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Reading glasses calculator: how many diopters do you need?

Enter your reading distance, the closest distance you can read clearly and your age to estimate the reading add in diopters and the clear range.

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Reading add needed

1.50 D

A low to moderate reading add, typical of early presbyopia
Typical reading add for your age
1.70 D
Focal length of the reading lenses
67 cm
Closest clear distance with the glasses
40.0 cm
Farthest clear distance with the glasses
67 cm

This calculator is for general information and education. It is not medical advice and does not replace the advice of your doctor or a registered dietitian.

Around the age of 40, the eye’s lens stiffens and close objects blur: this is presbyopia. Reading glasses fix it with a plus lens whose power is measured in diopters. This calculator estimates the add you need for a given reading distance and shows the range over which text stays sharp with the glasses.

How the calculation works

  • Vergence of the page = 100 ÷ reading distance in cm (diopters)
  • Vergence you can focus alone = 100 ÷ your closest clear distance in cm
  • Reading add = vergence of the page − vergence you can focus alone
  • Focal length of the lens = 100 ÷ add, in cm
  • Closest clear distance with glasses = 100 ÷ (100 ÷ closest distance + add)

The farthest sharp distance with the glasses is the focal length of the lens, because a relaxed eye then sees the page as if it were at infinity.

Example

You want to read at 40 cm and your closest clear distance without glasses is 100 cm. The page needs 100 ÷ 40 = 2.5 D and your eye can handle 1.0 D, so the add is +1.50 D. The lens has a focal length of 67 cm. With the glasses you read clearly from 40 cm to 67 cm. The typical add at age 52 is +1.70 D.

Why it matters

Reading distance changes the power you need. A person who holds a phone at 30 cm needs more add (+2.33 D for the same near point) than a person who reads a book at 40 cm. If your glasses are too strong, the clear zone shrinks and you must hold the book very close, and if they are too weak the text stays blurred.

Good to know

This is an estimate for plain reading glasses, with no astigmatism or distance correction. Single-vision readers focus at one distance, while progressive and bifocal lenses combine distance and near in one pair. If you wear distance glasses, the add is on top of that prescription. Have your eyes examined: sudden changes in vision can be a sign of a health problem. This tool is for general information.

The thin lens calculator relates focal length and image distance, the lensmaker calculator computes lens power from the curvature, and the eyeglasses cost calculator budgets the purchase.

Frequently asked questions

What is a diopter?

A diopter is the unit of lens power: the inverse of the focal length in meters. A +2.00 lens focuses light at 0.5 meter, and a +1.00 lens at 1 meter. Reading glasses have positive power and make near objects clear.

What is a reading add?

The add is the extra plus power needed for near work, on top of your distance correction. It is the difference between the vergence of the book at your reading distance and the closest vergence your eye can still focus on its own.

How do I choose the strength of reading glasses?

A rule of thumb is about +1.00 at 45 years, +1.50 at 50, +2.00 at 55 and +2.50 at 60. An eye exam gives the exact value, because it depends on your reading distance, your distance prescription and how much focusing power you still have.

Why do I need more power as I age?

The lens of the eye stiffens with age, a condition called presbyopia, so it can no longer change shape to focus close up. The loss goes on until about 60, when the add levels off at around +2.50 to +3.00.

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Results are estimates provided for information purposes. They do not replace professional advice.

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