Ring Blank Length Calculator
The strip length to cut for a ring, measured at the neutral axis — and why cutting to the inside circumference finishes a whole thickness small.
Enter the diameter directly — measured off a mandrel or a ring that fits. Ring size charts vary between countries and between makers, and the conversion is not what this calculation depends on. Add a little for the saw kerf and for cleaning up the joint. Nothing is uploaded.
Measure the strip at mid-thickness
Metal bent into a ring stretches on the outside and compresses on the inside. Halfway between, the neutral axis does neither — and that circle is the one whose circumference equals the flat strip you started with. So the blank is the inside diameter plus one whole thickness: not two, and certainly not zero.
| Stock, for a 17 mm ring | Blank to cut | Inside circumference | Short by | Would finish at |
|---|---|---|---|---|
| 0.5 mm | 54.98 mm | 53.41 mm | 1.57 mm | 16.50 mm |
| 1 mm | 56.55 mm | 53.41 mm | 3.14 mm | 16.00 mm |
| 1.5 mm | 58.12 mm | 53.41 mm | 4.71 mm | 15.50 mm |
| 2 mm | 59.69 mm | 53.41 mm | 6.28 mm | 15.00 mm |
| 3 mm | 62.83 mm | 53.41 mm | 9.42 mm | 14.00 mm |
Cutting to the inside circumference is the natural mistake, and it comes out short by exactly π times the thickness. On 3 mm stock that is 9.42 mm of strip, and the ring closes at 14 mm instead of 17 — a loss of exactly one thickness of diameter, which is several sizes.
The error does not depend on the ring size
π times the thickness, whatever the diameter — so it is proportionally worst on small rings
in heavy stock. A 0.5 mm band comes out only
0.5 mm under and you might not notice; a 3 mm one
is out by 3 mm and unwearable. The same cut list cannot be right
for both, and the blanks differ by
7.9 mm for the identical finished
size.
Each extra millimetre of thickness adds exactly π millimetres to the blank, which is a
pleasant thing to be able to do in your head: about 3.14 mm of strip per millimetre of stock,
regardless of what size ring you are making.
How to use
- Enter the finished inside diameter in millimetres.
- Enter the thickness of your stock.
- Cut the blank to the length given.
- Add a little for the kerf and cleaning up the joint.
Frequently asked questions
How long should a ring blank be?
Pi times the inside diameter PLUS one thickness. Metal bent into a ring stretches on the outside and compresses on the inside; halfway between, the neutral axis does neither, and that circle is the one whose circumference equals the flat strip you started with.
What if I cut to the inside circumference?
The ring comes out short by exactly pi times the thickness, and closes one whole thickness under the diameter you wanted. On 3 mm stock a 17 mm ring finishes at 14 mm — several sizes, and unwearable.
Or the outside circumference?
Too long, by exactly as much as the inside is too short. The blank sits precisely halfway between the two, which is a useful check on any figure you are given.
Does the error depend on the ring size?
No — it is pi times the thickness whatever the diameter. That makes it proportionally worst on small rings in heavy stock: a 0.5 mm band is half a millimetre under and you might not notice, while a 3 mm one is out by three.
How much extra per millimetre of thickness?
Exactly pi, about 3.14 mm of strip per millimetre of stock, regardless of ring size. That is a pleasant thing to be able to do in your head when adjusting a cut list.
Why does the tool not do ring size conversion?
Because size charts vary between countries and between makers, and the conversion is not what the calculation depends on. Measure a mandrel or a ring that fits, and enter the diameter — that removes a source of error rather than adding one.
Should I add anything for the saw?
Yes — the kerf, plus whatever you take off truing the ends for the joint. A blank a whisker long can be filed down; one a whisker short is scrap.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.