How far (technically) is a "stone's throw?"

There is no technical definition of a 'stone's throw'.

It's a vague idiom.

OED says:

The distance that a stone can be thrown by the hand; vaguely used for a short or moderate distance.

http://www.oed.com/view/Entry/190832

It's the distance that a person could throw a stone - make of that what you will. The size and weight of the stone and the strength of the person are not specified.

So a mile? No. Too far.

A few feet? Also not a stone's throw. Too short. If you can reach out and touch it then it's not a stone's throw away.

Somewhere in between the length of a tennis court and the length of a football pitch, I'd say. Whatever kind of football you like. :)

I'm not aware of any difference between UK and US usage.

The phrase "a stone's throw" is not related to the unit of weight the stone. The phrase has a biblical original and predates the weight unit.

The phrase means 'the distance you can throw a stone', and so is deliberately intended to be vague, since people throw stones for different distances, and stones weigh different amounts. It's usually taken to mean roughly the size of stone you can hold in your hand.

It's valid to use it with any distance that a person might reasonably throw such a stone. A mile is probably too long, and a few feet probably too short.You would normall find another phrase for each of those. However people do use the phrase vaguely and sometimes exaggeratedly. (I remember a cartoon of the proprietor of a 'seaside' hotel showing off a complex trebuchet-like stone-throwing engine, pointed at the distant beach, and saying to a disgruntled guest: "Yes I did advertise my hotel as being a stones-throw away from the beach").

Thanks to DJClayworth for noting the Biblical origins. Here are the particulars and an interpretation of them.

Luke 22:41 (New International Version):

He withdrew about a stone's throw beyond them, knelt down and prayed,

From the same link, described as a quote from "Gill's Exposition of the Entire Bible":

about a stone's cast; fifty, or sixty feet from the place where they were

I'd say that in current usage it isn't as precise as that, but I haven't heard people describe distances to the next town as a stone's throw, unless the context makes it clear that they're exaggerating. So the phrase itself in my mind would refer to a distance of between a few dozen feet (50 feet is reported as a manageable throwing distance for the average, non-athletic MIT arm when throwing a baseball for another student to catch (2.009 Demise of the Pony Express)) and a few hundred feet. The baseball record goes to Glen Gorbous who threw a baseball 445 ft. 10 inches on August 1, 1957. (Guiness Book of Baseball World Records)