Barry Commoner, the well known environmental writer and theorist, first developed the equation in the 70s, later modified into the form I=P.C.T. In this formula, I is the environmental impact, P is population, C is consumption per head and T is the environmental impact per unit of consumption. The implication of this formula, taken at face value, is that increases in personal consumption and population will increase the (negative) environmental impact. However, if T (also known as the Environmental Impact Coefficient, EIC, or environmental intensity) is reduced at the same time as P and C are going up, the negative effects of these increases can be mitigated.

Calculations using the Commoner-Erlich Equation. (Source: Ekins 2000)

The equation has the form:

I=P.C.T

If we assume:

PH is the population in the advanced capitalist, high income countries, PL the equivalent figure in low income countries, CH the consumption per head in the high income countries, CL the consumption figure for low income areas

Then I=[PH.CH + PL.CL] T

At the moment PH=902 million, CH =$24930, PL=4771 million, CL=$1090 million, therefore,

I= [902 x 24930+4771×1090] T

Rearranging the equation gives the current value of environmental impact per unit of consumption,

T=I/ [902.106 x 24930+4771.106 x 1090]= I/27.69×1012

Now consider the three cases mentioned in the main article:

- If there is no growth in population or consumption and it is assumed that I must be reduced by 50% for sustainability, then it is clear from the equation that T must also be reduced by the same amount i.e. 50%.
- If there is a factor of four growth in consumption per head, the population of the high income countries rises to 1186 million and the low income countries to 10160 million, and the environmental impact is reduced by 50%, then the new value of T needed for sustainability will be:

Tnew=0.5 x I/[1186.106 x 4 x 24930+10160.106 x 4 x 1090]=0.5 x I/162.6×1012

Dividing this figure by the current estimate for T made above gives:

Tnew=0.09 x T. i.e. T must be reduced by 91% for sustainability. - If the population rises as in example 2 above, and consumption for the entire world is 50% higher than currently found in the industrialised countries then the new value of T for sustainability will be:

Tnew=0.5 x I/[11346.106 x 1.5 x 24930]= I/8.5×1014

Dividing this figure by the current estimate for T made above gives:

Tnew=0.033 x T. i.e. T must be reduced by 97% for sustainability

How reliable are these predictions and where did they come from? Firstly, is a 50% reduction in environmental impact necessary for sustainability? The first point that needs to be made is that the Commoner-Erlich equation must be applied to each source of pollution separately. To aggregate the results over many disparate environmental threats would produce very arbitrary results. However, to take the example of global warming, the most serious problem of all, the Intergovernmental Panel on Climate Change (IPCC) said that CO_{2} emissions need to be cut by 60% to stabilise its concentration in the atmosphere. Other greenhouse gasses need to be cut by an average of more than 70% according to the IPCC. These statistics indicate therefore, that a 50% reduction in environmental impact is a conservative figure to use in predicting the conditions for sustainability due to global warming.

(A complicating factor in applying the equation is that it assumes there is no relationship between the variables of P, C and T. However, it has been claimed, using the USA as an example, that intensity in the use of resources (T) falls with rising consumption per head. This would tend to increase the cut in I needed for sustainability, compared to the case where the variables are assumed not to be related. Other examples, however, could be given which bend the results in the opposite direction. Also it is important to separate the calculations for the ‘North’ and the ‘South’, because their levels of consumption are vastly different).