TV screen sizes: what “65-inch class” means, and what the diagonal and cabinet figures on a spec sheet show

TV sizes are sold by class: a “65 Inch Class” set, in LG's naming. The class is not the measured screen. LG's US spec sheet for the 65-inch class OLED C6, a 2026 model, lists a size of 65 inches and a diagonal of 64.53 inches. This guide explains where the class convention leaves buyers, what the Federal Trade Commission's old screen-size rule required before it was repealed, and how the class, the listed diagonal and the cabinet dimensions on a spec sheet differ. It uses LG's published 2026 figures as the worked example, because LG's US pages list both a size and a diagonal for each model.

Class and diagonal on LG's 2026 spec sheets

On the eight LG pages we read, the class figure was above the listed diagonal in every case, by between 0.3 and 0.5 inch. The table adds the screen width and height that follow from each diagonal, assuming the 16:9 shape of a 3,840 by 2,160 4K screen, and sets them against the cabinet dimensions LG lists without a stand.

The rounding itself costs little picture. By our arithmetic, a 64.53-inch diagonal gives about 1.4% less screen area than a full 65 inches would, and the gap ranges from about 0.8% to 2.2% across the eight models. The bigger differences in the table are in the last column. The two 65-inch class sets have practically the same screen, yet the Gallery TV, which LG sells with a frame, is an inch wider and about an inch taller than the C6 as listed. The class name describes neither the screen nor the cabinet: the outer size of each set is in the cabinet dimensions LG lists separately.

How much more picture each size step adds

Screen area grows with the square of the diagonal, so a size step adds more than its inches suggest. Using LG's listed diagonals, our calculation gives these increases:

The rule the FTC dropped

For more than 50 years, a federal rule governed how screen sizes could be advertised. The FTC adopted its Picture Tube Rule in 1966, when sets used curved cathode ray tubes and part of the tube could sit hidden behind the casing. The rule required size claims to be based on the horizontal width of the actual viewable picture, unless the advertiser clearly disclosed another method, such as the diagonal, next to the size.

In a final rule published on October 9, 2018, and effective January 7, 2019, the FTC repealed it. It found that virtually all TVs now have flat screens whose picture covers the whole surface, that marketers uniformly advertise the diagonal, and that no commenter had shown deceptive screen-size claims. Its search of consumer complaints over the previous five years found none about such claims. The rule no longer appears in Title 16 of the Code of Federal Regulations, the eCFR showed on November 10, 2026.

The repeal did not leave screen sizes unregulated. The FTC said it keeps the authority to deal with unfair or deceptive screen-measurement practices case by case under the FTC Act, and it declined a trade group's request to declare state action preempted, saying states have authority under their own laws. The convention the FTC relied on, the diagonal, is what LG's pages list next to the class name.

Other figures on a spec sheet are covered in our guides to reading a TV spec sheet, from HDMI 2.1 to the EnergyGuide label, and to anchoring or mounting a new TV.