How many ants are there on Earth?

How to get close to this number

This is an order-of-magnitude question, so guess the exponent rather than the number. Start from the human population, ask how many ants there might be per person, and recognise that the answer is going to be millions rather than thousands — ants are small, ubiquitous and colonial. The explanation gives the ratio directly, and reasoning in "per human" terms is far more tractable than reasoning in absolute counts.

Where intuition fails is in the relationship between size and number. Biomass is roughly conserved across a habitat, so the smaller an organism is, the more of them a given area supports, and the relationship is steep. The comparison in the explanation — that the combined dry weight of ants exceeds that of all wild birds and mammals — is the reframing that makes a number with sixteen zeros feel reasonable.

This is a published scientific estimate built from thousands of field samples extrapolated to global land area, not a count, and the authors quote it with substantial uncertainty. That is normal and it is the honest way to answer a question like this. An estimate that is right to within a factor of two on a number this large is a genuine achievement, and it should be read with that precision in mind.