Dimensions Guide

The following average ring dimensions are for the US, although other countries very likely use them too. If necessary, check the shop for their size equivalent.

Engagement Ring Sizes: ½ to 3 1/2

Size ½ is 15/32 in or 11.95 mm in diameter. That’s size A in the UK. US size 1 is 31/64 inches in diameter (12.37 mm; UK size B). Size 1 ½ is ½ inch or 12.78 mm (UK size C). Size 2 is 33/64 inches in diameter (or equal to 13.21 mm; UK D).

The US size 2 ½ is 17/32 niches (equivalent to 13.61 mm or UK size E). Size 3 is 35/64 inches in diameter (14.0 mm, size F). Size 3 ½ is 9/16 inches in diameter (that is 14.45 mm).

Engagement Ring Sizes: 4 to 6 ½

The US size 4 is 37/64 inches in diameter (equal to 14.86 mm in diameter or size H ½ in the UK). Size 4 ½ measures 19/32 inches in diameter (15.27 mm; UK size I ½). US size 5 is 5/8 inches in diameter (15.70 mm or J ½ UK size).

The 5 ½ is 41/64 inches in diameter (16.10 mm; UK size L). The US size 6 measures 21/32 in diameter (16.51 mm or M). The 6 ½ size is 43/64 inches in diameter (16.92 mm; UK size N).

Engagement Ring Sizes: 7 to 9 ½

The US size 7 is equal to 11/16 inches in diameter (17.35 mm in diameter. The UK equivalent is size O). Size 7 ½ is 45/64 inches in diameter (17.75 mm in diameter; UK size P). US size 8 is equal to UK size Q (23/32 inches or 18.19 mm in diameter).

Size 8 ½ is 47/64 inches in diameter (18.53 mm). Size 9 is ¾ inches in diameter (or 18.89 mm). The 9 ½ size is 49/64 inches in diameter (that is 19.41 mm in diameter). There is no specific UK equivalent for this size.

Engagement Ring Sizes: 10 to 13

Size 10 is the same as the UK T ½, which is 25/32 inches or 19.84 mm in diameter. US size 10 ½ is equal to UK size U ½ (51/64 inches / 20.20 mm in diameter). US size 11 and the UK size V ½ are 13/16 inches / 20.68 mm in diameter.

The US size 11 ½ is 53/64 inches / 21.08 mm in diameter. Size 12 is the same as UK size Y (27/32 inches / 21.49 mm in diameter). US size 12 ½ is 55/64 inches / 21.89 mm (UK size Z). US size 13 is 7/8 inches / 22.33 mm in diameter.

Tips

Measure the finger when the day’s about to end. That’s the time the finger is biggest. Fingers are small when they are cold.
 
Finding out the correct engagement ring size is easy. Put a piece of string around the finger. Note the point where the ends join up. Set the string on a ruler. When you get the circumference, divide it by 3.14. Now you have your finger size.