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Use this watch mainspring calculator to estimate the correct mainspring size from your barrel internal diameter, barrel depth and arbor diameter. The tool gives an estimated spring height, thickness and length to help with watch repair and replacement mainspring selection.
These measurements are estimates and should always be checked against the original spring and supplier specifications before fitting.
Horological tool
Mainspring calculator
Enter the barrel dimensions to estimate a suitable mainspring, or verify whether a known spring is likely to fit.
RedRoosterUK
1 Barrel measurements
Quick fill
Measure the inside diameter of the barrel drum.
mm
Internal depth of the drum, used to recommend spring height.
mm
Outer diameter of the barrel arbor post.
mm
Optional, but useful when copying or printing the result.
2 Calculator mode
Used to scale the estimated spring length.
hours
mm
mm
mm
Recommended mainspring
Estimates only. Always confirm with supplier catalogues and the original spring where possible.
Fitting notes
- Spring height should usually leave about 0.10 to 0.20 mm of barrel clearance.
- A practical barrel fill target is roughly 45 to 55 percent.
- Automatic movements normally use slipping bridle mainsprings.
- Do not force an overfilled barrel; it can bind or damage the movement.
i Typical thickness reference
Ladies watch0.05 - 0.08 mm
Gents manual0.09 - 0.12 mm
Gents automatic0.08 - 0.11 mm
Pocket watch0.12 - 0.18 mm
Small clock0.20 - 0.35 mm
Mantle / wall clock0.35 - 0.55 mm