Reading an Eyeglass Prescription

Below is a breakdown of the parameters in an eyeglass prescription and what they represent, along with their common values:
 

Parameter	Description	Common Range
SPH / SPHERE	Overall lens power for distance (–) or near (+)	0.00 to ±5.00 Lens Thickness Chart
CYL / CYLINDER	Correction for astigmatism	0.00 to ±3.00
AXIS	Lens rotation or direction of correction. Only present if there's a CYL value.	0 to 180° (May start with '0' or '00')
ADD	Additional magnifying power (+) for reading (Used for bifocals & progressives)	+1.00 to +3.00
PRISM	Horizontal/Vertical Correction for eye alignment issues, being a Diopter value and Base Direction	0.25 to 2.0 BU (Base-UP), BD (Base-Down), BI (Base-In), BO (Base-Out)
OD	Right Eye
OS	Left Eye
OU	Both eyes	Same value for both eyes
PD	Pupillary Distance Measurement	58 mm – 64 mm Not always included

 

 Remember: 

• Blank fields don't apply and can be ignored
• If no plus (+) or minus (–) sign appears with a number, it is assumed to be plus (+) by default
• Prescription powers across all parameters (except AXIS) are 0.25 steps or in quarter diopter increments
• PL, PLANO, DS, SPH notations are placeholders that all indicate a zero (0.00) or no value, and can also be left blank
• Sometimes the doctor is in a rush and can leave out the decimal points in a diopter value by mistake (Example: -100 is the same as -1.00)
• Eyeglass prescriptions are universally written or understood the same across all countries

A standard single vision prescription includes three main parameters: SPH, CYL, and AXIS. Some may not apply, so they are left empty or have placeholder notations.

In this example:

• The right eye (OD) has astigmatism, so it also includes a CYLINDER and AXIS value
• The left eye (OS) does not have astigmatism; hence, it is just a SPH value with DS notation
• The OS AXIS is left empty because there is no OS CYL value
• DS is just a placeholder for 0.00 or no value

A bifocal prescription is the same as your single vision prescription, but it includes an additional parameter called the ADD power.  This more advanced type of prescription is used for bifocals, progressives, or trifocal lenses, as all these lens types utilize the same magnifying ADD power.

In this example:

• There is no change in the single vision (distance) portion of the prescription
• There is a magnifying ADD power of +2.50
• The ADD power is always a plus (+) value that is the same in both eyes

Note: If multiple ADD power values are listed, only one value is provided for both eyes, typically the lower number for bifocals and the higher number for progressives.
 

The PRISM field is often left blank in most prescriptions. When present, it can include both a Diopter and a Base Direction. A Prism can exist in both single vision and bifocal prescriptions.

In this example:

• Each Base Direction includes a Diopter amount (ie, 1.5 BU)
• The Base Direction is usually abbreviated and will start with the letter B, which means Base, followed by the first letter of the direction: UP/DOWN for Vertical or IN/OUT for horizontal
• The right eye (OD) utilizes both Base Directions: UP and IN (Vertical and Horizontal)
• The left eye (OS) only utilizes a single Base Direction: IN (Horizontal only)
• The Prism value of one eye can be in a single Base Direction or both Base Directions combined

Sometimes, a handwritten prescription may not be written or formatted as neatly as the prior examples, but it is read the same way. 

In this example:

• The top line is the right eye (OD)
• The bottom line is the left eye (OS)
• The SPH is written first, followed by the CYL and AXIS
• The CYL and AXIS are usually separated by an "x"
• The ADD power is the same in both eyes

A bifocal prescription can be converted into a near-only single vision prescription for reading glasses by using simple math and addition.

In this example:

• The ADD value is added to the SPH value (-2.25 + 2.50 = +0.25)
• The result (+0.25) is the new SPH value for Near vision, which is used for reading glasses
• In this conversion, only the SPH value is changed. The CYL and AXIS remain unchanged, but are still in the lenses
• The ADD value is omitted from the lenses
• See Example 2 for the original unconverted prescription

Still need help? Use the Online Conversion Tool