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What is the mathematical relation between the girth of the penis (defined as the maximum circumference) and the nominal width of the condom (an industry label) that should be used? I see a lot of contradicting information on the internet. My question is about latex condoms. All sources below are either about latex condoms or condoms in general.

On the one hand, Healthline and Very Well Health say that you should divide the girth by 3.14 to get the width.

You can figure out the width of your penis the same way you’d determine the diameter of a circle. To do this, divide your girth measurement by 3.14. The resulting number is your width.

To calculate the width, divide the girth by 3.14. For example, if your penis girth is 5 inches, then the penis width would be 1.59 inches (5 inches ÷ 3.14 = 1.59 inches).

On the other hand, many webites provide tables in which the ratio is between 2 and 2.5: condom-sizes, condomerie, postcondooms. Consider this quote from condomerie.com:

Regular sized condoms usually range from around 52 – 54mm, meaning they’ll fit those with a girth of around 110 – 120mm.

The ratio is here around 115/53=2.2; this is much smaller than 3.14. There is also a Reddit post with 565 upvotes, stating that nominal width should be 2.36 times smaller than penis girth:

How do you compute the "nominal width" that you require? There's a simple calculation that most people aren't taught. You measure your wrap-around circumference and divide by 2.36.

So what is the correct method, dividing by 3.14, or dividing by something like 2.36?

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  • Are you referring to latex, polyisoprene, polyurethane or even animal skin or intestines (or bladder) condoms? Different materials naturally have different stretchiness. Commented Nov 10, 2024 at 15:17
  • It's about latex condoms. I will edit my question to reflect this Commented Nov 10, 2024 at 17:26
  • 3.14 isn't some random number, it's an approximation for pi and is the literal conversion between diameter and circumference. My understanding is that 1) different manufacturers measure their sizing differently, 2) latex condoms are stretchy, one size is meant to accommodate a broad range of anatomy, and 3) in the literature, the focus is on reliable use which means finding a comfortable size that doesn't slip. There's little emphasis made on anything but personal preference because the perceived most important variable in protection is consistent use. Commented Nov 11, 2024 at 15:53
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    Additionally, penises are not cylindrical, and you're not going to be able to summarize "fit" in one number. I think it probably does not make sense to approach this question with the quantitative specificity you're looking for, it's more sensible to consider fit from the perception of each individual. Commented Nov 11, 2024 at 15:59
  • @Bryan Krause, although pi is the ratio between the diameter and circumference of a circle, this does not mean that it is the ratio between condom width and circumference. When you flatten out a condom, the ratio could be 2 instead of 3.14. According to condom-sizes.org: 'To calculate its circumference we would have to multiply that number with 2, for example, a condom with a 2.05 inch flat width would have a circumference of around 4.1 inches.' condom-sizes.org/condom-sizes/condom-sizes#ref Commented Nov 11, 2024 at 19:01

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The root of this confusion is the relation between penis circumference and condom width. Intuitively, it seems logical that the condom width equals the diameter of the penis, which is 3.14 (pi) times smaller than the circumference. However, this is not how the condom industry defines condom width. The number that you can see on condom packages is the nominal width. This is called the `flat width' by the International Organization for Standardization. On page 5 of the ISO standard it says (I made the part bold):

For the purpose of this test, the mid-body width is the mean flat width rounded to the nearest 0,5 mm of 13 condoms measured in accordance with Annex E at a point (75 ± 5) mm from the closed end excluding the reservoir tip.

When a condom is laid flat, its width equals half its circumference. The only source cited by the Healthline article that talks about condom width, is condom-sizes.org. This website has a page where they explain the difference between flat width and diameter:

Flat width = Circumference/2

Flat width ≈ 1.57*diameter

We have now arrived at a factor 2. The reason for using a slightly larger factor than 2, is that condom circumference should be slightly smaller than penis circumference. This makes sure the condom is tight enough. Another source that confirms this, is condom manufacturer Durex. The Dutch Durex website contains a table of penis circumferences and condom sizes, where the ratios are between 2 and 2.2.

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