How to Read a Glasses Prescription: A Parent's Guide

After your child's eye exam, the optometrist hands you a slip of paper filled with abbreviations and decimal numbers. Most parents stare at it and wonder: is -2.50 bad? What does CYL even mean? And what should I actually do with this information?

A glasses prescription is one of the most important documents in your child's eye health journey — especially if they are myopic. Understanding it helps you track whether their vision is stable or worsening, evaluate whether myopia control treatment is working, and have more informed conversations with your optometrist. This guide walks you through every number on that prescription, what the values mean in practice, and when it matters most.

What Is a Glasses Prescription?

A glasses prescription (also called a spectacle prescription or Rx) is a set of measurements that describes the optical correction your child's eyes need to see clearly. It is written by a registered optometrist or ophthalmologist after a refraction test — the part of an eye exam where you look through different lenses and say which one makes letters clearer.

Prescriptions are written in a standardised format across Singapore, Malaysia, Hong Kong, and most of the world. Once you learn the abbreviations, you can read any prescription from any clinic.

The Key Abbreviations Explained

OD and OS: Right Eye and Left Eye

Every prescription is split into two eyes:

OD (oculus dexter) = right eye
OS (oculus sinister) = left eye

Some charts use R and L instead. Some add a third row — OU (oculi uterque) — for a reading that applies to both eyes together. The right eye is always listed first.

It is common for children to have different prescriptions in each eye. If one eye is significantly weaker than the other (a difference of more than 1.00 D is clinically relevant), the optometrist will want to monitor for amblyopia (lazy eye) in younger children.

SPH: Spherical Power

SPH is the main lens power and the number that tells you whether your child is myopic or hyperopic.

A negative SPH (e.g., -1.75, -3.00) means myopia (short-sightedness). The child can see close-up but struggles with distant objects.
A positive SPH (e.g., +1.00, +2.50) means hyperopia (long-sightedness or far-sightedness). The child can often compensate for mild hyperopia, but higher amounts cause eyestrain and blur.
Plano or 0.00 means no spherical correction is needed in that eye.

The unit is dioptres (D). A prescription of SPH -2.50 means the lens must have 2.50 dioptres of minus power to correct the focusing error.

In Singapore, around 65% of children are myopic by the time they finish primary school, and the SPH value in their prescription is the figure that most parents focus on — understandably. But SPH alone does not tell the full story.

CYL: Cylindrical Power (Astigmatism)

CYL stands for cylindrical power and corrects astigmatism — an irregular curvature of the cornea or lens that causes blur at all distances, not just far away. Many myopic children also have some degree of astigmatism.

A negative CYL (e.g., -0.75, -1.25) is the most common way astigmatism is written in optometry prescriptions in Singapore and most of Asia.
If CYL is blank or 0.00, there is no astigmatism to correct.

Mild astigmatism (CYL ≤ -0.75) often does not need correction in children with otherwise good vision. Moderate astigmatism (CYL -1.00 to -2.00) typically does require a cylindrical lens component, and significant astigmatism (above -2.50) should always be corrected.

AXIS: Orientation of the Astigmatism

The AXIS value (written as a number between 1 and 180) describes the angle of the astigmatism — in which direction the eye's curvature is irregular. This number has no unit.

Axis only has meaning when CYL is present. If there is no astigmatism correction, the axis is irrelevant. The axis is used by the optician who grinds and cuts the lens so that the cylindrical correction is placed in exactly the right orientation.

From a monitoring perspective, a significant change in axis over successive exams (more than 20 degrees) can indicate changes in corneal shape and is worth discussing with the optometrist.

ADD: Near Addition (For Reading or Bifocal Lenses)

ADD is the extra magnifying power added to the bottom of bifocal or progressive lenses to help with near vision. It is always a positive number.

For children, ADD power is occasionally prescribed in the context of:

Bifocal or progressive lenses for myopia control — some research suggests that reducing the accommodation demand for near work can slow myopia progression slightly in children with significant near exotropia (a tendency for the eyes to drift outward when reading).
Accommodative esotropia in younger children who need plus power for near tasks.

Most standard myopic children's prescriptions will have no ADD value. If your child's prescription includes an ADD, ask the optometrist what it is for — it is not commonly prescribed without a specific clinical reason.

PD: Pupillary Distance

The PD (pupillary distance) is the distance in millimetres between the centres of the two pupils. It is used by the optician to position the optical centre of each lens directly in front of the correct pupil.

PD is technically a fitting measurement rather than a prescription value, but it often appears on the same form. For children, PD changes as the face grows, so it should be re-measured at each new pair of glasses.

Understanding the Numbers: What Is "Bad" Myopia?

Parents often want to know whether their child's prescription is mild, moderate, or severe. The clinical categories are:

Myopia Level	SPH Range
Low myopia	-0.50 to -3.00 D
Moderate myopia	-3.25 to -6.00 D
High myopia	Greater than -6.00 D

High myopia (above -6.00 D) carries significantly elevated risks of serious eye conditions later in life, including retinal detachment, glaucoma, myopic macular degeneration, and early cataracts. This is why myopia control — slowing progression before it reaches high levels — is a public health priority in Singapore and across Asia.

The rate of change from one prescription to the next is also critical. An increase of -0.50 D or more per year is considered fast progression and typically warrants active management. In Singapore, children with moderate-to-fast progression are commonly offered atropine eye drops, orthokeratology (Ortho-K), or myopia control spectacle lenses such as DIMS or HALT.

A Sample Prescription, Explained

Here is how to read a typical prescription for a myopic child:

        SPH      CYL      AXIS
OD:    -2.50    -0.75      180
OS:    -3.00    -1.00      170

Reading it:

Right eye (OD): -2.50 D of myopia plus -0.75 D of astigmatism, with the astigmatism correction oriented at 180 degrees.
Left eye (OS): -3.00 D of myopia (slightly worse than the right) plus -1.00 D of astigmatism at 170 degrees.

This is a moderate myopic prescription with mild-to-moderate astigmatism in both eyes. The 0.50 D difference between eyes is within the normal range, but the overall level of myopia at, say, age 9 or 10 would indicate that the child could potentially reach high myopia by adulthood without intervention.

How Myopia Prescriptions Change Over Time

One of the most important things parents can do is track the SPH value from year to year. Ask your optometrist for a copy of every prescription and keep a simple log:

Date	Age	OD SPH	OS SPH
Jan 2024	8	-1.00	-1.25
Jan 2025	9	-1.75	-2.00
Jan 2026	10	-2.50	-3.00

In this example, the child progressed by -0.75 D in the right eye and -0.75 D in the left eye in each 12-month period — slightly below the Singapore school-age average of around -0.88 D/year, but still fast enough to warrant attention. By age 10, they are already approaching the threshold of high myopia if progression continues unchecked.

Knowing this trajectory helps you and the optometrist make informed decisions about whether to start or escalate myopia control treatment, and it gives you a way to evaluate whether treatment is working.

When to Ask About Myopia Control

If your child's prescription increases by -0.50 D or more in a year, or if they are already at -3.00 D or above, it is worth explicitly asking the optometrist whether myopia control is appropriate. Treatment options used in Singapore include:

Low-concentration atropine eye drops (0.01% or 0.025%), used nightly, which slow the biochemical signals that cause the eye to elongate.
Orthokeratology (Ortho-K) lenses, worn overnight, which temporarily reshape the cornea and have been shown in multiple trials to slow axial elongation.
DIMS or HALT spectacle lenses (such as MiYOSMART), which have a special lens design that creates myopic defocus in the peripheral retina to slow eye growth.
Multifocal or extended-depth-of-focus contact lenses such as MiSight, worn during the day.

Each treatment has different suitability criteria, efficacy rates, and costs. There is no single "best" option — the right choice depends on the child's age, lifestyle, degree of myopia, and parental preference.

Using Your Child's Prescription With a Myopia Calculator

Once you have the prescription, one of the most useful things you can do as a parent is use it to project where your child's vision might be heading. The CarrotByte Myopia Progression Calculator allows you to enter your child's current prescription and age, and see a projected trajectory to age 17 — both untreated and with different control options applied.

This gives you a concrete, evidence-based visualisation of why optometrists recommend starting treatment early. A child who is -2.00 D at age 8 is projected to reach much higher levels by age 17 without intervention than one who starts atropine or Ortho-K at that same -2.00 D point.

The calculator is free and designed for both parents and optometrists. You do not need to create an account to use it.

Frequently Asked Questions About Glasses Prescriptions

Can a child's prescription improve on its own? For myopia, improvement (a reduction in the minus prescription) is uncommon in school-age children and adolescents. Myopia tends to progress until the late teens or early twenties when the eye stops growing. Significant "improvement" in a young child is sometimes a sign that the initial prescription was measured incorrectly (e.g., without cycloplegic drops to relax accommodation), not that the eye has recovered. Always ask your optometrist to clarify.

How often should children have their eyes tested? For a child who is already myopic, the Singapore Association of Optometrists and most clinical guidelines recommend yearly eye exams as a minimum, and every six months if their prescription is changing rapidly.

Is a higher SPH always worse? Yes, in the sense that higher minus powers mean more myopia and greater long-term risk. But remember that a prescription of -2.50 at age 7 is more worrying than the same prescription at age 15, because a younger child has more years of potential progression ahead of them.

Does wearing glasses make myopia worse? No. A common myth is that wearing glasses "makes the eyes lazy" and worsens myopia. The prescription itself does not affect how the eye grows. What influences myopia progression is primarily genetics, axial length (the physical length of the eye), near work habits, and light exposure — not whether the child wears glasses.

Can I use the prescription from one country in another? Yes. The prescription format (SPH, CYL, AXIS in dioptres) is internationally standardised. A prescription written in Singapore will be readable by any optical shop or optometrist in the UK, Australia, or elsewhere.

Summary: What to Do With Your Child's Prescription

Keep every prescription and track changes year over year.
Focus on the SPH value as the primary indicator of myopia level and change.
Note the rate of change: -0.50 D/year or more is fast and warrants discussion of myopia control.
Ask about CYL: astigmatism above -1.00 D is significant and can sometimes independently affect vision quality.
If SPH is already at -3.00 or above, have a direct conversation about long-term risk and treatment options.
Use tools to help visualise the trajectory. The CarrotByte Myopia Progression Calculator and Myopia Risk Calculator are free resources built on published clinical data, designed to help parents and optometrists have that conversation with confidence.

Understanding your child's prescription is the first step to being an active participant in their eye health. The numbers on that slip of paper tell a story — and once you know how to read them, you are in a much better position to help write a better outcome.