How to calculate fast insulin

I would like to know how to calculate the rapid insulin that I have to eat.I do it by eye.

Hello, I am not an expert and quite new, 2 years.They will tell you, but I have been proof and error until I put on the freelink and I have learned to put on what I will eat, the glucose that has etc .. And every day I learn because many times in the same situation andThe same data, I feel unequal, although every day I learn or little more

@ Cristóbal Castro Martínez: It's following link can help you
Link

@Cristóbal Castro Martínez, the fast dose is calculated according to carbohydrates.
You have foods or food rations, which the diabetes educator or the endocrine gives you.
A fast unit is usually needed for every 15 g of hydrates, but your doctor has to give you the guideline, because it depends on the sensitivity you have to insulin.