Iron is a metal, the most present by abundance and multiple organisms use it for different purposes. In the case of humans, it's a very important element creating issues if there's not enough in some circumstances.
After multiple studies and experimentation, international organisms provide interesting data for having references with presumably accuracy.
What is curious is how the female in reproductive age needs almost triple than the male, so women are more prone to experience shortage.
The logical approach would be searching for good sources and keep up insisting on them each week. Here we are not going to recommend those coming from autonomous and intelligent organisms, but we should mention clams, oysters, lentils, soy beans, spinach, chard, between the most important. There are cereals with added nutrients like iron that ensure the daily intake.
The reference for taking care: 8 mg to an adult male and 18 mg for a female in reproductive age.
Kids usually 11 mg, a bit less —7 mg— in the baby age, until 14-16 years where the female needs more. After the reproductive age the female also needs 8 mg.
These are not the ultimate quantities of course and each person is different. However, in this case a bit more is better than less. Specially people that make intense efforts or in specific conditions.
A good check tool comes with a blood sample. The overall feeling also helps because without iron the person feels somewhat weak.