Estimation of energy absorption buildup factors (EABFs) of some human tissues for photon energies commonly used in brachytherapy (BT) and external beam radiotherapy (EBRT). A five parameter geometric progression (G-P) fitting approximation was used to estimate EABF at different energies and penetration depths (up to 20 mfp). MCNP6.1 was used to obtain EABF for comparison. For radiation sources with lower mean energies (MEs), adipose tissue, which has lowest , has highest EABF values while bone, which has highest , has lowest EABF values. EABF reaches to a maximum value (EABF∼550) for Ir-192 BT source, which has a mean energy of 0.35 MeV. The radiation sources (MEs from 300 keV to a few MeV) were found to have higher values of EABF. At higher energies, e.g. 5 MeV, the annihilation photons contribute EABF at higher penetration depths, e.g. 20 mfp. In general, both MCNP and GP fitted EABF values along with available experimental data were found to agree well. EABFs of various human tissues have been investigated at radiation source energies relevant to BT and EBRT extensively. The presented data on EABF for the human tissues should be useful for accurate dose estimation at BT and EBRT.