Pressure equals force/area.
(p=f/a)
To answer your question, find the area of the hair (length x height).
Next, you have to find the force that was used. Typically a spring scale is used. Place the hair over the hook at the bottom. hold the top with your left hand. Pinch both sides of the hair, and gently pull down, watching the spring. Observe what tick the top of the spring said when the hair broke.
Now, use your formula and fill in what you know. (an example is listed below.)
Pressure = force/area
Pressure = 1N/10mm2
Pressure = 0.1 Pa.
Symbols:
N - The standard unit for force, indicating Newtons.
2 - The standard unit for area, indicating 'squared'
Pa - The standard unit in which Pressure is measured, indicating Pascals. If you have 1000 Pa, you can also say it is 1KPa (1 Kilo-Pascal)
Hope it helped!

Yours truly,