Edited from Lewan and Simmons (2001).

Footprinting essentially accounts the use of the planet's renewable resources (its 'interest' rather than its 'capital'). Non-renewable resources are accounted for only by their impact on, or use of, renewable, bioproductive capacity.
The footprint deals only with demands placed on the environment. It does not attempt to include the social or economic dimensions of sustainability. The footprint is a 'snapshot' estimate of biocapacity demand and supply usually based on data from a single year. Both available biocapacity and the eco-efficiency of the economy can change over time which is why it is not possible to forecast or 'backcast' footprints from current data although it is possible to make assumptions about future consumption and thus create informative, but speculative, scenarios.
The use of bioproductive area as an aggregate unit makes it a powerful and resonant means of measuring and communicating environmental impact and sustainability. In this sense it is comparable to many economic indicators such as the Retail Prices Index (RPI ) and GDP.


An Additive Model
The basic ecological footprint is an additive model. It sums several mutually exclusive uses of bioproductive area; arable, forest (for both wood products and carbon sequestration), pasture, degraded or built land, and sea space. Exceptions to the additive model have been made for footprinting certain types of pollution and water catchment where spatial uses overlap.

A key issue in the calculation of ecological footprints and biocapacities is the method used to aggregate areas of different quality facilitating international comparisons. Areas of generally different productivity (arable, pasture, forest, sea) are 'normalised' by multiplying them by equivalence factors relating to their bioproductivity. The equivalent areas are then expressed as standardised hectares of world average productivity (more recently referred to merely as 'area units').
Use of fossil fuel-derived energy is typically accounted for in terms of its carbon dioxide emissions although it is also possible to assess ecological footprints of energy use in terms of the land area required to sustainably derive biofuel alternatives. The former results in a more conservative estimate of the impact of fossil fuel use and have thus been the more common method.

Biocapacity
For calculation of national/regional biocapacity, local yield factors are introduced.
These factors show how much higher or lower the yield per local ha is compared to
the yield per area unit. There is always the possibility of converting ha of unit area
into ha of national/regional average productive space for both supply and demand.
Thus it is possible to answer two questions; How many planets would it take to if
everyone consumed as much as the average resident of Region X and How many
Region X's would it take to satisfy the current demands of that Region. This
calculation was performed for the Isle of Wight (Chambers et al 2000). Using local
yield values it was shown that two additional Islands would be needed to sustainable
support consumption. Using global yield factors it was shown that, if everyone lived
like the average Islander, 1 ? extra planets would be required. The approach of using
local yields is also favoured by a number of the studies reviewed in this report.
Some biocapacity must be set aside for non-human use. The necessary amount of
pristine habitat is not known but, as a general rule in footprint calculations, not more
than 88% of the existing biocapacity is considered 'available' for human use. The
Living Planet Report 2002 accounts for biodiversity as a percentage of the footprint
(demand). Previously biodiversity area has been subtracted from the available
regional supply.

References and further reading

Chambers, N., Simmons, C. & Wackernagel, M. (2000) Sharing Nature's Interest:
Ecological footprints as an indicator of sustainability. Earthscan, London.

Lewan, L. & Simmons, C. (2001) The use of Ecological Footprint and Biocapacity Analyses as Sustainability Indicators for Sub-national Geographical Areas: A Recommended Way Forward.
http://www.prosus.uio.no/english/sus_dev/tools/oslows/2.htm

World-Wide Fund for Nature International, United Nations Environment Programme,
World Conservation Monitoring Centre, Redefining Progress & Center for Sustainability Studies (2002) Living Planet Report 2002 (World-Wide Fund for Nature, Gland, Switzerland).

Wackernagel and Rees (1996) Our Ecological Footprint, New Society Publications,
CA.

Wackernagel, M., Onisto, L., Bello, P., Callejas Linares, A., Lopez Falfan, I., Mendez
Garcia, J., Suarez Guerrero, A. & Suarez Guerrero, G. (1999) Ecol.Econ. 29, 375–
390.

Wackernagel M., Silverstein J. (2000) Big things first: focusing on the scale
imperative with the ecological footprint, Ecological Economics, Vol.32,No.3,pp391-
394.