If you have both contact lenses and glasses, you may have noticed that your prescriptions for each product differ slightly. Similarly, glasses-wearers may wonder if their regular prescription will work for contact lenses as well. Most people will have a different prescription for each product. As the conversion between a glasses prescription and a contact lens prescription is complicated and delicate, you should verify your prescription with a doctor before ordering your new contact lenses.

1. Examine your glasses prescription. You will notice that the prescription has two categories: OD and OS. OD is your right eye, while OS is the left eye. The number following each abbreviation will be in diopeters, which is written as D (this may be written or simply understood). A prescription marked DS stands for diopters sphere, meaning that the patient has no astigmatism. The number that follows each abbreviation will be marked + or - ; + prescriptions are for farsighted patients, while – prescriptions indicate nearsightedness. If the patient does have an astigmatism, a second number will follow the first to note how severe this condition is.

2. Take into account the different position of a contact lens. Your glasses sit several millimeters away from your eye, while a contact lens (placed directly onto your eye) will be slightly weaker. You will need to have this prescription measured by an optometrist to get the exact power difference, as various factors will affect your prescription. This is especially true if your prescription is greater than +/- 4D.

3. Ignore any astigmatism if you have hard contact lenses. A hard contact will correct the shape of the eye by making it more spherical. Therefore, the lens will be of similar strength as your glasses prescription.

4. Take astigmatisms into account if you have soft contact lenses. You will need a “toric,” or differently-shaped, soft contact lens if you have at least .75D astigma. For this reason, hard contact lenses can be a better choice for those with severe astigmatisms.

5. Visit your eye doctor. After considering various factors, you must visit your eye doctor to determine the exact contact lens prescription you will require. Factors like the curvature of your eye, eye health conditions, and other factors can be determined only by the specialized machines she has in her office.