Jeweler's Finger Ring Gauge

Narrow rings are the most common, but if you are buying a wider ring it is best to try a wider set like the one's shown on the left, or anticipate that you will need a quarter or half size larger if the finger is sized using narrow sizing rings.

The dash marks following every other number on the rings indicate half sizes
7- = 7 1/2

Reality Check

Rings and gauges are hard metal objects that can be measured exactly, but your finger is flesh and bone. Your finger is not round and it cannot be measured with the same degree of accuracy as the ring. At the end of the day, the right ring is one that will stay on comfortably. Average ring finger sizes may be useful as a point of reference for people who are not very good at measurements. These are offered to help you know if your calculations are reasonable.

Average Adult Women Size 6 to 8 or L to Q

Most American Jewelers 
Get Women's Rings in Size 7
Average Adult Man Size 9 to 11 or J to W

Most American Jewelers
 Get Men's Rings in Size 10 

Finger Sizes Sometimes Do Not Correspond To Body Size. Finger Sizes Are Prone To Change With Temperature, Humidity, Exercise, What you have been eating, Time of day, Time of the Month, Pregnancy and Other Factors. Dominant hand is Usually 1/2 Size Larger, So If You Are Right Handed and Have a Size 7 Ring that Fits The Left Hand it is Reasonable to Guess That a 7 1/2 Will Fit The Right Hand. Quarter Sizes Are Offered to Help You Get The Best Fit, However Quarter Sizes Make more Difference on Smaller Fingers Than On Large Fingers.

The Best Size Is Usually The Ring That Fits Snugly and Gives a Little Resistance When You Take it Off. Wider Rings Should Be a Little Looser than Narrow Rings. 

Men Take Note That if You Are Not Used To Wearing a Ring, The Proper Size Will Probably Feel Tight at First. Many Men lose Their Wedding Rings Because They Choose Sizes That Are Too Loose.

There Are Two Common Methods of Reading A
Ring Gauge, Also Called a Sizing Stick.

This Ring is a 10 1/2 If Sized By The True Diameter Method
or 10 1/4 If Sized by The Center Line Method.
We Would Call This Ring a Size 10 1/2

True Diameter Method
This is The Method We Use.
The Mark is Read At The Point At Which The Wider Side Of The Taper Meets The Edge Of The Ring. Wide Rings Are Occasionally Tapered and The Smaller Of The Edges Of The Ring Should Be Taken As The True Size Since This Is The Diameter Of The Largest Finger That Can Fit Into It.

The Center Line Method
Using This Method The Mark At The Center Of The Ring Is Judged To Be The Size. Using This Method a Wider Ring Will Be Measured At a Size Slightly Smaller Than It's True Diameter, Due To The Taper Of The Sizing Gauge. This method Automatically Compensates Somewhat For The Fact That On The Same Finger A Wider Ring Will Fit Tighter.

If The Ring Is Narrow, 3mm or Less, The Difference Between The Two Methods is Negligible. Wider Rings Gauged By These Two Methods Will be Called a Slightly Smaller Size if The Center Line Method is Used. Because a Ring Size is a Definition of Diameter We Think that It is Proper That a Ring Actually Measures The True Diameter Rather Than Contain a Factor for Width. Your Local Jeweler May Disagree, but Calibration of Gauges From Different Manufacturers May Also Disagree.