Gear Size Calculator
Enter the teeth on the pinion and the gear and their module. The calculator gives the gear ratio, the pitch, outside and root diameters of both gears and the distance between their shafts.
- Pitch and outside dia
- Gear ratio
- Centre distance
Standard 20° full-depth spur gears. For diametral pitch (inch gears), module = 25.4 ÷ DP.
Module ties the size of a gear to its number of teeth. Module is the pitch diameter in millimetres divided by the number of teeth. Two gears mesh only if they share the same module. Once you know it, every other dimension follows from simple multiplication.
Gear dimension chart
Module 2. Search for a tooth count.
| Teeth | Pitch dia | Outside dia | Root dia |
|---|---|---|---|
| 12 | 24 | 28 | 19 |
| 15 | 30 | 34 | 25 |
| 20 | 40 | 44 | 35 |
| 25 | 50 | 54 | 45 |
| 30 | 60 | 64 | 55 |
| 40 | 80 | 84 | 75 |
| 50 | 100 | 104 | 95 |
| 60 | 120 | 124 | 115 |
| 80 | 160 | 164 | 155 |
| 100 | 200 | 204 | 195 |
Pitch dia = m × z. Outside dia = m × (z + 2). Root dia = m × (z − 2.5).
Teeth and gear size
20 tooth pinion, module 2. Slide the gear teeth.
The drawing shows real meshing teeth. Every tooth on the big gear is the same size as on the small one.
How to size spur gears
Module first. Step through it.
Pick a module
From load and size.
Bigger modules give stronger teeth. Choose module by the torque the gears carry, then set the teeth for the ratio.
Spur, helical or worm
Gear types. Compare them.
Straight teeth, parallel shafts. Simple and efficient.
Angled teeth, smooth and quiet.
Screw driving a wheel. Big ratios at right angles.
Helical gears are quieter but push sideways on their shafts, so they need thrust bearings.
Before making gears
Design checks. Tick each item.
Tick each item as you go.
Leave a little backlash, the play between teeth, so gears do not bind when they warm up.
What you get
Full gear geometry. Each tile explains one result.
Speed and torque.
Both gears.
Blank size.
Tooth base.
Shaft spacing.
Circular pitch is the distance from one tooth to the next along the pitch circle: π × module.
Frequently Asked Questions
No question matches that search.
How do I calculate gear pitch diameter?
Module × number of teeth.
What is gear module?
Pitch diameter in mm divided by teeth. It sets tooth size.
How do I find centre distance?
Module × (teeth 1 + teeth 2) ÷ 2.
What is the outside diameter of a gear?
Module × (teeth + 2).
How do I convert diametral pitch to module?
Module = 25.4 ÷ DP.
What is the minimum number of teeth?
17 for a standard 20° gear without undercut.
Can gears with different modules mesh?
No. They must share the same module and pressure angle.
Gears that mesh
Pick a module, set the teeth for your ratio and get every diameter and the centre distance. With matched gears and a little backlash, they will run smoothly.
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