Action figures come in all shapes and sizes and although it's easy to break out a ruler and simply measure one of your action figures, that's not really going to help you to understand the scale references often used by toy companies and long-time collectors. Besides, it's way cooler to tell a fellow collector on an action figure forum that you picked up some nice 1:10 scale figures. It's all part of the lingo.
The term scale refers to the size ratio to a normal sized object. In this case, we're talking about smaller representations of the human figure. For standardization purposes, toy companies refer to the usual ideal human figure as being 6 feet tall (we're usually talking about heroic figures such as Batman or Darth Vader, hence the height). Therefore an action figure that is also 6 feet tall (which I have never seen) would have a 1:1 ratio. A 3 foot tall action figure (again, never seen one) would have a 1:2 ratio and so on.
Over the years, some standards scales have been used in the action figure world and we'll take a look at the most common, starting from largest to smallest.
1:4 Scale (aprox. 18")
This scale is one of the largest common scales for action figures and is technically reserved for dolls, if we're going by the doll/action figure definition, as they often have "real" hair or cloth clothing. Examples of this format can be found in Sideshow's Premium Format figures and the superhero dolls made by Tonner.
1:6 Scale (aprox. 12")
This scale holds a special place in action figure history as it was the original size of the very first figure to sport the "action figure" moniker, G.I. Joe. This was the reigning scale for action figures during the first decade or so after G.I. Joe hit the market, and many company's toyed with 12" figures of their own, including the 12" Star Wars dolls from Kenner in the late '70s and Mego's 12" line of superhero dolls, featuring Batman, Superman and a TV tie-in version of Wonder Woman.
1:9 Scale (aprox. 8")
This scale is pretty much exclusive to the Mego toy company's World's Greatest Heroes line of 8" action figures that ruled the toy aisles in the late '70s and early '80s. Other toy companies were quick to follow, such as Ideal with their famous monsters and Evel Knievel lines. This size became so popular, in fact, that G.I. Joe himself shrunk down to this size (although a petroleum shortage needed to make plastic didn't help, either).
1:10 Scale (aprox. 7")
For today's collectors, the 1:10 scale seems to be the king of the hill. Several action figure lines are being produced in this format from Mattel's DC Universe to Marvel's Lengends line. Mattel has made an even bigger splash with their Masters of the Universe Classics line that re-imagines the old school fantasy figures in a newer 7" scale. This size tends to be more popular with adult collectors than with children looking for play value.
1:18 Scale (aprox. 3.75" - 4")
Mego started it with their Pocket Heroes line in the late '70s, followed shortly thereafter by Fisher Price with their Adventure People line, but it would be Kenner's massively popular Star Wars collection that would set the unshakable standard for action figures of this scale for almost 20 years. Figures in this scale were less expensive, easier to fit into vehicles and loads of fun to collect. Toy lines such as G.I. Joe: A Real American Hero did extremely well in this size as did many movie figures, such as Indiana Jones and Tron and figures based on TV shows, like The Dukes of Hazzard and ChiPs.
Although 3.75" was the standard in this scale, a proper 1:18 scale figure should be 4", and a recent resurgence in popularity of this size has brought us several new figure lines from Marvel, DC Comics, G.I. Joe and Star Wars.
1:12 Scale (aprox. 5" - 6")
Chances are, if the figure was based on a movie made during the '90s (Last Action Hero, Congo, Jurassic Park, Super Mario Brothers) their action figures were this size. Throughout the 1990s, figures ranging between 5-6" took over the action figure world, pretty much killing the 3.75" figures and setting a new standard. Although not very popular today, figures in this scale will never be forgotten.
1:48 Scale (aprox. 2")
It should be noted that with the rise in popularity and collectability of figures such as Lego "minifigs" and other miniature, yet fully articulated, figures from other building sets such as Mega Bloks, the 2" tall figure is starting to hold its own and is beginning to command respect in the collectibles world. These figures may very well one day be the reigning scale standard in the not too distant future, so they get an honorable mention here.