You will need: Thin bendable cardboard (preferably not paper).
You will also need sticky-tape/sellotape;
a pen or pencil,
a surgical blade/small scissor; and
a ruler.
On the cardboard, draw a rectangular strip measuring: 80 x 5mm.
Cut out the shape.
Curl both ends with the use of a scissor or a surgical blade.
Add a small piece of sticky-tape to the one end of the cardboard strip; it shouldn’t be wider than 5mm.
Using your thumb, keep the one end of the cardboard strip in place (on the “no sticky-tape” side).
Wrap it around your finger. Stick it into position. The fit should be snug, but not tight.
Remove the cardboard “ring” from your finger. You should have to wiggle it over the knuckle, otherwise the fit is too loose.
Test the fit by putting the “ring” back on again, wiggling it over the knuckle. If necessary, make the needed adjustments to the size at this point before proceeding to the next step.
Cut through the ring where the two ends of the cardboard strip overlap. The cut should be straight across at a 90 degree angle.
Lay the cardboard strip flat on a level surface.
Place a ruler over the length of the cardboard strip, pressing it in place.
Read the exact millimeter measurement (measure as precisely as possible; up to the first decimal. Example; 16.2mm).
Convert the measured millimeter length to a ring size. It is important to note that all of the ring sizes on this website is listed using the UK standard (Alphabetic sizes).

(For a custom ring size not listed above (R 1/2 or larger), please contact us for a quotation).