An unobtrusive, noninvasive and continuous pressure measurement is invaluable however, still being under research and development. There are many attempts proposing an appropriate relationship between pulse pressure velocity and pressure. Fifteen different formulas, both theoretical and experimental, describing relation between blood pressure and cross-sectional area of the vessel were examined. Using these formulas the relation between blood pressure and pulse transit time were derived. The results obtained show variety of dependences. For some of them an explicit derivation was not possible due to non-linear characteristics of the models. It follows from the study performed that depending on the assumptions accepted, even using the same approach, one can obtain contradictory results.