In this paper an algorithm is presented for estimating the maximum feasible penetration path length for neutron and synchrotron X-ray strain measurement instruments. This reflects the attenuation and scattering capability of the material under examination, the incident flux and detector arrangement, the likely background signal, the required strain measurement accuracy, the sampling volume and the diffracting geometry. Its validity and generality is examined through a consideration of data collected using a number of instruments on a variety of materials. Two criteria for the maximum feasible path length are examined: one based on the maximum acquisition time, the other based on the minimum acceptable peak height to background ratio. As demonstrated in the companion paper [part II: Withers (2004). J. Appl. Cryst. 37, 607-612], the algorithm can be used to delineate those conditions under which neutron and synchrotron X-ray radiations can provide useful information and to identify which is most suited to any particular measurement task.