart make any sense whatsoever? Commented Oct 29, 2019 at 0:07
• They are NOT calculating cross-sectional area, they are calculating, "circular mil area" CMA as described. You must measure in mils (thousandths of an inch) when doing the calculation. Commented Dec 14, 2021 at 1:37

You can also measure the resistance of a meter of cable using 4-point (kelvin) connection (http://en.wikipedia.org/wiki/Four-terminal_sensing)

After that, you can use any online tool to obtain the AWG from the resistivity

• By physically do you mean "not electrically"? If that's the case, I will delete my answer... Commented May 23, 2013 at 5:19
• Electrically IS physically : the question didn't specify mechanically!
– user16324
Commented May 23, 2013 at 11:05
• I more meant "mechanically", but I didn't think about it much, making it rather unspecific. Commented May 27, 2013 at 5:37

Depends on how accurately you need to know. The most accurate would come from the geometry as described by Connor. For "close enough for bench work", I take a wire stripper that's always been fairly accurate, and see which set of notches strips the wire cleanly. For somewhere in between, I suggest carefully tinning a sample, applying heat well away from the solder so it wicks well into the strands without adding to the diameter, and measure with calipers.

If it is stranded single material, like copper, I think the best way is to cut a foot (or more), strip it so it is pure metal, weigh it. Then calc what a single solid wire weighs. Compare.

Likely an obvious solution but one I didn't see mentioned: look at the insulation for marking... such as "18 AWG GXL 125C J1128" where:

• 18 AWG = 18 gauge wire
• GXL = cross-linked polyethylene (XLPE) insulation
• 125C = temperature rating (in Celsius) of the wire
• J1128 = SAE (or other governing organization) to which the wire complies

To prevent stranded wires from unpacking and changing diameter when the insulation is removed, first strip off about an inch or so from the end of the wire you want to measure, then strip another quarter inch of insulation without removing that piece from the end of the wire. Leave about a quarter inch gap of bare wire exposed between the insulation pieces - just enough to get a caliper gauge in for measurement. The insulation keeps the wire strands captive on both sides of the measured section, helping to prevent untwisting, unpacking, or squishing the wires together into an oval shape while measuring. Don't squeeze the caliper too tight when measuring.

Stranded wire diameters can vary quite a bit, depending on things like the number of strands, amount of twist and manufacturing processes. While tables exist on the web listing diameters for various stranded wire gauges, those tables vary quite a bit. If you know the manufacturer of the wire in question, go to their website for more accurate information. If that is not possible, then you will have to guess-timate where the measurement falls into the various lists on the web. If you must be absolutely positive of the correct answer, then you will need to get a micrometer, and follow the methods described above.

Something that hasn't been noted here yet: if you don't need an exact gauge but just something good enough to know which crimp terminal to use or the like, try seeing which hole on a closed pair of wire strippers the wire fits through.

For example, if you can fit all the strands through the 20 gauge hole but not the 22 gauge hole, the wire is probably 22 gauge (stranded wire has a slightly larger overall diameter than solid, just because the packing density isn't 100%). This certainly won't give you an exact diameter, if that's what you need, but it should tell you whether to reach for the blue or the red crimp terminals.

In theory, you could also make a set of holes like this in anything that you can precisely cut holes into--there's a set in my PCB ruler for exactly this purpose. But a wire stripper is something everyone working with electronics is likely to have, and it conveniently has holes just the right size for this application.