Reading glasses strength calculator
Three ways to work out a strength: from a prescription, for another distance, or from how close you can focus.
From a glasses prescription
Enter the numbers from your prescription. Leave cylinder empty if there is none.
Enter sphere and ADD to see the result.
For a computer or another distance
Farther away needs less strength. Enter the strength that works for reading and the new distance.
From your near point
Without glasses, bring small print closer until it just starts to blur. Measure from your eyes to the page and enter it here.
This is a screening aid, not an eye exam. It cannot detect eye disease.
How each calculation works
From a prescription
A prescription for progressive or bifocal lenses lists a sphere (SPH) for distance and an ADD for near. Reading strength is the two added together, for each eye. With SPH −1.00 and ADD +2.00 the reading strength is +1.00. With SPH +0.75 and ADD +1.75 it is +2.50.
Store-bought readers only work when both eyes come out the same, the result is between +1.00 and +4.00, and there is little or no cylinder. Otherwise ask an optician for prescription reading glasses, which cost more but match each eye.
For a computer or another distance
Focusing on something takes one diopter divided by its distance in meters. Reading at 14 in (35 cm) takes about 2.8 diopters. A monitor at 24 inches takes about 1.6. The difference, about 1.2 diopters, is how much weaker your computer glasses can be. That is why computer strength comes out near half of reading strength.
From your near point
The closest distance at which you can still focus shows how much focusing power your eyes have left. Eye doctors keep half of that power in reserve so that reading stays comfortable. The calculator takes what the reading distance needs and subtracts half of what your eyes can still do.
This method only works if your distance vision is clear, with glasses or contacts if you wear them.
Check the result
Whatever the calculator says, try the strength on with something you really read. If you are between two strengths, take the lower one. To see the result as print sizes, take the test.
Calculator questions
How do I convert my prescription to reading glasses strength?
Add the ADD to the sphere for each eye. SPH +1.00 and ADD +1.50 gives +2.50. If the sphere is negative, the result is smaller than the ADD: SPH −0.50 and ADD +2.00 gives +1.50.
Is the ADD on my prescription my reading glasses strength?
Only if your sphere is 0.00 (sometimes written PL or plano) in both eyes. Otherwise the sphere has to be added in.
Can I use store-bought readers if I have astigmatism?
With a small cylinder, up to about 0.50, many people manage fine. With more, print stays slightly blurred through store-bought readers and prescription lenses are the better choice.
What strength do I need for a computer?
For a screen at 24 inches, about 1.25 diopters less than your reading strength. Someone who reads with +2.00 needs about +0.75 for the monitor. See reading glasses for computer use.
My prescription has two different eyes. What strength do I buy?
If the two results differ by a quarter diopter, the lower one is usually a workable compromise. If they differ by more, store-bought readers will leave one eye blurred. Order prescription reading glasses instead.