Ecological Footprint
"The Ecological Footprint of a person is calculated by considering all of the biological materials consumed, and all of the biological wastes generated, by that person in a given year. These materials and wastes each demand ecologically productive areas, such as cropland to grow potatoes, or forest to sequester fossil carbon dioxide emissions. All of these materials and wastes are then individually translated into an equivalent number of global hectares."

Calculate your Ecological Footprint - After the program loads use your cursor to point at the U.S. and then on English. The program goes through quite a few screens with default settings which you can change with buttons. As you go along watch the gray box on the upper right to see the effect of different choices - whether it increases or decreases the default numbers. Report how many acres and planets that you need to support your lifestyle if everyone in the world lived as you do.

Personal Environmental Impact Calculator -  another calculator.

Eco footprint standards - these pages also have a calculator, but is not very precise in making various adjustments.  It does explain a little bit on how the calculations are made.

Eco footprint history - Wikipedia - This shows that there have been a lot of research and work to back up the calculations.
The Most Harmful Consumer Activities:
From the Union of Concerned Scientists
Cars and Light Trucks :

The manufacture and, more important, the use of consumers' vehicles cause more
environmental damage--especially air pollution and global warming --than any other single
consumer spending category.
Meat and Poultry:

Meat and poulter production requires large amounts of water and causes 20 percent of the
common (as opposed to toxic) water pollution related to consumer expenditure. It also
uses a significant share of the nation's land--800 million acres for grazing livestock and an
additional 60 million acres to grow animal feed. Red meat causes especially hight amounts
of environmental damage for the nutrition it delivers.
Fruit, Vegetables, and Grains:

Irrigated crops grown to meet consumer demand use an enormous quantity of water (30
percent of consumer-related water use). pesticides and fertilizers cause 5 percent of
consumer-related toxic water pollution. Food crops also use substantial amounts of land.
Household Appliances and Lighting Electricity seems clean and nonpolluting
when it's used in the home, but most of it is generated by burning polluting fossil
fuels, especially coal. Appliances and lighting are responsible for 15 percent of the
greenhouse-gas emissions related to consumer expenditures and 13 percent of
consumer-related common air pollution.
Home Heating, Hot water, and Air Conditioning:

Cooling and heating homes and water has an impact on global warming and air
pollution similar to that of appliances and lighting. Systems that rely on
electricity or oil contribute heavily to both problems. Most fireplaces and wood
stoves are especially high air polluters.
Home Construction:

The land and wood used for new homes are responsible for about a quarter of
consumers' impact on wildlife and natural ecosystems. Six percent of
consumer-related water pollution comes from manufacturing the materials for
new homes and disturbing the soil during construction.
Household Water and Sewage:

Despite advances in sewage treatment, municipal sewage remains a major source
(around 11 percent) of water pollution, especially affecting coastal areas and
estuaries. Interestingly, households' home water use is only 5 percent of th total
compare d with nearly 74 percent for food production and distribution.