Clock Mainspring Size Calculator
Calculate mainspring dimensions — width, thickness, length, and strength — for mechanical clock repair and restoration.
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
Mainspring Thickness Formula (Dennison Gauge)
The optimal thickness depends on the barrel and arbor sizes:
Thickness = (Barrel ID - Arbor OD) / (2 × Number of Coils × π) × Correction Factor
A simpler empirical rule used by clockmakers:
Thickness (mm) ≈ (Barrel ID - Arbor OD) / 16
This assumes the spring fills approximately 50% of the available barrel space when fully wound (the “rule of thirds” — one-third arbor, one-third spring, one-third empty space).
Rule of Thirds (Barrel Space Allocation)
| Zone | Percentage | Purpose |
|---|---|---|
| Inner third | ~33% | Arbor hub and inner coils |
| Middle third | ~33% | Active spring coils (working range) |
| Outer third | ~33% | Clearance for full unwinding |
If the spring fills more than 50% of the barrel, it will bind when wound and potentially crack. If it fills less than 30%, it lacks the power to drive the clock reliably.
Mainspring Length Formula
Length (mm) = π × N × (Barrel ID + Arbor OD) / 2
Where N = number of effective coils (typically 8–14 for clocks, depending on run duration).
Alternatively, using the empirical formula:
Length (inches) ≈ Barrel ID (inches) × 12 to 16
The multiplier depends on the clock type:
| Clock Type | Length Multiplier | Run Duration |
|---|---|---|
| 30-hour movement | 12–13× barrel diameter | 30–36 hours |
| 8-day movement | 14–16× barrel diameter | 8–9 days |
| 400-day (anniversary) | 10–12× barrel diameter | ~400 days (very weak spring) |
| Cuckoo clock | 12–14× barrel diameter | 1–8 days |
Mainspring Width
Width = Barrel Drum Height - 0.3 mm clearance (each side)
Measure the barrel drum interior height with calipers. Subtract 0.3–0.5 mm for clearance so the spring can expand and contract freely without binding against the barrel caps.
Worked Example — 8-Day Mantel Clock
Barrel ID: 38 mm. Arbor OD: 8 mm. Barrel height: 18 mm.
Thickness = (38 - 8) / 16 = 1.875 mm → use 1.9 mm (nearest standard size). Width = 18 - 0.6 = 17.4 mm → use 17.0 mm (round down to standard). Length = 38 × 15 = 570 mm (approximately 22.5 inches).
Standard Mainspring Thickness Sizes (metric)
| Gauge | Thickness (mm) | Common Use |
|---|---|---|
| 16 | 0.95 | Small mantel clocks |
| 18 | 1.10 | Mantel, wall clocks |
| 20 | 1.30 | Wall clocks, 8-day |
| 22 | 1.50 | Large 8-day movements |
| 24 | 1.75 | Grandfather clocks |
| 26 | 2.00 | Large grandfather, tower clocks |
| 28 | 2.25 | Heavy-duty movements |
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.