Shoe size chart question

Hello everyone! I need your help.
We are often asked about the shoe size of our models, and I'm not sure about the numbers because we have Russian sizes here, and I found two different conversion charts, which is confusing for me.

For example, medical student Irina has Russian size 38, which is about average. (Her height is 5'9) In one chart it's American size 9, but in another one it's 7.5
How can I know which is right?

American Men's size 7.5 is the same as American Women's size 9, so maybe that is the source of the confusion.

Let x represent the American shoe size for women. Then the size of the foot in inches (sorry about that, but we use inches here) is (x+20.5)/3.

Let x represent the American shoe size for men. Then the size of the foot in inches (sorry about that, but we use inches here) is (x+22)/3.

To convert from inches to centimeters, multiply by 2.54.

So women's size 7.5 is 28/3 = 9.333 inches = 23.706 cm.
And women's size 9 is 29.5/3 = 9.833 inches = 24.977 cm.

Thank you for your answer. Well, I guess it wasn't about the men women confusion, it was clearly stated there it's about Women's sizes.
Still I'm not sure, because too many different charts in the web.

For example this website is listed in wikipedia article on shoe sizes.
http://shoesizeconversion.net
It differs from your numbers.

Maybe anyone could just tell me what better suits the average women's foot size - 7.5 or 9? Russian 37-38 is average for ladies. 40 for example is a large foot - in one chart it's 9, in other - 10 or more.

In adidas chart: Russian women's 38 is 7.5 and 24.2cm
And for example Russian 40 is 9 and 25.5cm

In Reebok Russian 38 is 7.5 and 24.5cm
Different everywhere..

I guess I'm gonna use adidas size chart:
http://www.adidas.com/us/help-section-size_charts.html

FeatherEndeavor said:

https://www.google.com/search?q=shoe+size+chart+women&tbm=isch&imgil=O3T5jaWdnsn4rM%253A%253BpljVz8-HvBYBBM%253Bhttps%25253A%25252F%25252Fwww.pinterest.com%25252Fpin%25252F563161128386374084%25252F&source=iu&pf=m&fir=O3T5jaWdnsn4rM%253A%252CpljVz8-HvBYBBM%252C_&usg=__Fe5r-IR9MOM90V_qDOmpES-DQO8%3D&biw=1920&bih=950&ved=0ahUKEwjU6L3C3cvTAhVH6YMKHavcDNIQyjcIPw&ei=C5wFWZS3IsfSjwSrubOQDQ#imgrc=O3T5jaWdnsn4rM:

Click to expand...

Thank you for the chart. However if you noticed there are many more charts in google and they are different. That's the problem
But I like this one and I'll probably use it, because European sizes in it are actually Russian, not EU sizes.
Considering this chart our ticklish friends ring girl Tanya and medical student Irina are both Russian size 38 - US 7.5

I just worry about misunderstanding if people use different charts.