What a ring size actually measures
Every ring sizing system describes the inside of the band, either its circumference or its diameter, and the two are linked by pi: circumference equals diameter times 3.1416. The European system is the most transparent, because an EU size simply is the inside circumference in millimeters, so EU 54 is a 54 mm band. The US and Canadian scale starts near 44.2 mm at size 3 and adds about 2.55 mm of circumference per full size, roughly 0.81 mm of diameter, with half sizes in between. The UK and Australian scale uses letters, advancing one letter per US half size, so US 6 is L, US 7 is N, US 8 is P and US 9 is R. Japan and China number their sizes from 1 upward, and those numbers work out to about twice the US number, so US 7 is Japan 14.
The anchors in the table are the standard ones: 51.9 mm circumference and 16.5 mm diameter is a US 6, UK L and EU 52; 54.4 mm and 17.3 mm is US 7, UK N and EU 54; 57.0 mm and 18.1 mm is US 8, UK P and EU 57; 59.5 mm and 18.9 mm is US 9, UK R and EU 59.5. Enter any one of those figures and the rest fill in.
How to measure at home
The most reliable home method uses a ring that already fits the correct finger. Lay it on a ruler and measure the inside edge to inside edge across the widest point in millimeters, then enter that as the inside diameter. A 17.3 mm inside diameter is a US 7. Measuring on a screen is not reliable, so use a physical ruler.
If you have no ring to copy, wrap a strip of paper about 5 mm wide snugly around the base of the finger, mark where the end overlaps, lay the strip flat and measure to the mark in millimeters. That length is the circumference; enter it directly. Do not use string, which stretches, and do not pull the paper tight enough to dent the skin. Check that the loop can slide over the knuckle before you commit, since the knuckle, not the base, sets the smallest workable size. Fingers swell in heat and after salty food, so measure at the end of a normal day at room temperature and take two or three readings on different days.
Fit, band width and resizing
A ring should slide on with light pressure and need a small tug over the knuckle. If it spins freely or comes off when your hands are cold, it is too big. Body temperature, pregnancy, exercise and time of year all move finger size by up to half a size, which is why jewelers recommend sizing in the afternoon rather than first thing in the morning.
Band width changes fit more than most people expect. A wide band presses against more of the finger and feels tighter, so for a band 6 mm or wider, order a quarter to a half size up from your measured size. Comfort-fit bands, which are domed on the inside, run slightly large and often need a quarter size down. Plain gold, silver and platinum bands can usually be resized one or two sizes by a jeweler, but eternity bands set with stones all the way around, tension settings, titanium, tungsten and some inlaid designs cannot be resized at all, so measure carefully before ordering one of those. When buying a surprise gift, a US 6 to 6.5 is the most common womens size and a US 9 to 10 the most common mens size.