How to work out your reading glasses strength at home

Ready-made reading glasses are sold in dioptres, printed as a plus number: +1.00, +1.50, +2.00 and so on up to about +3.50. The number describes how much focusing power the lens adds. Higher numbers bring the point of clearest focus closer to your face and, importantly, also shrink the range over which anything is sharp.

That last point is the one most people miss. Buying a stronger lens than you need does not give you better vision, it gives you a narrower window of usable distance, and the fatigue that comes from holding your head at exactly the right spot.

The age table, and why it is only a starting point

Accommodation declines predictably enough that age alone gives a decent first guess. Broadly: early forties, +1.00; mid to late forties, +1.50; early fifties, +2.00; late fifties, +2.50; sixties and beyond, +2.50 to +3.00.

Two things distort this. If you are naturally short-sighted, you may be able to read comfortably without any correction well past fifty, because your uncorrected eye already focuses at close range. If you are long-sighted, symptoms arrive early, sometimes in the late thirties, because you have been spending accommodative effort just to see the far wall clearly.

A test you can do with a newspaper

Find text about the size of ordinary newsprint in good, even light — daylight from a window is ideal, and a dim room will skew everything. Cover one eye and hold the page at the distance you would naturally read at, roughly 35 to 40 centimetres.

Try the weakest strength you think might work. Read a full paragraph. The right lens does three things: the text is sharp without you adjusting the page distance, you can hold the page at a comfortable arm bend rather than up near your face, and nothing feels tight behind the eyes after a paragraph.

Now move the page slowly closer and then further away. Note the range over which the text stays readable. With a correctly chosen strength this range is comfortably wide, perhaps fifteen centimetres. If it is a narrow band and you find yourself hunting for the sweet spot, the lens is too strong. If the text only sharpens when you push the page out past a comfortable arm bend, the lens is too weak.

Repeat with the other eye. If the two eyes want noticeably different strengths, ready-made readers are the wrong product for you, because both lenses carry the same power. Book an eye test.

Choosing between two candidates

When two adjacent strengths both seem acceptable, take the weaker one. A slightly weak lens leaves some work for your own accommodation, which keeps a wider comfortable range and delays the point at which you need the next step up. A slightly strong lens does the opposite and tends to feel fine for ten minutes and tiring at forty.

One exception: if most of your close work happens at a genuinely short distance — detailed handwork, fine assembly, sewing — the stronger option is the right call, because you actually need the closer focal point.

Matching the strength to how you will use them

A single-strength reader is set for one distance. If your day is mostly a printed page, that is fine. If you move between a phone at 30 centimetres, a laptop at 60 and a face across the table, one power cannot serve all three, and you will take the glasses on and off constantly.

This is where progressive readers change the calculation. Choose the strength using the newspaper method above, because that is the power that sits at the bottom of a progressive lens where reading happens. The lens then supplies the weaker intermediate powers automatically as your eyes travel up the corridor. Check whether the model you are buying has zero power at the top, which suits normal distance vision, or half power at the top, which is optimised for desk range but blurs the far wall.

Signs you got it wrong

Headaches at the temples or brow after twenty minutes usually mean the lens is too strong or the two eyes are mismatched. Text that is sharp only when the page is held unusually close means the lens is too strong. Leaning forward towards the page means it is too weak. A pulling sensation when you look up quickly is normal in the first week with progressives and abnormal after two.

Finally, expect the number to change. Presbyopia progresses until roughly the mid-sixties, when the lens has stiffened as far as it will go. Moving up one step every two or three years is ordinary, not a sign that the glasses damaged your eyes. Wearing readers does not weaken the eye; it simply makes the ongoing change more noticeable, because you now have a clear baseline to compare against.