Clock Mainspring Size Calculator

The mainspring is the power source of a mechanical clock. It is a coiled strip of tempered steel (or in modern clocks, a cobalt alloy like Nivaflex) that stores energy when wound and releases it gradually to drive the gear train. Selecting the correct mainspring requires matching the barrel diameter, arbor diameter, and the power requirements of the movement.

Key Mainspring Dimensions

• Width (height): The vertical dimension of the spring strip, must match the barrel drum height minus a small clearance (0.2–0.5 mm)
• Thickness: The cross-section of the steel strip, determines the spring’s strength (torque)
• Length: The total uncoiled length, determines how long the clock runs between windings
• Barrel diameter: The inner diameter of the barrel drum that houses the spring

The half-full rule

Everything here follows from one fact: a mainspring is a flat spiral of steel occupying the ring of space between the arbor and the barrel wall, and it should fill about half of that ring.

Half, not more, because the spring has to have somewhere to go. Fully wound it packs tight against the arbor; fully run down it lies open against the barrel wall. A spring that fills much beyond 60% binds against itself at full wind, which is how springs crack. One filling much under 35% simply does not store enough turns to run the movement for its stated period.

The ring’s area is straightforward:

Annulus area = π/4 × (Barrel ID² − Arbor OD²)

and the steel in the spring is just its length times its thickness. Setting the second to about half the first is the whole calculation:

Length ≈ 0.48 × Annulus area ÷ Thickness

Thickness

Thickness is the one figure you cannot derive from the barrel alone, because it trades against length: a thinner spring of the same steel area gives more turns and less torque. As a starting point, clock mainsprings run near one-hundred-and-twentieth of the barrel diameter, so a 55 mm barrel takes roughly 0.46 mm steel. Movements that must run longer between windings go thinner and longer than that; a 30-hour movement can afford to be thicker and stronger.

Treat the thickness this calculator suggests as a starting point and check it against a supplier’s chart before ordering. Mainsprings are sold in fixed sizes, and the practical answer is usually the nearest stocked size rather than the computed one.

A sanity check on the numbers: a common 8-day American movement uses a spring about 0.46 mm thick and 2,440 mm long in a barrel near 55 mm. Put those into the annulus formula and you get 49% fill, which is exactly where the rule says it should land.

Mainspring Width

Width = Barrel drum height − 0.6 mm (about 0.3 mm clearance at each end)

Measure the drum interior height with calipers, subtract the clearance, then round down to a stocked size. The spring must not rub against the barrel caps.

Worked Example — 8-Day Mantel Clock

Barrel ID: 38 mm. Arbor OD: 8 mm. Barrel height: 18 mm.

Annulus = π/4 × (38² − 8²) = 1,084 mm² Thickness = 38 ÷ 120 = 0.32 → use 0.30 mm Width = 18 − 0.6 = 17.4 → use 17.0 mm Length = 0.48 × 1,084 ÷ 0.30 = 1,734 mm (about 68 inches)

That length surprises people the first time. Mainsprings are far longer than the barrel suggests, because the steel is thin and wraps many times.

Standard Mainspring Thickness Sizes (metric)

Springs are usually sold by imperial thickness, which is why the inch column matters: 0.018 in is the size you will see quoted most often.

Safety Warning

A fully wound mainspring stores significant energy. ALWAYS use a mainspring winder and let-down tool. Never remove a barrel cap without first letting down the spring. An uncontrolled release can cause serious hand injuries and damage the movement.