# Why is a 1" sensor actually 13.2 × 8.8mm?

I'm wondering how sensor sizes are calculated. The Sony RX100 reportedly has a sensor size of 1" which is 13.2 x 8.8 mm according to the press release and various sites.

How does that work? That's a pretty small inch.

The Wikipedia page on sensor sizes has this chart:

which doesn't really help. (The 1" format is labelled Nikon CX. Chart by Moxfyre under the CC-BY-SA license.)

If full frame/35 mm refers to the horizontal size, what does 1" refer to?

• Full frame sensor is 36 mm wide. The 35 mm is the width of film, including perforations on each side needed for moving the film inside a camera. – Esa Paulasto Aug 2 '13 at 7:39
• Exacerbating the confusion is that some camera makers refer to their sensors as 1" while others refer to theirs as 1"-type. Are they the same size or it there really a full 1" sensor, too? – John Cilmi Dec 10 '16 at 15:01
• See also: dpreview's "Making (some) sense out of sensor sizes" article. – inkista Dec 11 '16 at 19:09

Digital camera sensor format-size names have their roots in television camera tubes. These were measured in inches diagonal, but for various practical reasons, the entire circle isn't used. So, from way back then, there's a concept called "the rule of 16", which says that the usable, actual sensor diagonal for a 1" tube is 16mm. (Yes, it mixes imperial and metric measurements.) So, for each "inch" in a sensor format designation, translate that to approximately 16mm of sensor diagonal. Or, for formats smaller than an inch — very typical, e.g. `1/2.5"` — use the corresponding fraction of 16mm.

This rule matches the 1"-format designation for this sensor: 13.2mm × 8.8mm has a diagonal of 15.9mm, and you can see how it roughly applies to the other typical compact digicam formats as well. Usually, there's a little variation and sensor-makers round to the nearest somewhat-standard fraction, but occasionally, as with the 1/1.83" Nokia N8, a very-specific number is given, in which case it's almost certain that they're following the 1" = 16mm rule literally.

More background in this archived article.

Confusingly, 1" does not refer to the size of the sensor, it is instead a "type designation". The 1" refers to a certain size of TV camera tube, of which the usable image area occupied the inner two thirds. The use of such cameras is completely obsolete now but the sizing system remains.

It's arguable whether this is due to doggedly sticking to tradition or an conscious attempt by manufacturers to make it seem like their sensors are larger than they actually are. Probably a bit of both.

For a table of system "types" and their actual physical sizes see

• Great answer thanks, covered everything I wanted to know. – Carsten Jul 5 '12 at 11:50
• I would say that this is a matter of tradition--it's easier to compare sensor sizes this way. – bwDraco Jul 5 '12 at 22:58
• Great answers. This is an extremely confusing way to specify size, there may be a culture that wants to keep imperial measurements rather than convert to a much easier to use metric system. – RobG Dec 22 '15 at 6:58
• @RobG even going metric won't help. The relationship between 35mm and 36x24mm isn't at all obvious either. – Mark Ransom Mar 23 '17 at 16:35

It's a matter of unit conversion/Pythagorean Theorem (or Pythagorean Theorem/unit conversion.) The diagonal measure is (in historical/effective terms) 1 inch. It is similar to purchasing a tv, laptop, screen of any type. The stated size is a diagonal measurement.

• No, of course not. My comment is just about the historical source of the measurement. At one time, it was the actual diagonal dimension of the thing being dedctibed, and the algorithm is what I gave. – Mark R Russell Nov 24 '17 at 16:37