Cut lengths, build marks & predicted strength for 12-strand hollow HMPE (Amsteel Blue). One engine — fid 21·d, full bury 72·d.
Standing length is the finished line below the eye (set to 0 for just the spliced end).
Assumes two equal pieces joined in the middle. You lose one bury length (72·d) of overlap.
The finished size depends on how hard you dress the knot — the model's baseline assumes a firmly-set knot. Tell it your style, then dial it in exactly from one test build.
Rough starting points. Tightening harder pulls more rope into the knot, so the finished shackle comes out shorter unless you cut longer.
Measured at the diameter currently selected above. Calibration is stored on this device and applies to every size.
Numbers above are your measurements from the calculator — they update with diameter, size and calibration. Tap a tool chip to jump to the toolbox.
Video break-tests & technique comparisons: HowNOT2 · L-36 testing
Length: one geometric engine. Every 12-strand splice scales off the fid length = 21·d (Samson standard) and a full bury = 72·d (≈ 3.5 fid). Brion Toss, Evans Starzinger and Samson independently converge on 72·d for HMPE.
Strength: capstan / constriction mechanics. A buried tail is a Chinese-finger-trap — under load the hollow braid constricts on the tail, so grip is self-energizing and a 72·d bury hits 90–95% efficiency with no knot. A taper adds ~20% by removing the abrupt stiffness step.
Strength is a range, not a number. Published break tests scatter a lot (knot efficiency is genuinely a random variable), so this tool shows a band and sizes the working load off the conservative end:
Bury length — two valid schools. The long-bury method (72·d, above) relies purely on constriction grip. Gleistein's Brummell lock instead traverses the rope through itself — it "is its own whipping" — so it holds full strength on a shorter bury. Either way, a splice keeps nearly all the line's strength while a knot loses "up to fifty percent" (Gleistein). For a Brummel eye, add 2·d to your marked eye size — the lock pulls the crossovers together and shrinks it (this tool does it for you).
Why there's no tucked eye splice here. Gleistein documents a fast, no-tools eye made by tucking the tail back through the braid four times in alternating directions — but explicitly not for high-modulus rope. On slippery HMPE those tucks never develop the constriction grip a long bury does, so for Dyneema the Brummel lock or a whipped bury is the right call.
Lengths round to 1/8″ (or mm). Final dimensions depend on how tightly you dress the knot — use Calibrate to your technique to lock it in from one measured build.
Sources: Gleistein Splicebook · Splicing Dyneema (Sailing Mag) · Samson 12-Strand Class II Eye Splice · L-36 break-test data · HowNOT2 break tests · Cordage Institute CI-1500