Spike's Calculators

US / Canadian Ring Sizes from Inside Diameter

Calculate your US / Canadian ring size from an inside ring diameter measurement in inches.
In addition to the ring size, this calculator will provide the inside circumference of the ring.

The sizes are based on an increase of 0.032 per full number. A US number 3 ring has an inside diameter of 0.554 decimal inches, a number 4 ring size 0.554 + 0.032 = 0.586 decimal inches.

To get the right size will take a certain amount of accuracy in measuring. If you have a tape measure with 1/64" graduations you will be able to it fairly accurately.

To get the decimal amount from a fractional inch you divide the numerator of the fraction by the denominater.
11/64" = 11 ÷ 64 = 0.171875"
15/16" = 15 ÷ 16 =  0.9375"
1/2" = 1 ÷ 2 = 0.5"
A ring size, 0, has an inside diameter of 0.458 decimal inches. That is very close to 29/64"
A ring size 10 has an inside diameter of 0.778 decimal inches. 50/64"

The increase of a half size ring number equals 0.016" That is very close to 1/64" in a fraction.

US / Canadian Ring Sizes

Inside ring diameter dec.in


Diameter in decimal inch dec. in
Inside ring circumference dec. in
Inside ring circumference in
US ring size #


  1. enter the inside diameter of the ring


  1. the inside diameter in decimal inch
  2. inside ring circumference in decimal inches
  3. inside ring circumference in inch plus a fraction
  4. the size of the ring
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