The concept of TNT equivalence is often invoked to predict the blast parameters generated from explosions, in which high explosives other than Trinitrotoluene (TNT) are used. This paper discusses the TNT equivalence concept for bare charges using Pentaerythritol tetranitrate (PETN). At first, a review of different methods to derive TNT equivalence factors is given. After that, numerical simulations are conducted in order to obtain overpressure and impulse data, which is fitted by using the curve-fitting method. Meanwhile, the equations representing the fitted curves of peak overpressures and maximum impulses are employed to calculate the TNT equivalence factors for overpressure and impulse, respectively. Fitting these data, formulae are developed to calculate the TNT equivalence factors of PETN for overpressure and impulse. It is found that a single value, i.e. 1.258 for overpressure and 1.272 for impulse, can be used to convert the charge mass of PETN to TNT in the far field. This statement is confirmed by a series of free field air blast measurements. However, the TNT equivalence factors of PETN vary significantly in the near field. The value can be as large as 2.6 times the one in the far field.