Presbyopia, the gradual loss of the eye's ability to focus close up, starts for almost everyone in their forties. As a rough guide: early forties +1.00, mid forties +1.25 to +1.50, around fifty +1.75 to +2.00, mid fifties +2.00 to +2.50, sixty and over +2.50 to +3.00. If you are already long-sighted you will usually need a little more; if you are slightly short-sighted, a little less or none at all.
Hold something with small print at the distance you normally read, usually 35 to 40cm. If you have to push it further away to read it, you need more strength. If a pair of readers makes you bring the text closer than is comfortable, it is too strong. If you are between two strengths, the weaker one is usually more comfortable for longer periods.
Ready-made readers have the same strength in both lenses and no correction for astigmatism. If your eyes are different from each other, if you have astigmatism, or if readers give you headaches, you need an eye test and made-to-prescription lenses. An eye test also checks the health of your eyes, which readers cannot.
You can find a comfortable ready-made reading strength yourself, using your age and the distance test above. You cannot check your eye health that way, so the NHS recommends an eye test at least every two years.
It will not damage your eyes, but you will have to hold things closer than is natural, which is tiring and can cause headaches.
sight.direct is run by the team behind Eyeconic. Shop links go to Eyeconic. Guides are written to be useful whichever retailer you buy from.