# Mainspring Finder - Calculate Spring Dimensions for Watch Movements
0.08-0.18 mm Typical spring thickness
5-10 turns Standard winding range
150-450 mm Common spring length
# How mainspring dimensions are calculated
The mainspring occupies the annular space between the barrel wall and the arbor. The spring thickness is estimated as (barrel ID - arbor OD) / (2x turns + 1.5), where the extra 1.5 accounts for the spring-end attachment and the space the spring occupies against itself. The spring length follows L = pi x turns x (barrel ID + arbor OD) / 2, which gives the total length of the ribbon if laid flat. The spring height equals the internal barrel height minus a small clearance (typically 0.1 mm) to prevent rubbing against the barrel lid.# How to measure a barrel for mainspring selection
- Barrel Inner Diameter
- The inside diameter of the barrel drum measured with a caliper. This is the largest circle the mainspring can occupy. Typical range: 8-30 mm.
- Arbor Diameter
- The diameter of the barrel arbor at its widest point where the inner coil of the mainspring hooks. Smaller arbors allow longer springs for the same barrel.
- Barrel Height
- The internal height of the barrel drum. The mainspring height must be slightly less (0.05-0.15 mm) to allow free rotation without friction on the lid.
- Number of Turns
- The number of complete winding turns the movement provides. Manual wind calibers typically offer 5-8 turns, automatics 6-10 turns.
# Spring strength and what it means for your movement
The spring thickness is the primary factor determining the torque delivered to the gear train. A thicker spring (greater than 0.14 mm) provides high torque suitable for movements with complications like chronographs or striking mechanisms. Medium springs (0.10-0.14 mm) are standard for most time-only and date calibers. Thin springs (below 0.10 mm) are found in small ladies movements or ultra-thin calibers. Always match the original manufacturer specification when available; an overly strong spring can damage the barrel arbor or gear train pivots.Using the wrong mainspring can damage your movement
Installing a spring that is too thick increases friction and torque beyond design limits, potentially wearing the barrel arbor hole, bending the centre wheel pivot, or even cracking the barrel. A spring that is too thin will not provide enough amplitude, causing the watch to run slow or stop before reaching its full power reserve. Always cross-reference the calculated dimensions with known supplier catalogues before ordering.
# Commercial mainspring sizing systems
Mainspring suppliers catalogue springs by length x height x thickness in millimetres. Common metric sizes follow the General Resources or GR system. Inch-based systems are still used by some American and British suppliers. The tool displays both systems so you can search catalogues regardless of the supplier. When the exact calculated size is not available, choose the closest commercial size that matches the height exactly and the thickness within 0.005 mm, then adjust the length by selecting a spring from the same height/thickness family.# Vintage Caliber Considerations
- Swiss calibers (ETA, FHF, AS, Unitas) - well-documented in the GR system. Most have known mainspring references.
- French calibers (LIP, France Ebauches) - often use non-standard barrel proportions. Measure carefully.
- American calibers (Waltham, Elgin, Illinois) - inch-based systems. Use the imperial output to cross-reference.
- Japanese calibers (Seiko, Citizen, Miyota) - metric system with good catalogue coverage for post-1960 movements.
- Chinese calibers (Sea-Gull, DG, Tongji) - often clone Swiss designs. The Swiss GR reference usually applies.
Key takeaways for mainspring selection
Measure barrel ID, arbor OD, and barrel height accurately with a precision caliper
Use the calculated dimensions as a starting point, not as an absolute specification
Cross-reference with manufacturer technical sheets or known supplier catalogues
Match the spring height exactly to the barrel height minus 0.05-0.15 mm clearance
Match the thickness within 0.005 mm of the calculated value for optimal performance
Verify the spring length is adequate for the expected number of winding turns