In short: To make a ring to a given UK size, find that size's inside diameter, add the thickness of your metal, and multiply by π: blank length = (inside diameter + thickness) × 3.1416. For a UK size N, at roughly 17.1 mm inside diameter, in 1.5 mm stock that is about 58.4 mm. Measure your stock rather than trusting its label, and make a copper test ring before cutting gold or silver.
Cut a ring blank to the inside circumference of the finger and the finished ring will be too small. Cut it by eye and it will be a different size every time. There is a simple formula that gets you within a fraction of a size, and a few bench habits that close the remaining gap. Here is how to work out ring blank length for any UK size, with a worked example and the corrections that matter.
Why the inside circumference is not enough
When you bend a flat strip into a ring, the inside surface is compressed and the outside surface is stretched. Somewhere in the middle of the metal is a line that neither stretches nor shrinks. Its length is what your blank needs to be, and it sits roughly halfway through the thickness. That is why the formula adds the full thickness to the inside diameter: half a thickness on each side of the ring.
The formula
Blank length = (inside diameter + metal thickness) × π
The order matters. Add the thickness to the diameter first, then multiply by π (3.1416). If you multiply the diameter by π and then add the thickness, the blank comes out short — the thickness needs multiplying by π too.
A worked example: UK size N
- UK size N has an inside diameter of about 17.1 mm (published charts vary by a few tenths of a millimetre).
- Your strip measures 1.5 mm thick.
- 17.1 + 1.5 = 18.6 mm.
- 18.6 × 3.1416 = 58.4 mm.
Cut the blank at 58.4 mm, file the ends square and you are close to a size N before soldering.
Getting the inputs right
The inside diameter
UK sizes run from A to Z, with half sizes between, and there is no single official chart: retailers' published figures for the same letter differ by up to about half a millimetre. The answer is to use one reference consistently. A set of steel A–Z finger ring sizing gauges gives you the customer's size, and the mandrel you true rings on should be the same reference you check finished rings against. If you work from a ring that already fits, measure its inside diameter with calipers at two or three angles and use the average.
The thickness
Do not trust the label on the packet. Rolled or drawn stock varies, and a ring blank cut from "1.5 mm" strip that is really 1.6 mm will come out a little large. Measure it. A 0–25 mm outside micrometer with 0.01 mm graduations settles the question, and a wire and sheet thickness gauge is a quick check for stock marked in gauge numbers. For round wire, the thickness is simply the wire's diameter.
Corrections that close the gap
Wide bands
Wider strip resists bending more and tends to come out slightly small. One widely used supplier sizing guide adds 0.5 mm to the blank when the band is over 4 mm wide. Treat that as a starting point and confirm it with your own test rings.
Your own fudge factor
The formula assumes the neutral line sits exactly in the middle of the metal. In practice it shifts with metal, temper, profile and how you form the ring, so some makers adjust the multiplier slightly for heavy stock. Rather than guessing, make a test ring: cut a copper blank by the formula, form and solder it, and check it on your mandrel. If it comes out half a size large, shorten future blanks in that stock by the difference. Write the result down; it will hold for that metal and thickness.
The solder seam and filing
A well-fitted seam with a thin line of solder adds almost nothing. Filing the ends square takes a little away. Most of the time these cancel out, but if your seams are generous, allow for it.
From blank to ring
- Anneal the blank and file both ends square.
- Bend the ends towards each other with half-round forming pliers, then spring them past each other and back so they meet under tension.
- Solder, pickle and rinse.
- Slide the ring onto a ring mandrel and tap it round with a soft mallet, turning it over often so both edges are worked equally.
- Check the size against your reference. If it is slightly small, you can stretch it a little on the mandrel; if it is large, cut a section out and re-solder.
Our measuring & marking collection gathers the gauges, calipers and dividers that make every one of these steps repeatable.
A quick reference habit
Keep a notebook page per stock: metal, profile, thickness, the size you aimed for, the blank length you cut and the size you got. After a handful of rings it becomes your own sizing chart, tuned to your hands and your tools, and far more accurate than any number on the internet.
Frequently asked
How do I calculate the length of a ring blank?
Add the metal thickness to the ring's inside diameter, then multiply by π (3.1416). For example, a 17.1 mm inside diameter in 1.5 mm stock needs a blank of 18.6 × 3.1416, which is about 58.4 mm.
Why is my ring coming out too small when I use the inside circumference?
Because bending compresses the inside of the metal and stretches the outside. The blank needs to match the length of the line through the middle of the metal, so you must add the metal thickness to the inside diameter before multiplying by π.
What is the inside diameter of a UK size N ring?
About 17.1 mm. Published UK ring size charts differ by up to about half a millimetre for the same letter, so choose one reference, such as a set of ring sizing gauges, and use it consistently.
Do wide ring bands need a longer blank?
Usually a little. One widely used supplier guide adds 0.5 mm to the blank for bands over 4 mm wide. Confirm it with a copper test ring in your own stock and note the result.
Related guides
- How to Measure Your Ring Size at Home (UK A–Z & mm)
- How to size and stretch a ring without distorting the metal
- How Much Metal Do I Need? Calculating Weight & Cost
The formula gets you close; a test ring and a notebook get you exact. Free UK delivery from £30, and unused tools can be returned within 30 days.