Physicists at the Institute of Physics have devised a formula that, based on your shoe size, tells you the maximum height of heel you can wear without toppling over or suffering agonies. And it is:

h = Qx(12+3S/8)

h is the maximum height of the heel (in cm)
S is the shoe size (UK ladies sizes). This factor makes sure that the base of support is just good enough for an experienced, and sober, high-heel wearer not to fall over

Q is a sociological factor.
It equals px(y+9)xL divided by (t+1)x(A+1)x(y+10)x(L+£20)M

pis the probability that wearing the shoes will help you "pull" (in a range from zero to one, where one is a certainty and zero is stick to carpet slippers). If the shoes are a turn-off, there's no point wearing them...
y is the number of years experience you have in wearing high heels. As you become more adept, you can wear a higher heel. Beginners should take it easy...
L is the cost of the shoes, in pounds. Clearly, if the shoe is particularly expensive, you can put up with a higher heel
t is the time since the shoe was the height of fashion, in months (0 = it's the "in thing" right now). One has to suffer for one's art, and if the shoes are terribly fashionable, you should be prepared to put up with a little pain...
A is units of alcohol consumed. If you're planning on drinking, be careful to give yourself a little leeway for reduced co-ordination.

"Although at first glance our formula looks scary," said Dr Paul Stevenson, of the University of Surrey, who carried out the research. "It's actually pretty simple as it's based on the science you learned at school and which you never thought you would use in real life, in this case Pythagoras' theorem. Applying this to shoes can tell us just how high the heel of the foot can be lifted above the ground."

So now you know…