Internal explosion that occurs within a structure causes multiple reflections of the blast wave and a multiimpact effect of the wave on the structure is observed. This leads to a greater blast load that acts for an extended time duration than compared to when the explosion occurs externally under an open environment. This paper illustrates the calculation of different parameters of the pressure wave produced as a result of internal and external explosion. Then, the behaviour of a typically reinforced concrete structure subjected to a blast load of 50 kg TNT placed both internally and externally was analytically determined. The time for which the structure experienced the internal pressure was 7.58 times more than that for external explosion. The results showed that maximum deformation for internal blast for all structural components was several times that of for an external blast. The minimum difference of deformations for the two blast scenarios was observed in the front wall, where the maximum deformation for the internal explosion was 9.97 times more. Whereas, the maximum difference was at the slab where the internal blast produced a deformation that was 34.89 times more than the corresponding value for external explosion. Also, deformations exceeded the maximum limits of deflections as per IS 456:2000 for internal blast loading while they were within limits for external blast loading.