Distance per count is the wheel circumference divided by the counts per wheel revolution. Enter the encoder, gearbox and wheel to get the numbers your odometry code needs.
Measure the real counts by turning the wheel by hand ten times. Gear backlash and tyre squash change the result.
How it is calculated
Counts per wheel revolution = encoder pulses per motor revolution × decoding factor × gear ratio. Distance per count = π × wheel diameter ÷ counts per wheel revolution. Decoding is 4 when your code counts both edges on both channels of a quadrature encoder.
A worked example
A motor encoder with 11 pulses per revolution, read at 4× decoding through a 30:1 gearbox gives 11 × 4 × 30 = 1,320 counts per wheel revolution. A 65 mm wheel travels π × 65 = 204.2 mm per turn, so each count is 204.2 ÷ 1,320 = 0.1547 mm, or about 6,464 counts per metre.
Common mistakes
- Counting only one channel and forgetting the decoding factor.
- Putting the encoder after the gearbox and still multiplying by the gear ratio.
- Using the wheel diameter without allowing for the tyre squashing under load.
Related
wheel speed calculator, gear train calculator, ROS 2 for beginners.
Last reviewed: November 2026. The calculator runs in your browser and sends nothing anywhere. Results are ideal-case figures; add margin before you order parts.