Necklace Length Calculator
Two tape readings and the number on the tube. Nothing you type leaves the browser.
A chain is sold by one number and worn against two: the measure round your neck uses part of the loop before any of it is free to hang — half the neck goes round the back and the rest hangs in two strands — and where the lowest point lands is arithmetic, not opinion.
This tool runs that arithmetic: the depth of the front drop, the share of the loop the back takes, and the chain length a wanted drop requires. Every rule is the published one from this site's necklace guide, and every intermediate figure is printed, so the working can be checked with your own tape.
Your chain and your neck
The number on the tube — the whole loop, as sold.
Around the neck where the chain would sit, snug rather than tight.
Where the lowest point should sit, measured below the line where the chain leaves the sides of the neck.
Fill the chain length and the neck measure to get the drop.
Calculated in your browser — the numbers you type are never sent to a server.
What the tool runs, step by step
Free front = L − N ÷ 2. Half the neck's measure is spent going round the back, so a 24-inch chain on a 340 mm neck has 439.6 mm of free front, running as two equal strands of 219.8 mm.
Depth = √(f² − (N ÷ 2π)²), where f is (L − N ÷ 2) ÷ 2. The two runs leave the neck one diameter apart — N ÷ π across — giving 133.0 mm of drop for an 18-inch chain on a 340 mm neck, and 213.0 mm for 24 inches.
A chain begins to hang free at all only when L passes (1 ÷ 2 + 1 ÷ π) × N: a boundary of 245.5 mm at a 300 mm neck, up to 343.7 mm at 420. Below it, the front lies against the neck.
The identity read backwards gives the chain a depth needs: chain = N ÷ 2 + 2√(d² + (N ÷ 2π)²). A drop of one whole face length — 142.8 to 224.4 mm at the working width — needs 475.4 to 631.7 mm of chain on a 340 mm neck, 156.2 mm of that opened by the face alone.
Where these rules come from
Every rule above is published on this site, spelled out in the necklace guide — including the half-wrap assumption, which the guide declares as its own parameter rather than a measurement: