💍 What Is A Ring Diameter Chart?
A Ring Diameter Chart matches the internal width of a ring with an estimated ring size. If you already own a ring that fits comfortably, measuring its inside diameter is one of the simplest ways to estimate the size of another ring for the same finger.
The diameter must be measured across the inside opening of the ring. Measure from one inner edge directly through the centre to the opposite inner edge. Do not include the metal band itself.
In the UK, ring sizes are normally expressed using letters such as K, L, M, N, O and P. A diameter chart allows you to translate a physical measurement such as 17.12 mm into an approximate British letter size.
Use the Ring Diameter Size Finder for an instant result or browse the complete UK Ring Diameter Chart below.
Enter Your Ring's Inside Diameter
Use millimetres or inches to find the closest British letter size.
Measure Straight Across The Empty Centre
Use the internal edges of the ring rather than its outside width.
Use A Ring That Fits
Choose an existing ring worn comfortably on the same finger and hand.
Measure The Inside
Measure from one internal edge to the opposite internal edge through the centre.
Use Millimetres
Millimetres make close comparison between neighbouring ring sizes easier.
Match The Letter
Compare your measurement with the diameter chart or use the finder above.
| UK Ring Size | Inside Diameter (mm) | Inside Diameter (in) | Circumference (mm) | Circumference (in) | Approx. US | Size Range |
|---|
Chart note: These values are practical ring-sizing references. Individual jewellery retailers may use slightly different rounding, so always compare your result with the seller's own ring size chart before ordering.
Ring diameter is the straight-line distance across the circular opening inside a ring. The line passes through the centre and begins and ends at the inner edges of the metal.
This is different from the outside diameter. The outside measurement includes the thickness of the ring band, so it will always be larger and should not be used with a ring diameter chart.
For example, if an existing ring has an internal width of approximately 17.12 mm, the chart on this page places it close to UK size N. The physical outside width of that same ring could be considerably greater depending on the thickness of the metal.
Diameter is particularly useful when you already have a correctly fitting ring but do not know its UK letter size.
Method 1 — Ruler
Choose The Correct Ring
Use a ring that already fits the exact finger on which the new ring will be worn.
Lay It Flat
Place the ring on a flat surface over a ruler marked clearly in millimetres.
Find The Widest Internal Point
Position the ring so you measure straight through the centre.
Read Inner Edge To Inner Edge
Do not begin from the outside metal edge. Record only the empty internal opening.
Repeat The Measurement
Measure more than once to make sure the ring has not shifted on the ruler.
Method 2 — Digital Callipers
Set To Millimetres
Use the millimetre setting for easier comparison with the diameter chart.
Use The Inside Measuring Jaws
Position the correct measuring surfaces against opposite inner edges.
Measure Through The Centre
An angled measurement will not represent the true internal diameter.
Use Gentle Contact
Do not force the callipers outward against the ring.
Compare With The Chart
Find the closest millimetre diameter and read the corresponding UK size.
Inside Diameter
The straight distance from one internal edge of the ring to the opposite inner edge through the centre.
Inside Circumference
The total distance around the inside opening of the ring.
UK Ring Size
The British letter assigned to approximately corresponding internal measurements.
Because the inside opening of a standard ring is approximately circular, diameter and circumference can be converted mathematically using pi.
Millimetres are usually easier to work with when measuring ring diameter because neighbouring sizes differ by relatively small amounts. A figure such as 17.12 mm is easier to compare precisely than a heavily rounded inch value.
However, you may encounter inch measurements when buying from international retailers or using an imperial ruler. The chart therefore includes both millimetres and inches.
If possible, use a ruler or callipers with millimetre markings and keep at least one or two decimal places before matching the result to the chart.
Your measured diameter may fall between two chart values. This can happen because the ring itself has manufacturing tolerances, because the measurement was rounded or because your ideal size genuinely falls between full UK letters.
Start by measuring again. A difference of only a fraction of a millimetre can change which row is closest.
If your result remains between two sizes, check whether the retailer offers half sizes. British sizing commonly includes options such as L½, M½, N½ and other intermediate values.
The final choice should also consider ring width and fit preference. A broad band can feel tighter than a slim ring of the same nominal size.
The measured internal diameter describes the ring opening, but two rings with the same nominal diameter can still feel different if their widths and internal profiles are different.
A wide wedding band covers more of the finger and may feel tighter. A comfort-fit ring has a rounded internal profile, which can also affect how it slides over the finger.
Therefore, use the Ring Diameter Chart as a measurement guide but check the manufacturer's specific recommendations for broad or unusual rings.
Ring sizing labels vary internationally. UK jewellery commonly uses letters, whereas US and Canadian sizes generally use numbers. European jewellery may use numerical sizing closely related to circumference.
For international shopping, diameter can be a particularly useful reference because it gives you a physical measurement rather than relying entirely on converted labels.
If a UK N converts approximately to a particular US or EU size, compare the actual diameter shown by the retailer as well. Different charts can round international conversions differently.
If you already have a ring that fits, measure its internal diameter before shopping online and keep the millimetre result available.
When viewing a product, compare your diameter with the retailer's own chart. This can be more useful than relying solely on the letter or international conversion.
For example, if your existing ring measures approximately 17.1 mm inside, look for a retailer size with a similar internal diameter. Then check the associated UK letter and any guidance regarding half sizes or band width.
If the seller's chart differs slightly from a general Ring Diameter Chart, follow the seller's measurements for that particular product.
Outside Diameter
Measuring across the outside edges includes the metal band and gives an incorrect sizing diameter.
Measuring At An Angle
The diameter line must pass through the centre of the circular opening.
Ruler Misalignment
If the ring or ruler moves, a small measurement error can affect the nearest size.
Rounding Too Early
Rounding 17.12 mm to 17 mm removes useful precision before size comparison.
Wrong Finger's Ring
Use an existing ring worn on the exact finger and hand intended for the new ring.
Assuming Charts Match
Jewellers can use slightly different rounding and conversion values.
- ↔ Always measure from inner edge to inner edge.
- 🎯 Make sure the measurement line passes through the centre.
- 📏 Use millimetres and retain one or two decimal places.
- 🔁 Take multiple measurements and compare the results.
- 💍 Use a ring that already fits the intended finger comfortably.
- ✋ Do not assume a ring from the opposite hand is automatically the same size.
- ↔️ Consider band width before making the final purchase.
- 🏪 For valuable jewellery, confirm the size professionally.
A diameter chart is an excellent tool when you already have a correctly fitting ring, but professional sizing is preferable when the final fit is particularly important.
Consider professional sizing for engagement rings, wedding bands, high-value jewellery, very wide bands or rings made from materials that may be difficult to resize later.
A jeweller can compare your finger with calibrated metal sizing rings and consider factors such as knuckle width and ring profile that cannot be fully captured by diameter alone.