But surely people who don't have kids are already part of calculating what the fertility rate is? Why would you manually exclude parts of the population, only to add a skew factor later to your number to effectively reinclude them?

I totally believe you that 2.1 is the standard, but I don't understand why based on the explanations in this thread.

Shouldn't "people who don't have kids" be accounted for by lowering the overall measure of "children a woman will give birth to on average"? So if half of women in a country have exactly four kids, and the other half have zero kids for whatever reason, the average number per woman is 2.

The reason is that TFR is not the average number of children per woman over her lifetime.

Instead, it is calculated by measuring the average number of children born to women at a specific age (e.g. 15, 16..49). Adding all of these up results in the TFR. Thus mortality rates are not included in the TFR.

> Kids that “want” to have kids but for whatever reason are unable to

The opposite is also likely. People may have "accidental" kids, or they could change their mind.

(I still agree that 2.1 is a good approximation, but it is also likely to vary on cultural and natural effects)