Abstract The energy balance method previously described gives values for the elastic modulus of single crystals of nylons that are reasonably close to experimental data, where these are available, apart from nylon-6. Force constants were derived from Urey Bradley (UBFF) and valence (VFF) force fields. Modulus values, in GN/m2, are as follows: Nylon-6 244.1 (UBFF) 262.8 (VFF) 164.8 (exp) Nylon-11 243.5 (UBFF) 241.4 (VFF) — Nylon-6, 10 185.6 (UBFF) 189.6 (VFF) 196.2 (exp) Nylon-6, 6 180.9 (UBFF) 192.9 (VFF) 172.6 (exp) Nylon-7 180.2 (UBFF) 188.6 (VFF) — Consideration of the cross-sectional area and repeat distance of the polymer chain indicates that the modulus will decrease in the order given. The extensibility (f value) of each polymer was within the expected range (0.4 to 0.5nN). The method is applicable to similar polymers, e.g. poly(tetramethylene hexamethylene sulphone) has a modulus of 176.7 GN/m2 and an f value of 0.403nN.