[[File:Cardiothoracic ratio.jpg|thumb|220px|Cardiothoracic ratio =
(MRD + MLD)/ID}
<br>where:<ref>{{cite web|url=http://www.ohsu.edu/xd/education/schools/school-of-medicine/departments/clinical-departments/diagnostic-radiology/pediatric-radiology-normal-measurements/chest-measurements.cfm|title=Chest Measurements|website=[[Oregon Health & Science University]]|accessdate=2017-01-13}}</ref>
<br>MRD = greatest perpendicular diameter from midline to right heart border
<br>MLD = greatest perpendicular diameter from midline to left heart border