As I know, there are about 8 spectacled people
If you have already had glasses' prescription, you need to have the optometrist measure your eyes and write the prescription specifically for contact lenses because glasses and contacts are not the same things, let along the prescriptions. Please take the different positions of these two products into account, you will find out that glasses sit several millimeters away from your eyes, while the contact lens is placed directly onto your eye, then you can imagine that the contact lens will be slightly weaker when regarding to the power.
Next, we also have to take the K-readings into consideration to calculate a proper base curve for the contact lenses. The diameter of the lens is also very important. This measurement is from one side of the lens to the other and is also in millimeters. Diameter is important for the fit of your lenses because it determines where the lens will sit on your eye.
Last but not least, we should put the astigmatisms into account. If you have hard contact lenses, please ignore any astigmatism, because a hard contact will correct the shape of the eye by making it more spherical. However, if you have soft astigmatism, you will need a 'toric' or a differently-shaped soft contact lens, because there are some differences between them.
If you have something confused after finish reading this article, you must visit your eye doctor to make sure the contact lenses are exactly suitable for your eyes. Then you can successfully convert your glasses' prescription to the contact lens power.