Mainspring Calculator — Barrel & Arbor

Measurements

Barrel Internal Diameter (mm) Measure inside the barrel drum

Arbor External Diameter (mm) Measure the barrel arbor hub

Barrel Height / Depth (mm) Internal depth of barrel drum

Mainspring Alloy

Thickness

Length

Height (width)

Approx. Coils

Theoretical Turns

Practical Length (−12%)

This calculator uses the Dennison/Grossmann formula combined with the rule of thirds:

Rule of thirds: The barrel cross-section should divide equally into spring, arbor, and free space.
Thickness: t = (D - d) / (2 × N × k), where k is a packing factor for the alloy.
Length: L = π(D² - d²) / (4t) — the Grossmann formula.
Height: Barrel internal depth minus ~0.15mm clearance.
Practical length: 10-15% shorter than theoretical (bridle, inner coil attachment).

These are starting-point calculations. Always verify against manufacturer specs or a known-good mainspring when possible.

Free tool from Barrel & Arbor. Calculations are theoretical starting points — always verify against manufacturer specifications. Terms of use