This web page helps you find the 35mm/full-frame equivalent to a focal length in a different sensor- or film-format.
Like a lot of photographers, you might be familiar with focal lengths in the 35mm/full-frame image format and what field of view that focal length represents (24 mm = wide angle, 50 mm = “normal” etc.). Different formats, however, require different focal length lenses to produce an equivalent field of view.
With this site, you can look up these focal lengths and their 35mm-equivalents.
The equivalent focal lengths are calculated based on the diagonal of each imaging format.
35mm format diagonal ≈ 43.3 mm.
APS-C format diagonal ≈ 28.8 mm
Diagonal ratio: 43.3 mm / 28.8 mm ≈ 1.5
50 mm in APS-C ≈ 75 mm in 35mm/full-frame format, because 50 mm * 1.5 = 75 mm.
Using a diagonal-based calculation (instead of width- or height-based) is not ideal when the aspect ratios of the compared formats vary (greatly), as is for example the case when comparing full-frame with its 3:2 aspect ratio to, say, 6x6, which is a square format. However, this still seems to be the most popular and useful way to make this comparison.