How many cells do animals have?

It is actually really difficult to count the number of cells that an animal has because there are just so many! As an estimate, we think there are something like 40 trillion cells in an average adult human, but it does depend on the size of the animal, so this number will vary for different people as well as for different animals and different plants. That means I can’t really give you a number for plants or animals generally because there is such a difference in size between a mouse and a whale or between grass and a tree! I think most cells across plants and animals are roughly the same size with some exceptions, so larger organisms have to be made of more cells rather than bigger cells.

There is one animal that I can give you an exact number for though. Caenorhabditis elegans, which is a really tiny worm (about 1 mm long), has been studied by scientists for a long time. One thing that scientists have done with this worm is count up how many cells there are and have found out exactly how each cell division happens during growth to adult size. There are 1031 cells in the adult male. I expect there are other small animals like this that have a certain number of cells, but most animals have varying numbers of cells and the cell divisions don’t happen in the exact same way each time.