Long liquid slugs, with sizes reaching 500 pipe diameters or more, may form in gas-liquid horizontal pipe flow at intermediate liquid loadings. Such slugs cause serious operational upsets due to the strong fluctuations in flow supply and pressure. Therefore, predicting the transition from short (hydrodynamic) to long slug flow regimes may play a significant role in preventing or reducing the negative effects caused by the long slugs. In this paper we introduce a growth model for calculating the average slug length in horizontal and near horizontal pipes. The model applies a volumetric balance between the front and tail of the slug in order to calculate the slug growth rate. The dynamic behaviour of the liquid at the tail is described by a linear kinematic relation between the slug downstream and the wave upstream. For the validation of the model we performed measurements in a 137 m length air-water horizontal pipe flow of an internal diameter (i.d.) of 0.052 m. The measurements provide a detailed flow map of the long slug regime and sub-regimes. Furthermore, we compared predictions by the model with available data for a range of 0.019-0.095 m i.d. pipes to investigate the effect of varying operation pressures, different inlet conditions, different fluid properties and slight inclinations. The model predicted the transitions from hydrodynamic to long slugs with satisfactory agreements, however it underpredicts the average slug length at relatively large mixture velocities. (c) 2009 Elsevier Ltd. All rights reserved.