Sprocket Size Calculator
Enter the teeth on the front and rear sprockets, the chain size, the wheel diameter with its tyre and the rpm of the front sprocket. The calculator gives the ratio, the sprocket diameters, the road speed and what one tooth of change does.
- Final drive ratio
- Pitch diameter
- Road speed
For a motorbike, the front sprocket rpm is engine rpm divided by the primary and gearbox ratios. For a bicycle, it is your cadence.
Sprocket teeth set the trade between pull and top speed. The final drive ratio is the rear teeth divided by the front. A 15 tooth front and 45 tooth rear give 3 : 1. Dropping one tooth at the front gives about 7% more pull, the same as adding three teeth at the rear.
Gearing chart
15 tooth front, 630 mm wheel. Search for rear teeth.
| Rear teeth | Ratio | Speed (km/h) | Rear pitch diameter, 520 chain (mm) |
|---|---|---|---|
| 36 | 2.4 | 148 | 182.1 |
| 40 | 2.67 | 134 | 202.3 |
| 42 | 2.8 | 127 | 212.4 |
| 44 | 2.93 | 121 | 222.5 |
| 45 | 3 | 119 | 227.6 |
| 46 | 3.07 | 116 | 232.6 |
| 48 | 3.2 | 111 | 242.7 |
| 50 | 3.33 | 107 | 252.8 |
| 52 | 3.47 | 103 | 262.9 |
Speed at 3,000 front sprocket rpm.
Rear teeth and gearing
15 tooth front sprocket. Slide the rear teeth.
The drawing shows both sprockets to scale with the chain between them.
How to change gearing
Sprocket swap. Step through it.
Count teeth
Front and rear.
Changing sprocket sizes may need a longer or shorter chain. Check the adjuster still has room.
Smaller front, bigger rear or both
Gearing changes. Compare them.
Big step.
Same step.
Fine tuning.
A smaller front sprocket wears the chain faster because the chain bends more sharply.
Before you swap
Sprocket checks. Tick each item.
Tick each item as you go.
Replace the chain and both sprockets together. A worn chain wears new sprockets quickly.
What you get
Sprocket gearing. Each tile explains one result.
Rear ÷ front.
Pitch circle.
Road speed.
Metres a second.
Of one tooth.
Lower gearing means more teeth at the rear or fewer at the front, for quicker acceleration.
Frequently Asked Questions
No question matches that search.
How do I calculate a sprocket ratio?
Divide the rear sprocket teeth by the front sprocket teeth.
What does a smaller front sprocket do?
It gives quicker acceleration and a lower top speed at the same rpm.
How many rear teeth equal one front tooth?
About three, depending on the sizes.
How do I find the sprocket pitch diameter?
Divide the chain pitch by the sine of one hundred and eighty degrees divided by the number of teeth.
What chain pitch is a five twenty chain?
Five eighths of an inch, or fifteen point eight seven five millimetres.
Do I need a new chain when changing sprockets?
Often, if the change is large or the chain is worn. Replace chain and sprockets as a set.
Will gearing change my speedometer?
Yes, if it reads from the gearbox output. Wheel based sensors are not affected.
Gear it your way
See exactly what each tooth changes before you buy sprockets, and set your bike up for pull or for top speed.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.