The “average” amount of blood lost during a period can be very different from person to person. What is “normal” for one will seem too little or too much for another. Some can bleed more in one day than another would over their entire period. Some people’s flow lasts 3 days, some lasts 10 days. It varies greatly.
Not all of what makes up menstrual flow is actual “blood”, only about half of it is, and the “average” amounts are calculated using the blood content, not total volume of what you loose as menstrual flow. Making it difficult to work out exactly how much “blood” loss there is.
The “normal” amount of menstrual blood loss is on average 30-40 mL. With a “normal” range being 10mls-80mls. Anything above 80mls is considered “excessive” and is said to run the risk of Anemia. These amounts are calculated using the alkaline hematin method – where the amount of blood content is measured, not just the volume of menstrual fluids.
Some informal polls have been done in a few communities, asking people to calculate their flow. This is generally collected in Menstrual Cups, where exact volumes can be measured. The results are quite interesting, and show that even if half of what is measured is not “blood”, many still have a blood loss over the 80mls that would be considered “excessive”.
http://community.livejournal.com/menstrual_cups/392108.html?nc=13
A side topic on heavy bleeding from that LJ post is here
If your bleeding is affecting your quality of life, or you think it is excessive,
it might be worth a trip to the Dr to have a checkup.
References:
http://www.med.monash.edu.au/ob-gyn/research/menorr.html
http://www.ncbi.nlm.nih.gov/pubmed/4650663?dopt=Abstract
http://www.blackwellpublishing.com/isth2003/abstract.asp?id=7605
Reusable Menstrual Products 