This was a question asked elsewhere which turns out to be quite tricky. I only have half an answer.

Firstly, there are two distinct ways to count 'living things':

  1. How many species are there?
  2. How many individuals are there?

Unfortunately, the first question is partly known for only a third of life. That is, the Eukarya are relatively well documented - zebra, monkeys, soy, humans are all well known species. Insects make up by far the majority, but there are many birds and fish and so on. The other two thirds of life - the bacteria and the archaea - are not catalogued as extensively. E.coli, S.aureus, H.influenzae and other parasitic or human-related prokaryotes tend to be the exception.

So the numbers of species may be millions or tens of millions, but each species has many individuals. Homo sapiens - a particularly prolific mammal - has around 6 billion (6 times 109) members (and each one carries around many millions of E.coli cells). However each human is made of many cells, so in terms of cell count the small numbers of large multicellular organisms start to approach the vast numbers of unicellular organisms. Say each human has a couple of billion cells - that makes around 10 billion billion (1 times 1019) human cells around.

However, the only estimates I can provide that aren't made up on the spot come from a paper that (maybe controversially) puts the numbers of prokaryotes at about 5 thousand billion billion billion cells (that's 5 times 1030). Stars in the sky and grains of sand on the beach spring to mind. This kind of relies on estimates of populations in the ocean and deep subsurface. This latter environment is a relatively novel idea - that there are prokaryotes living at depths in the earth previously thought uninhabitable.

What, then, is the final number? 1030prokaryotes + 1020?eukaryotes? 1050, 1060? Who knows. I, for one, shall not be counting them.

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