In the mist inferior aspect of the septum width averaged 1.2 mm thick in children and 1.8 mm thick in adults.
Mean thickness
Overall mean by Transesophageal US
anterior region 2.56 ± 0.92 mm
fossa ovalis 1.89 ± 0.75 mm
posterior region 2.95 ± 0.97 mm

68 M LAE preablation LAE
Ashley Davidoff MD

4 chamber of the heart on MRI shows a normal interatrial septum. Three measurements range between 3 to 3.7 mm. The middle portion is the region of the septum primum.
Ashley Davidoff MD
130607L

66-year-old male presents with dyspnea
Non gated axial CT through the mitral valve suggests left ventricular hypertrophy. The interatrial septum measures 1.8 mms which in this region is upper limits normal (1.8mms)
Ashley Davidoff MD
Links and References
Qadir et al Measurement of interatrial septal thickness by echocardiography in patients with moderate to severe rheumatic mitral stenosis undergoing percutaneous balloon mitral valvuloplasty Int J Cardiol Heart Vasc. 2018;22:35–38. Published 2018 Dec 9.
Sweeney et al The normal anatomy of the atrial septum in the human heart Am Heart J. 1979 Aug;98(2):194-9.