Mean Coil Diameter Calculator
Mean coil diameter is the wire-center diameter of the coil: D = OD - d = ID + d. Use the working spring index calculator to convert OD, ID and mean diameter and to check the spring index and Wahl factor they imply.
The conversions
All three coil diameters differ by one wire diameter:
D = OD - d = ID + d
Why mean diameter matters
Spring formulas use D, not OD or ID: rate goes with 1/D^3 and stress with D, so using the outside diameter by mistake understates stress and overstates rate. The spring index C = D/d is the manufacturability and stress-concentration screen.
C = D/d
Fit checks
Work in mean diameter, then convert back to OD for a bore fit or ID for a rod fit, allowing for coil growth under compression and tolerance.
Frequently asked questions
Is mean coil diameter measured or calculated?
Usually calculated. Measure OD with calipers and wire with a micrometer, then D = OD - d. Measuring ID directly is less reliable on small springs.
What spring index should I aim for?
Most production compression springs land between C = 4 and C = 12. Lower is hard to wind and raises the Wahl factor; higher gets flimsy and tangles.
Related pages
- Spring index calculator - working OD/ID/mean conversion with index and Wahl factor.
- Spring rate calculator - rate from wire size, mean diameter and material.
- Compression spring calculator - full rate, stress and travel check.
- Spring wire size calculator - back-solve wire diameter from a target rate.