How to Use This Calculator
Measure your barrel with a digital micrometer or caliper and enter the values above. All inputs use the same unit system — toggle between mm and inches at the top. The results update instantly.
- Barrel Internal Diameter: The inside diameter of the barrel drum, measured with the lid off.
- Arbor Hub Diameter: The diameter of the arbor collet — the wider section the spring hooks onto, not the pivot diameter.
- Barrel Internal Depth: Measure from the barrel floor to the top rim, inside.
- Barrel Lid Thickness: Measure the lid. Typically 0.2–0.5 mm for wristwatch barrels.
- Known Spring Thickness: If you have a spring in hand, enter its measured thickness to check whether it matches the barrel. Leave at 0 to use the ÷87 estimated value.
What the Outputs Mean
Recommended Thickness (÷87)
Dividing the barrel internal diameter by 87 is a widely used bench rule that gives the correct spring strength for most going-barrel watch movements. Théorie de l'Horlogerie approximates this as 1% of barrel ID (÷100). The ÷87 rule gives a marginally stronger spring and is favoured by practising watchmakers.
Spring Width (Height)
The spring must fit inside the barrel depth without binding on the lid or floor. The calculator subtracts the lid thickness plus your chosen clearance from the barrel internal depth. A typical clearance is 0.1 mm; some watchmakers use 0.05–0.15 mm depending on the movement.
Theoretical Length — Area Method (+20%)
The Theory of Horology (§4.2.5) defines correct length as the quantity of spring that fills exactly half the cross-sectional area between barrel wall and arbor. A footnote in that text observes that practical experience shows the spring should be 20% longer than the theoretical figure — this calculator applies that adjustment automatically. You can compare it against the quick ×30/×35 multiplier rules shown below the main figure.
Turns
Estimated number of turns the barrel makes from fully wound to run-out. Traditionally wristwatch barrels deliver around 6–8 turns. Fewer turns produce a steeper torque drop; more turns yield a flatter, more consistent power delivery.
Fill %
The ratio of the spring's cross-sectional area (length × thickness) to the available area in the barrel. The Theory of Horology target is 50%. The calculator flags values outside 40–60% — too little fill and the spring won't store enough energy; too much and the spring binds as it winds.
Arbor Ratio k
k = arbor hub radius ÷ spring thickness. A value of 10–14 means the spring is not bent too sharply at the inner coil, which would cause stress fracture. Below 10 risks breaking the spring; above 14 the arbor is wastefully large and reduces turns count.
When to Use This Tool
- You're servicing a vintage watch with a missing, broken, or unknown mainspring and need a starting specification for ordering a replacement.
- You suspect the previous watchmaker installed the wrong spring (low amplitude, short power reserve, or very fast rate when freshly wound).
- You want to verify that a spring pulled from the parts bin is within acceptable parameters before installing it.
- You're building or designing a movement and need to work out the barrel/spring geometry from scratch.
Common Mistakes
- Measuring the wrong arbor diameter: Use the hub/collet OD (the shoulder the spring hooks onto), not the pivot stub diameter.
- Ignoring lid thickness: A spring that fits the barrel depth without lid correction will bind on the lid when assembled.
- Assuming the existing spring is correct: Post-Covid, many movements have been opened and incorrectly reassembled by hobbyists. Always verify against calculated values, not just what's in the barrel.
- Mixing metric and imperial: Toggle the unit system at the top; all fields update together. Never use a value in one unit when the tool is set to another.
Frequently Asked Questions
How do I calculate watch mainspring thickness from barrel diameter?
The practical rule is barrel internal diameter ÷ 87 = recommended thickness (same units, usually mm). The Theory of Horology uses ÷100. The ÷87 rule gives a slightly stronger spring and is standard at the watchmaker's bench. For a 10 mm barrel this gives ~0.115 mm; for a 14 mm barrel ~0.161 mm.
What is the 50% area rule for mainspring length?
Theory of Horology §4.2.5 states the spring should occupy 50% of the cross-sectional area between barrel wall and arbor. Formula: L = (π/2 × (R_barrel² − R_arbor²)) ÷ thickness. A footnote adds that springs need about 20% more length in practice, which this calculator applies automatically.
What should the arbor ratio k be for a watch mainspring?
k = arbor hub radius ÷ spring thickness, and should fall between 10 and 14. Below 10 risks fatigue fracture at the inner coil; above 14 wastes barrel space and reduces power reserve. Most vintage Swiss wristwatch movements fall in the 10–14 range.
How many turns should a watch mainspring have?
Traditionally 6–8 turns for a wristwatch going barrel. Fewer turns means a steeper torque drop-off across the power reserve, which degrades rate consistency. More turns gives flatter, more isochronal output. Some modern long-power-reserve movements exceed 10 turns by using narrow barrels with unusually thin springs.
What is the difference between American and Swiss mainspring length multipliers?
The quick-reference bench rule multiplies barrel ID by 30 for American pocket and wrist movements, or by 35 for Swiss. Swiss movements traditionally used longer, thinner springs for a flatter torque curve. Both are rough guides; the area-method formula gives a more precise figure.
What mainspring width should I use?
Spring width (height) = barrel internal depth − lid thickness − clearance (≈ 0.1 mm). Use the calculator's "Barrel Internal Depth" and "Barrel Lid Thickness" fields; it computes width automatically.