Spur gear calculator
Calculate pitch diameters, center distance, ratio and outside diameter for a pair of spur gears.
How it is calculated
Pitch diameter d = m × z. Center distance a = m × (z1 + z2) ÷ 2. Ratio = z2 ÷ z1. Outside diameter = m × (z + 2).
Worked example
m 2, 20 and 40 teeth: d1 = 40 mm, d2 = 80 mm, center distance = 60 mm, ratio 2:1.
Steps
- Enter the module of the gear pair.
- Enter the tooth count of each gear.
- Use the center distance to set the housing.
Frequently asked questions
What is the module?
Pitch diameter divided by teeth number, in mm. Both gears of a pair must share the same module.
Does this include backlash or profile shift?
No. It gives standard gears with no profile shift.
Is this for helical gears?
No, helical gears need the normal module and helix angle.
References
ISO 21771 (involute gears, module definitions) and standard spur gear geometry.
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.