Inner Bore (Internal Diameter)
The straight-line distance in millimeters across the inside opening of a ring, excluding the thickness of the metal band.
METROLOGY KNOWLEDGE BASE & GLOSSARY
Frequently Asked Questions & Jewelry Sizing Glossary
Direct, bench-jeweler answers to the most common questions about 1:1 optical screen calibration, international ring size conversions, watch wrist proportions, and GIA diamond dimensions.
1. Frequently Asked Sizing Questions
How many millimeters is one full US ring size?
Each full US/Canadian ring size step equals exactly 0.8128 mm (0.032 inches) of internal diameter, or approximately 2.55 mm of inner circumference. A half size equals 0.4064 mm of diameter, and a quarter size equals 0.2032 mm.
Why does my ring feel tighter in the morning and summer?
Finger volume fluctuates due to fluid retention overnight, vasodilation in warm weather, exercise, and dietary sodium. Always measure your finger at room temperature in the late afternoon or evening when your hands are at their stable baseline.
Can eternity bands, platinum rings, or tungsten rings be resized?
Full diamond eternity bands, tension settings, and alternative metals like tungsten carbide, titanium, and ceramic cannot be resized because cutting the shank disturbs the stone settings or shatters hard alloys. Platinum and 14k/18k gold solitaire or plain bands can easily be resized by 1 to 2 sizes.
How do I convert my US ring size to European (EU / ISO) or Cartier size?
Multiply your ring’s internal diameter in millimeters by π (3.14159) to get your European ISO 8653 circumference size. For example, a US size 6 (16.51 mm inner diameter) equals an EU/Cartier size 52 (51.87 mm circumference).
What is the difference between Comfort-Fit and Standard-Fit rings?
A Standard-Fit (flat court) ring has a flat interior wall that maintains full surface contact with your skin. A Comfort-Fit ring features a slightly domed, convex interior that reduces friction over the knuckle, making wide bands (5 mm+) feel roughly 0.25 US size looser.
Why is a credit card used to calibrate the screen?
Every Visa, Mastercard, driver’s license, and bank card worldwide is manufactured to the strict ISO/IEC 7810 ID-1 physical standard of 85.60 mm × 53.98 mm. Matching our on-screen frame to your physical card locks the exact pixel-per-millimeter pitch of your display.
What is the difference between watch case diameter and lug-to-lug length?
Case diameter measures the round dial housing (typically 36 mm–42 mm), whereas lug-to-lug measures the total vertical span from the top strap lug tip to the bottom lug tip (typically 44 mm–51 mm). Lug-to-lug determines whether a watch overhangs the edges of your wrist.
Does a 2.00 carat diamond look twice as large as a 1.00 carat diamond?
No. Because carat is a 3D weight unit (200 mg per carat), doubling the weight increases top-down diameter by the cube root of 2 (~25%). A 1.00 ct Round Brilliant is 6.45 mm wide, while a 2.00 ct Round is 8.10 mm wide.
2. A–Z Jewelry & Horology Metrology Glossary
ISO 8653 Standard
The international horology and jewelry standard defining ring size by the internal circumference in millimeters (e.g., Size 52 = 52.0 mm inner circumference).
Wheatsheaf Gauge (UK / Australian Scale)
The alphabetical ring sizing standard (A to Z+) used in the United Kingdom, Ireland, Australia, and New Zealand, advancing in 1.25 mm circumference half-steps.
ISO/IEC 7810 ID-1
The international engineering specification for standard identification and payment cards, mandating exact physical dimensions of 85.60 mm × 53.98 mm.
Lug-to-Lug Distance
The vertical tip-to-tip measurement across a watch case’s strap horns. The primary determinant of whether a timepiece sits flat within the width of a human wrist.
Spread (Face-Up Footprint)
The visible top-down surface area (in mm²) of a faceted gemstone when mounted in a ring setting, independent of its total carat weight.