Fiber minimum bend radius calculator

It is the tightest curve a cable can take before the glass starts to leak light or the cable is damaged. Bend a fiber past its rated radius and you get microbending loss, where light escapes at the bend and the link loses power. Push it further and you get a kink, which is permanent: the loss stays even after you straighten the cable, so the only fix is to cut it out and re-splice.

The common industry rule (GR-20 and most cable spec sheets) is that a cable being pulled, under load, should never bend tighter than 20 times its outside diameter, and once it is installed and at rest it should not bend tighter than 10 times its diameter. So a 0.5 inch cable wants a 10 inch minimum radius while you pull it and a 5 inch minimum radius after. The multipliers can differ by cable, so confirm them against the spec sheet.

Size a stored slack or service loop so the coil never bends tighter than the loaded minimum radius. That means the loop diameter should be at least twice the loaded minimum radius: for a 0.5 inch cable at the 20x rule, the loaded minimum radius is 10 inches, so the loop diameter should be at least 20 inches. Bigger is fine, tighter is not.

First you lose light. A bend tighter than the rating bleeds optical power at the curve, which shows up as extra loss on an OTDR or power meter and can drop a link out of budget. Bend it harder and the fiber kinks or the buffer tubes crush, which is mechanical damage that does not recover. That is why crews watch bend radius at pull boxes, sheave wheels, and inside cabinets, not just on the long run.

Yes. While a cable is under tension during the pull it is more fragile, so the loaded limit is larger, typically 20 times the diameter. Once it is placed and the tension is off, it can sit at a tighter radius, typically 10 times the diameter. Design your pulls and sheaves to the loaded number, then dress the slack and racking to the unloaded number.