What many people don’t realize is that measuring cervical dilation is not an exact science. Two people will often give different results. For this reason, in a hospital birth, different nurses might have different opinions of how dilated you are, which is one reason why they try to have the same person do your exams to keep their results a bit more consistent.
Now, doctors may learn a different technique but as for midwives, the typical learning technique is to start by measuring the distance between the index and middle finger when they are stretched apart.
On my hand, they are about 6cm apart. This distance will be different for each of you.
Next, you can start a “guess and check system” : take common items, such as cans, salt shakers, or anything else you have handy that’s a small circle. Make a guess, then measure first with a tape measure, then with your fingers.
Then, when you are guessing correctly more than not, reverse the order... fingers, guess, then measure.