What is your mass in kilograms?

Take whatever you weigh in pounds and divide by 2.205.

Explanation:

Because weight is a measure of the force you exert on the earth, with some simple manipulation of Newton's second law we can get your mass in kilograms. 2.205 is just a nice constant that does that for you, but the more in-depth version is that

F = ma

The equation for weight is thus

W = mg, where W is your weight in pounds, m is your mass, and g is the acceleration due to gravity (9.80 m/s^2)

Thus, your mass in kilograms is m = W / g.