A magnifying glass of focal length f centimetres magnifies about 25 / f times: a 10 cm lens gives about 2.5, and a shorter, more curved lens gives more.
The eye cannot focus comfortably closer than about 25 cm, its near point. A lens held with the object just inside its focal length makes a virtual image far away that the relaxed eye can look at, and the object can then be brought up close: its apparent size is multiplied by 25/f. If the image is brought in to the near point the figure is 25/f + 1. A magnifier marked in dioptres works the same way: a 10-dioptre lens (f = 10 cm) is about 2.5 times, which is why some makers quote the dioptres divided by four.
It assumes a thin lens held close to the eye, a normal near point of 25 cm (older eyes are further, so the same lens helps more) and a small object near the axis. Makers use different conventions, 25/f or 25/f + 1, so two lenses labelled alike may differ by one. At high powers aberrations and a tiny field of view limit what is useful, and a magnifier held further from the eye gives a smaller field, not a bigger magnification.
Estimate with the rule, then check it against the calculator that models it properly.
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