Many newer "15mm" Sci-Fi figures are actually closer to 18mm scale now.
Some used to be measured in total height, but now many people reference this scale as being measured to the figures' eyes, for more consistency, since if they are wearing hats or helmets, that throws off the total figure height measure.
18mm figs, measured to their eyes are really about 20mm in overall height (sans hats/helmets), so are really very close to 1/72nd - 1/76th scale.
You can also look at the older 25mm figs (now many companies do 28mm - 32mm ones), which some claim are "1/72" scale, but to me are closer to 1/64th, or larger, depending upon their size and heft.
28mm figs are about 1/56th according to many, again, with allowances for different sculptors, figs, heft, etc.. Some use these with 1/48th vehicles too, instead of with the 1/56th ones, especially if they are mounted on taller, thicker bases.
Confused yet?
I hope that helps some.
15mm/18mm figs are a real bargain at about 50 cents - 75 cents each, depending upon the manufacturers, and there are a very, very wide range of sculpts, vehicles, buildings, and other items to go with them, so I highly recommend this scale.
25mm/28mm are very nice too, for games with smaller numbers of forces and vehicles, and/or where you don't need/want as much room on the tabletop to maneuver as desired with the smaller minis.