O-Ring Cord Cut Length Calculator
Free O-Ring cord cut length calculator. Enter the inside diameter and cross-section, or an outside diameter, and get the exact length of cord to cut and splice.
Making a ring from cord stock? Give it the inside diameter and cross-section, the way an O-Ring is specified, and it gives you the length to cut. Working from an outside diameter off a print instead? Switch it over. It works backwards too: enter a piece of cord you already have and it tells you what size ring it will make.
What do you know?
The thickness of the cord itself. Measure it if you are not sure, a caliper across the cord is all it takes.
Leave the adjustment at zero to cut dead on the calculated length, which is right for most jobs. Some shops take a half percent or so off on big diameters so the ring sits under a little tension and stays put in the groove.
Cut length
Cut length is π × (ID + cross-section). Inside diameter and cross-section are how an O-Ring is specified and the dimensions we certify, so they are what this works from. Cut the ends square and bond them. O-Ring cord stock ships by the foot, and we also make custom vulcanized rings if you would rather have the joint done properly.
Why the inside diameter is what we work from
An O-Ring is specified on inside diameter and cross-section. Those are the numbers on the print, the numbers we inspect, and the numbers that go on the paperwork. Every batch we ship is measured on the Olypsys® O-Ring Measurement System from The O-Ring Store LLC, an ISO 9001:2015 certified distributor, and what it certifies is ID and cross-section. So that is what this calculator asks for, and what you should be quoting when you order.
The cut length comes out longer than the inside diameter suggests, and that is the part that trips people up. Bend a cord into a circle and the outside has to travel farther than the inside, so the length you cut is set by the circle running through the middle of the cord, one cross-section out from the ID:
Cut to the ID alone and the ring comes out short by π × CS, because you left out the cord’s own thickness. On a 6 inch ring in 1/8 inch cord that is about 0.44 inches, plenty to leave you holding two ends that will not meet. Cut to the OD and it comes out long by the same amount, and a loose ring bunches up in the groove.
ID and OD are the same ring described two ways:
| Outside diameter | ID + 2 × cross-section |
| Inside diameter | OD − 2 × cross-section |
Cutting and joining
Cut the ends square. A clean 90 degree cut is most of a good joint. A razor blade or a proper cord cutter beats scissors, which pinch the cord oval and leave you a face that will not sit flat against its mate.
Two ways to close the loop, covered in more detail on Special Uses:
- Field splice. Bond the ends with a cyanoacrylate adhesive. Fast, cheap, and fine for low-demand static jobs at large diameters where a standard ring is not available or would be a nuisance to install.
- Vulcanized joint. For anything that matters, order a custom vulcanized O-Ring. The ends are hot-bonded with uncured rubber so the joint cures into the ring itself. It is stronger and more elastic than any glue line, and it is how oversized rings and odd drive belts get made.
Should you cut it exactly? For most work, yes. Cut dead on the calculated length. Some shops take a half percent or so off on very large diameters so the finished ring sits under slight tension and stays seated while the cover goes on. The adjustment box is there for that. What you do not want to do is cut long, because the extra has nowhere to go but into a bunch at the joint.
A spliced ring is not a molded ring. The joint is the weak spot, in tension and often in chemical resistance too. Keep spliced rings to static duty, and if the ring is small enough that a standard size exists, buy the standard size. The calculator warns you when the diameter gets tight relative to the cord, because a small ring in fat cord fights the bend and puts all that strain straight into the joint.
What you need to order
Cord stock ships by the foot, so the calculator gives you feet and inches alongside the decimal, and totals the whole job if you are making more than one ring. Buy a little extra: you will want a practice cut or two to get the ends square, and a trimmed retry costs less than a second order.
We stock O-Ring cord stock in Buna-N, Viton®, EPDM, and Silicone across the common cross-sections. Not sure which compound the job wants? Start with the materials guide, or give us a call and we will talk it through.
Need a standard size instead? The Master O-Ring Size Chart covers AS568, metric, JIS, BS, and more. Cutting the groove as well? Use the O-Ring Groove Calculator.
