Understanding prescriptions

What do the numbers and symbols all mean

Optometrists measure refractive errors – myopia, hyperopia, presbyopia and astigmatism – in units called dioptres.

Dioptres are a measure of the amount of refractive correction you need in order to see normally. One dioptre is the equivalent of a lens that can focus on an object one metre away.

The more short-sighted, long-sighted or astigmatic you are, the higher your prescription will be in dioptres. A typical prescription has three numbers – for example: -5.00 / -1.50 x 180.

  • The first number (-5.00) identifies your degree of short-sightedness or longsightedness. The minus sign in front of the number identifies you as short-sighted, whereas a plus sign means you are longsighted;
  • The second number (-1.50) identifies the amount of astigmatism you have. This is written with either a plus sign or a minus sign (usually minus in the UK);
  • The third number (180) indicates the axis in degrees, communicating the orientation of your astigmatism. An axis of 180 degrees, for example, means the astigmatism is horizontal.

So, a prescription of -5.00 / -1.50 x 180 indicates that the patient is moderately short-sighted, with a moderate degree of astigmatism in a horizontal direction.