About the Reading Glasses Strength Calculator
This reading glasses strength calculator estimates the magnifying power (in dioptres, such as +1.25 or +2.00) that over-the-counter readers should have for you. From around age 40 the eye’s lens gradually loses its ability to focus up close — a normal change called presbyopia — and readers add the focusing power you are missing for a chosen working distance.
You can estimate it two ways. The age method uses Hofstetter’s formula for the minimum focusing ability expected at your age. The near-point method is more personal: hold small print at the closest distance you can still read it sharply (without glasses, or with your distance glasses) and enter that distance. In both cases the calculator keeps half your remaining focusing ability in reserve, the standard optometry rule for comfortable reading, and rounds to the 0.25-dioptre steps readers are sold in.
Choose the distance you will actually use: about 40 cm (16 in) for books and phones, 50–70 cm for a computer screen. Readers suit people whose two eyes are similar and who do not have significant astigmatism; they are no substitute for an eye exam, which also checks for glaucoma, cataract and other conditions — especially if your near vision changes suddenly.
How to use the reading glasses strength calculator
- 1Choose whether to estimate from your age or from your near point.
- 2For the near point, hold small print as close as you can while it stays sharp and measure that distance.
- 3Enter the distance you want to read at — 40 cm for books, farther for a computer.
- 4Add your distance prescription sphere if you wear glasses for driving or TV.
- 5Try readers at the suggested strength and one step either side before you buy.
Formula and method
Focusing on an object d centimetres away requires 100 ÷ d dioptres of accommodation (2.5 D at 40 cm). Your amplitude of accommodation A is how much focusing your eyes can still do: either Hofstetter’s minimum expected amplitude for your age, 15 − 0.25 × age, or measured directly as 100 ÷ your near point in centimetres. Because working at your full limit for long is tiring, the usual clinical rule is to use only half of it, so the reading addition is the demand minus half the amplitude.
The result is rounded to the nearest 0.25 D and limited to 0 to +4.00, the usual range of ready-made readers. If you wear distance glasses, the near power is your sphere plus the addition. The approximate clear range runs from 100 ÷ (A + add) to 100 ÷ add centimetres.
- d
- Working distance in centimetres
- A
- Amplitude of accommodation in dioptres
- NP
- Near point: closest clear distance in centimetres
- Add
- Reading addition (glasses strength) in dioptres
Worked examples
50-year-old reading at 40 cm
At 50 the minimum expected amplitude is 15 − 0.25 × 50 = 2.5 D. Reading at 40 cm needs 2.5 D; half the amplitude (1.25 D) is kept in reserve, so readers of +1.25 make up the difference and give clear vision from about 27 to 80 cm.
60-year-old reading at 40 cm
By 60 Hofstetter’s minimum amplitude has reached zero, so the glasses must supply the whole 2.5 D needed at 40 cm: +2.50 readers.
Near point of 20 inches, reading at 16 inches
A 20 in near point is 50.8 cm, so the amplitude is 100 ÷ 50.8 = 1.97 D. Sixteen inches (40.6 cm) needs 2.46 D; 2.46 − 1.97 ÷ 2 = 1.48 D, which rounds to +1.50.
55-year-old at a computer 60 cm away
A screen at 60 cm needs only 1.67 D. With an expected amplitude of 1.25 D at age 55, the addition is 1.67 − 0.63 = 1.04 D, so +1.00 computer glasses — weaker than the roughly +2.00 this person would want for a book.
50-year-old who is slightly far-sighted (+1.00)
The reading addition is still +1.25 at 40 cm, but this person also needs +1.00 for distance, so the total near power is +1.00 + 1.25 = +2.25.
Frequently asked questions
What strength reading glasses do I need for my age?+
As a rough guide for reading at 40 cm: around +0.75 to +1.00 in your mid-40s, +1.25 to +1.50 around 50, about +2.00 in your mid-50s and +2.50 from about 60. Your near point and preferred distance can shift this by a step or two.
How do I test reading glasses strength at home?+
Hold small print and bring it closer until it blurs, then measure that near-point distance and enter it here. In a shop, choose the weakest readers that let you read comfortably at your normal distance for a few minutes.
Do I need different glasses for the computer?+
Often yes. A screen at 50–70 cm needs less power than a book at 40 cm, so full-strength readers can make a monitor blurry. Computer glasses are typically about 0.50 to 1.00 D weaker than your reading strength.
What happens if my readers are too strong?+
Readers that are too strong force you to hold things unusually close and shrink your clear range, which can cause eye strain or headaches. Too weak and you end up holding text at arm’s length. Choose the weakest pair that works.
Can reading glasses damage my eyes?+
No. Readers do not weaken your eyes or speed up presbyopia; needing stronger ones over time is the natural progression until about age 60. But do get regular eye exams, because readers do not correct astigmatism or detect eye disease.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.