Prescriptions for eyeglasses are measured in diopters. The measurement starts at zero ("plano"), with four quarters to a diopter: 0.25 (a quarter diopter), 0.50 (one half a diopter), 0.75 (3/4 of a diopter), and 1.00 (one diopter). The higher the numbers, the higher (stronger) the prescription.
Here are estimates for the approximate correction needed for each line seen on the Snellen chart:
Snellen
20/10
20/15
20/20
20/30
20/40
20/50
20/100
20/200
Estimated prescription
Plano (zero)
Plano
Plano to -0.25
-0.50
-0.75
-1.00 to -1.25
-1.75 to -2.00
-2.00 to -2.50
This estimation works best for myopia (nearsightedness) because hyperopic (farsighted) people can focus and compensate for a certain amount of their prescription. The Snellen fractions cannot be used for ordering a pair of glasses or contacts that will correct a person's vision.