What strength reading glasses do you need?

What presbyopia actually is

Inside each eye, behind the iris, sits a lens about the size of an aspirin. A ring of muscle around it changes its shape: relaxed for distance, squeezed rounder for near. That shape change is what focusing is, and the amount of it available is called the amplitude of accommodation, measured in dioptres.

The lens stiffens throughout life. It is not a disease and not a consequence of anything you did — it happens to everyone, it starts long before anyone notices, and the day you notice is simply the day the remaining amplitude stopped covering the distance you hold a book at. A twenty-year-old has around ten dioptres of it. By forty-five there are about four left, and reading at forty centimetres needs two and a half of them, which is when the arms start getting longer.

It stops. Somewhere in the early sixties the amplitude reaches zero, and from then on the reading addition is simply whatever the working distance requires and does not climb further. People who expect their reading power to keep rising indefinitely are usually moving the page closer without noticing.

The arithmetic

Focusing at a distance of d metres takes 1 ÷ d dioptres. That is the whole of the demand side. Forty centimetres is 0.4 m, so 1 ÷ 0.4 = 2.50 D. A phone at twenty-eight centimetres asks for 3.57 D. A desktop monitor at seventy asks for 1.43 D.

The supply side comes from Hofstetter's formula for the minimum expected amplitude: 15 − 0.25 × age. At fifty that gives 2.50 D; at sixty, zero. Clinical practice works from the minimum rather than the average, because prescribing from the average leaves half the people who walk in short of what they need.

The addition is the shortfall, with half the remaining amplitude held back in reserve:

Add = (1 ÷ d) − (amplitude ÷ 2)

The reserve is the part people skip, and it is the reason a lens that technically lets you read still aches after twenty minutes. Running any muscle at its absolute limit is uncomfortable, and the ciliary muscle is no exception. Keeping half the amplitude spare is the long-standing convention, and it is why a calculator that simply subtracts amplitude from demand always comes out too weak.

Worked through for a fifty-year-old reading at forty centimetres: the demand is 2.50 D, the amplitude 2.50 D, the reserve 1.25 D, so the addition is 1.25 D — which is dispensed as +1.25. At sixty-five the amplitude is gone, so the reserve is nothing and the addition is the full 2.50 D.

Reading addition by age and distance

Built from the same two formulas the calculator uses. Read across a row to see how much the working distance matters, and down a column to see the rise stop.

The column for twenty-eight centimetres is the phone column, and it is a full dioptre stronger than the book column at every age. This is the single commonest reason someone decides their new reading glasses are too weak — not the glasses, the distance.

Why one pair never covers everything

A reading lens has one plane of best focus. Either side of it there is a band that still looks sharp, and the width of that band shrinks as the lens gets stronger.

Wearing a +1.25, the clear band runs from roughly twenty-five centimetres out to about a metre. Wearing a +3.00, it runs from about thirty centimetres to forty. That is the difference between glasses you can leave on around the house and glasses that work for a book and nothing else. Nobody explains this at the point of sale, and it accounts for most of the disappointment.

The practical consequences follow directly. If you read books and use a laptop, those are two distances forty centimetres apart and one fixed lens will not serve both well. The options are two pairs, a progressive, or an occupational lens designed for the intermediate range — and the last of those is genuinely underused, because most people have never heard of it.

If you already wear glasses for distance

Then a rack of readers is not for you, and the reason is arithmetic rather than snobbery.

Your near power is your distance prescription plus the addition. A −3.00 myope needing +2.00 of add has a near power of −1.00. Put a +2.00 reader on that person and you have given them +2.00 where they needed −1.00, which is three dioptres wrong. The reader ignores the distance correction entirely because it was designed for someone who has none.

There is a pleasant exception, and it is the reason short-sighted people often reach their late fifties without buying reading glasses at all. A myope's far point with the glasses off sits at 1 ÷ |prescription| metres: fifty centimetres for a −2.00, thirty-three for a −3.00. That is reading distance. Taking the glasses off is a free pair of readers of exactly the right strength, and the calculator above checks whether yours land in a useful place before recommending anything else.

What the numbers cannot tell you

The formula describes a typical eye of a given age. Yours may sit either side of typical by half a dioptre, which is two steps of lens power, so treat the result as the middle of a range to try rather than a prescription.

It also assumes both eyes need the same thing, which is often untrue, and no significant astigmatism, which is frequently untrue. Made-up readers cannot correct astigmatism at all — if you have a cylinder of any size, that is why cheap readers never feel quite sharp no matter which power you pick.

And it assumes the near difficulty is presbyopia. Most of the time it is. But an uncorrected long-sighted prescription produces the same complaint a decade early, a convergence problem produces it with normal focusing, and some medical conditions announce themselves first as trouble reading. None of those are things arithmetic can distinguish.

This calculator applies a standard clinical estimate to figures you supply. It is not an eye examination, not a prescription, and not a substitute for advice from an optometrist or ophthalmologist. Anyone with a recent change in vision, a difference between the two eyes, headaches, or any eye pain should be examined rather than reaching for a stronger pair.

Related: read your prescription in plain words, work out how thick your lenses will be, measure your pupillary distance, or convert between 20/20 and logMAR.

Questions people ask

What strength reading glasses do I need?

It depends on two things, and the charts on the rack only use one of them. Your age sets how much focusing your eyes can still do on their own, and the distance you hold things at sets how much focusing the job requires. A chart that says +2.00 at fifty-five has quietly assumed you read at forty centimetres. Hold your phone at twenty-eight and you need nearly a dioptre more; read sheet music at eighty and you need over a dioptre less. The calculator above asks for both.

Can I just buy readers off the rack?

Only if you need nothing for distance. Over-the-counter readers carry one power in both eyes, no astigmatism correction and a fixed optical centre distance. For someone who sees well at distance and has a mild, even prescription, that is genuinely fine and always has been. If you already wear glasses for distance, readers are the wrong shape of answer: they ignore the distance part completely, and the lens you actually want is your distance prescription with the addition applied to it.

Why do my reading glasses only work at one distance?

Because a reader is a single fixed lens, and the stronger it is, the narrower the band of distances it focuses. A +1.25 gives you roughly twenty-five centimetres to a metre. A +3.00 gives you about thirty to forty. That is not a fault in the glasses, it is the geometry of a fixed lens, and it is the reason people end up with one pair for books and another for the computer. The calculator prints your band in centimetres.

I am short-sighted. Do I need reading glasses too?

Often not — you may already own the answer. A −2.00 myope has a far point at fifty centimetres with the glasses off, which is a book's distance. Taking the glasses off is a free pair of readers, and it is why short-sighted people frequently reach their fifties without ever buying any. It stops working once the prescription is too weak to reach the page or strong enough that the page has to come uncomfortably close. The calculator checks this before it recommends anything.

Does reading in dim light or wearing readers make my eyes worse?

No. Presbyopia is the gradual stiffening of the lens inside the eye; it progresses on its own schedule from around the mid-forties and stops in the early sixties, and nothing you do with your glasses changes that timetable. Wearing a stronger add than you need does not accelerate anything, though it will shrink the distance you can work at and feel unpleasant. The one thing worth acting on is a change that arrives suddenly or affects one eye more than the other, which wants an examination.

Why do I need a stronger pair every couple of years?

Because the eye's remaining focusing amplitude falls roughly a quarter of a dioptre per year through the presbyopic decades, and the glasses make up whatever is missing. The rise stops when the amplitude hits zero, somewhere around sixty, after which the addition is simply whatever your working distance demands and stays there. If your reading power keeps climbing well past that, the working distance has probably crept closer rather than the eyes having changed.

Can this replace an eye test?

No, and the reason is not caution for its own sake. A formula can estimate what a typical eye of your age needs at a given distance; it cannot measure your actual refraction, cannot check the two eyes work together, and cannot see the back of the eye. Difficulty with near vision is also the first thing some people notice about conditions that have nothing to do with presbyopia. Use this to understand the numbers and arrive at the counter informed, not to skip the appointment.