There is no direct conversion from focal length in millimeters to an “x zoom” number.

In common use, “x zoom” means the ratio of the lens’s longest focal length to its shortest focal length:

• 17–70mm = about 4x zoom (70 ÷ 17 ≈ 4.1)
• 100–400mm = also 4x zoom (400 ÷ 100 = 4)
• 18–90mm = 5x zoom (90 ÷ 18 = 5)

So the zoom rating only tells you how large the zoom range is, not how wide or how telephoto the lens actually is.

That’s why two lenses can both be “4x” but behave very differently:

• 17–70mm starts fairly wide
• 100–400mm is entirely telephoto

To compare actual angle of view, you need the focal length itself, often along with the sensor size or 35mm-equivalent focal length. The “x zoom” figure by itself does not indicate how much magnification you’ll get relative to another camera or lens.