That is an American measure. Imprecise as fuck.

The metric conversion is:

For an ISO standard size adult hand (adjust +/-13% to take into account hand size deviation from ISO standard):

For spaghetti sizes 5-6, make the ok sign with your hand, placing the fingernail of your index finger against the base of your thumb for one person, against the middle joint for 2 people, and to the tip for 3 people.

The pressure exerted by the index finger to the thumb joints should be 9.81 kPa +/- 15%.