Understanding Prescriptions

Understanding Your Prescription

What do all the numbers and symbols mean?

Your prescription measures the extent of your refractive error, be it myopia (short-sightedness), hyperopia (long-sightedness), astigmatism, and presbyopia. This is measured in units called dioptres.

More specifically, these measurements refer to the amount of refractive correction you require to achieve ‘normal’ visual acuity. For example, one dioptre is the equivalent of a lens that can effectively focus your vision on an object one metre away. The more severe your refractive error, the higher the numbers on your prescription.

A typical prescription features three numbers – for example: -5.00 / -1.50 x 180.

  • The first number (-5.00) identifies the degree of short-sightedness – indicated by a minus (-)symbol – or long-sightedness – indicated by a plus symbol (+). 
  • The second number (-1.50) identifies the extent of astigmatism: This may be preceded by a plus or minus symbol (usually a minus in the UK).
  • The third number (180) indicates the axis, in degrees, of your astigmatism. The ‘180’ in this example indicates that the astigmatism is horizontal.

When this is all put together, the example prescription tells us that the patient is moderately short-sighted with a moderate degree of astigmatism in a horizontal direction.