Reading add calculator
Age and reading distance in; an estimated add for near work out, with the range behind it.
For general information only — not medical advice. This page gives an educational estimate from an age formula; it is not a prescription. A prescription must come from an eye-care professional.
Typical distances: a book or phone about 16 inches, a laptop about 26. Leave the amplitude blank to use the average expected at this age. The add is on top of your distance prescription.
What a reading add is
Presbyopia is the gradual loss of the eye’s ability to change focus for near objects. The reading add is the extra plus power, in diopters, put on top of the distance prescription so that the near task comes into focus without strain. The number depends on how much the eye can still accommodate, called the amplitude, and on how close the task is held.
The method, variable by variable
- Amplitude of accommodation from Hofstetter (1950): minimum = 15 − 0.25 × age, average = 18.5 − 0.3 × age, maximum = 25 − 0.4 × age, all in diopters and never below zero. At age 50 they give 2.50, 3.50 and 5.00 D.
- Demand = 1 ÷ working distance in meters.
- Add = demand − (share of the amplitude used × amplitude), not below zero, rounded to the nearest 0.25 D. The share is ½ by default: half of the amplitude is left in reserve for comfort.
The reserve rule is our choice of default, not a published standard: the sources we could open describe several variants, and the add is described there as a starting point to be adjusted at the chart.
Worked example
A 50-year-old reads at 16 inches, which is 40.64 cm. Demand is 100 ÷ 40.64 = 2.46 D. The average amplitude is 18.5 − 0.30 × 50 = 3.50 D, half of which is 1.75 D. The add is 2.46 − 1.75 = 0.71 D, rounded to +0.75 D. At a laptop distance of 26 inches the demand falls to 1.51 D and the estimate to 0.00 D. Using the minimum amplitude of 2.50 D the add rises to +1.25 D, and with the maximum of 5.00 D it drops to 0.00 D, which shows how wide the spread behind a single age is.
Estimates by age at 16 inches
| Age | Average amplitude (D) | Demand (D) | Estimated add (D) |
|---|---|---|---|
| 40 | 6.50 | 2.46 | 0.00 |
| 45 | 5.00 | 2.46 | 0.00 |
| 50 | 3.50 | 2.46 | +0.75 |
| 55 | 2.00 | 2.46 | +1.50 |
| 60 | 0.50 | 2.46 | +2.25 |
| 65 | 0.00 | 2.46 | +2.50 |
This table is our calculation with the half-in-reserve rule, not a published prescribing chart. By the formulas the average amplitude reaches zero near age 62, so beyond that the estimate is the full demand.
Demand at common working distances
| Task | Distance | Demand (D) | Add at age 50 (D) |
|---|---|---|---|
| Phone or book held close | 13 in (33 cm) | 3.03 | +1.25 |
| Book or paperwork | 16 in (41 cm) | 2.46 | +0.75 |
| Laptop | 22 in (56 cm) | 1.79 | 0.00 |
| Desktop monitor | 26 in (66 cm) | 1.51 | 0.00 |
| Music stand or tablet on a stand | 32 in (81 cm) | 1.23 | 0.00 |
The distances are examples for illustration, not measured norms; measure your own. The demand column is exact (1 ÷ meters); the add column uses the average amplitude at age 50 and the half-in-reserve rule. It shows why a person who needs one strength for a book may need a weaker one for a screen, and why a lens designed for a single distance can feel wrong at another.
Limits and common mistakes
- The formulas are old and broad. Hofstetter’s equations come from two early surveys, by Donders (1864) and Duane (1922), and the methods behind them carried considerable error. Later studies of children and young adults have found measured amplitudes that differ from the averages.
- One age, a wide range. At the same age the minimum and maximum lines differ by about 2.50 D, which moves the add by more than a full step. That is why this page shows all three.
- Ignoring the task. Reading at 16 inches and working on a monitor at 26 inches need different adds; a single pair rarely suits both.
- Treating the add as a prescription. Antona and colleagues (2008) tested seven ways of choosing a tentative add in 69 people aged 40 to 60 and found every method needed adjustment: the limits of agreement with the final add were always wider than ±0.50 D. Distance correction, eye alignment and eye health also matter.
Common questions
At what age do most people need reading glasses?
The calculator’s age rule shows when the arithmetic starts to bite: at a 2.46 D demand (16 inches) the estimated add is 0.00 D at age 40 and +0.75 D at age 50. Real onset varies with the person, the task and the lighting, and people who are nearsighted may simply take their glasses off to read. Anyone who has trouble with near work should have an eye exam instead of relying on an age formula.
What is amplitude of accommodation?
It is the total focusing power the eye can add when it looks from far to near, in diopters. Young eyes have many diopters to spare; the supply falls steadily with age until, around the early 60s on Hofstetter’s lines, little is left. Presbyopia is that loss, not a disease.
How do you convert reading distance to diopters?
Divide 1 by the distance in meters. A book at 16 inches (0.41 m) needs 2.46 D, a laptop at 26 inches (0.66 m) needs 1.51 D, and a sheet of music at 40 inches needs 0.98 D. This is why a pair of reading glasses made for a novel can be too strong for a monitor.
Why does the estimate leave some accommodation in reserve?
Holding a fully strained focus for long makes eyes tired, so the add is chosen so that the eye supplies only part of the demand and keeps the rest in reserve. The page defaults to keeping half in reserve. Other conventions keep a third or two-thirds in reserve and give a lower or higher add: at age 50 and 16 inches the estimate is +0.75 D with half in reserve and +1.25 D with two-thirds in reserve. We could not find a single standard, so treat the choice as a clinical judgment.
Can this tell me what reading glasses to buy?
No. It estimates a starting point from an age formula. In a 2008 comparison of seven methods in 69 people aged 40 to 60, Antona and colleagues found that tentative additions agreed with the final prescribed add only to within wider than ±0.50 D, so each add has to be adjusted for the patient. Pairing the add with your distance prescription, the two eyes, and the task is the optician’s job.
Can I enter an amplitude I have measured?
Yes. If an eye-care professional gave you a measured amplitude, type it in the optional field and it replaces the age formula. A measured amplitude of 2 D at 16 inches gives an estimated add of +1.50 D with half in reserve, against +0.75 D from the average for age 50.
Related: the spherical equivalent calculator for converting a distance prescription, the visual acuity converter and the thin lens calculator, where the same diopter arithmetic (power = 1 ÷ focal length in meters) applies to any lens.
For general information only — not medical advice. This estimate is educational and is not a prescription; a prescription must come from an eye-care professional. Sources: H.W. Hofstetter, amplitude of accommodation equations (1950), as reported in the “Amplitude of accommodation” article of Wikipedia, which cites Donders (1864) and Duane (1922) as the underlying surveys (secondary source; the original was not opened); B. Antona et al., “Comparing methods of determining addition in presbyopes,” Clinical and Experimental Optometry 91:313–318 (2008), abstract only. How we check our calculators.