How do I convert encoder ticks to distance?

Distance per tick = (π × wheel diameter) ÷ (encoder CPR × decoding factor × gear ratio). A 65 mm wheel behind a 30:1 gearbox with a 12 CPR motor-shaft encoder at 4× quadrature decoding gives about 0.14 mm per tick.

What is the difference between PPR and CPR on an encoder?

PPR (pulses per revolution) counts pulses on one channel; CPR (counts per revolution) usually means PPR × 4 after quadrature decoding, because each pulse has two edges on each of two channels. Always check which one the datasheet quotes.

What encoder resolution does my robot need?

For basic dead reckoning, 0.5–1 mm per count is plenty. For smooth PID velocity control at low speed, aim for at least 500–1000 counts per wheel revolution so the loop sees fresh data every cycle.

Wheel Speed & Odometry Calculator — Encoder Ticks to Distance

How this calculator works

Counts per wheel rev = PPR × decoding × gear ratio
Distance per count = (π × wheel diameter) ÷ counts per wheel rev
Robot speed = π × wheel diameter × (motor RPM ÷ ratio) ÷ 60
Count rate = counts per wheel rev × wheel RPM ÷ 60

Everything downstream — dead reckoning, velocity PID, "drive exactly 500 mm" functions — depends on one honest number: how far the robot moves per encoder count. The multiplication chain above is short, but each factor is a classic source of the mysterious 2× or 4× error that makes a robot drive twice as far as commanded.

The classic bug: the datasheet says "11 PPR" but your library counts every edge of both channels, so the code actually sees 44 counts per motor revolution. If your measured distances are exactly 2× or 4× off, the decoding factor is the first suspect — not the wheel size.

The count-rate figure matters more than beginners expect: a fast motor behind a high ratio with 4× decoding can push tens of thousands of interrupts per second at a microcontroller. If that number lands above roughly 50k on an Arduino Uno–class board, use hardware counters, drop to 1× decoding, or read the encoder with a dedicated peripheral.

Once you trust the conversion, feed the measured wheel speed into a velocity loop — the PID Tuning Visualizer shows what happens next — and use the Gear Ratio Calculator if the RPM-to-speed relationship isn't giving you the top speed you wanted.

Common questions

Why is my odometry drifting even with correct math?

Wheel slip, tyre squish under load (effective diameter shrinks 1–3%), and unequal left/right diameters. Calibrate by driving a measured 2–3 m on the real floor and scaling your mm-per-count constant to match — every serious team does this.

Encoder on the motor shaft or the wheel shaft?

Motor-shaft encoders get free resolution (multiplied by the gear ratio) but can't see gearbox backlash or belt slip. Wheel-shaft encoders measure the truth with fewer counts. Motor-shaft is the right default for velocity control; wheel-shaft wins for precise positioning.

Parts this calculation leads to

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Encoder Gearmotors

Gearmotors with quadrature encoders already fitted — the easy path.

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Rotary Encoders

Add-on magnetic and optical encoders for existing shafts.

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Robot Wheels

Accurate, consistent wheels make odometry math come true.

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Microcontrollers

Boards with hardware quadrature counters for high tick rates.

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