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Google states that the US has a fertility rate of ~1.73 and a population growth rate of ~0.5%.

It seems like anything less than 2 for a fertility rate should result in population lessening. Is my understanding of this correct? If not, what factors am I missing here?

Seph Reed
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    Could it be immigration? – Dave May 01 '21 at 01:52
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    ... or lengthening lifetimes. Or changes in how the Census counts the population. Or adoptions and in vitro conceptions ;-). Because Google doesn't appear to explain what it means by "population growth," it's hard to attach any definite meaning to that figure. – whuber May 01 '21 at 18:49

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Fertility rate does not include immigration, which largely makes up for the relatively low fertility rate.

Back of the napkin calculations: the US has 330 million people. Assume half are women. That's 165 million women. With a fertility rate of 1.73, they would be expected to have roughly 285.5 million children.

The immigration rate to the United States as of right now is roughly half a million per year. But the average reproductive lifespan of a woman is roughly 30 years, so that's 15 million immigrants. We're up to 300 million when you combine a generation of births plus immigrants. That still isn't growth.

But life doesn't end when we stop reproducing. People live on past 45... And lifespan is increasing. That probably explains the rest of the growth. For example, the fertility rate is only 1.73, but in 2019, there were about 1 million more births than there were deaths despite the fertility rate being less than the amount needed to replenish the population. This is because lifespan wasn't constant for the last few generations, it has been increasing consistently for decades. Seems confusing, doesn't it?

This isn't a definitive answer, but I think it is enough to show that simply looking at fertility rate can be misleading when you're interested in explaining population growth at a given time.

AJV
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