Order smaller than you think.  I wear a women's nine but needed a small because my feet are thni to boot.  Didn't feel a real increase in my leg workout but they fit well.

Order a size smaller than you think, especially if you're feet are narrow

I usually wear size 10.5 or 11 in shoes, so I ordered the 8/9 fins.  They fit just the way I like them, snug, but not too tight.  Perfect size fin for lap swimming.

The fins are slightly shorter that I expected but they work just fine for me.  The fit is tight and I have narrow feet so if you have regular or wide feet, you'll have to go up a size or two.  The material is very flexible and after a few laps, I forget that I have them on.

I assumed that the fin sizes were men's size and ordered a size smaller than I normally would. The fins fit perfectly. Very happy with the fit and performance of the fins. I swim laps 2-3 times per week for fitness and the fins meet my needs exactly. I like the shorter fin. I get fin performance without a long floppy fin.

lightweight, nice short blades. I swim 25 laps daily and the fins improve my stroke form