Some report CTR as a percentage, however this is strictly incorrect, as it is a ratio.
The CTR is measured on a PA chest x-ray, and is the ratio of maximal horizontal cardiac diameter to maximal horizontal thoracic diameter (inner edge of ribs / edge of pleura). A normal measurement should be ≤0.5.
It should be noted that this measurement is a coarse marker of disease 4. If the patient is symptomatic then echocardiography is required but the yield of echocardiography is low if performed just for an increased CTR 3.