To find proper thimble sizing, you can either measure with calipers, or with a ruler or tape measure. Instructions for both are below, but note that there is nothing better than just trying the sizes on to see what fits, so there's still a certain amount of guesswork involved, even in these methods.
IMPORTANT: The thimble usually goes on the middle finger of the hand that you like to hand sew with. For many people that's the hand they write with (but not always). The thimble usually ends near or just below the base of your nailbed, so that's *where on the middle finger* you will measure.
Option 1. To measure with calipers set to measure mm, gently place the base of your middle fingernail - the part that's nearest your palm - into the calipers, so that the calipers rest on the sides of your middle finger. Do not press them tightly, but just let them touch the sides of your finger. Depending on how precisely you measure, this is likely to be an accurate measurement. Look at the thimble size list, and choose the thimble whose mm measurement (noted after each thimble size in the description) corresponds with the mm measurement shown on the caliper.
Option 2. To measure with a metric ruler or tape measure, lay the measuring device on a flat surface and gently place the base of your middle fingernail - the part that's nearest your hand - onto the tape measure. Do not press down, but just rest it or hover it over the ruler. Line it up so that your finger just covers a strong line (any one, such as the 1cm or 2cm line, etc). What measurement becomes visible on the other side of your finger? Take this measurement and then perhaps take away up to 1mm, since you want the thimble to be tight on your finger so it doesn't slip off continually. This is *likely* to be an accurate measurement, although there's also a fair amount of guesswork involved here, since the angle of viewing can so impact which lines become visible to you.
Because the measuring methods are imprecise and the fit needs to be precise, some people find it's a good idea to get a few sizes and then narrow it down after trying them on, either gifting, returning, or saving for a future sewing apprentice ;) the ones that don't fit.
