How much blood does the human body have?

The best rough guide for an adult is about 5 litres, or about 10 pints.

Actually, it's best expressed as a percentage of total body weight - clearly, a four foot man doesn't need as much circulating blood as a 7 foot man does! For adults, it equates to about 7% (which, for the legendary 70kg man, makes 4.9 litres). For infants, toddlers and children, this percentage is higher, though.

In turn, blood can be subdivided into roughly 40% red blood cells and 60% plasma (blood minus all cells). There are of course many other types of cells present in blood besides red blood cells, but these make up only a tiny percentage. Lastly, the usual warning with figures: the above numbers can vary quite a lot with weight, sex, and other factors.