RC Spur Gear Center Distance Calculator
Enter a matching pinion and spur pair to check its standard center distance and diameters. Or enter a fixed motor-to-spur shaft distance to find the nearest usable whole-tooth pinion.
Standard center distance24.342 mm (0.958 in)
- Gear ratio
- 3.6
- Working module
- 0.529 mm working module (48 DP)
- Pinion reference diameter
- 10.583 mm (0.417 in)
- Spur reference diameter
- 38.1 mm (1.5 in)
- Outside diameters
- Pinion: 11.642 mm (0.458 in); spur: 39.158 mm (1.542 in)
- Root diameters
- Pinion: 9.26 mm (0.365 in); spur: 36.777 mm (1.448 in)
- Base diameters
- Pinion: 9.945 mm (0.392 in); spur: 35.802 mm (1.41 in)
Assumes standard, unshifted external spur gears with a matching pitch and pressure angle.
How to use this calculator
- Choose whether you know both gears or need to select a pinion for a measured shaft distance.
- Choose diametral pitch or module, then enter the shared pitch value marked on both gears.
- Enter the spur tooth count and pressure angle; enter the pinion tooth count for a known pair.
- For reverse mode, measure center to center between the motor shaft and spur-gear shaft in millimeters.
- Check the mismatch and adjacent choices before setting final mesh and backlash.
How RC spur-gear center distance is calculated
Two standard external spur gears run at a center distance equal to half the sum of their reference diameters. The pinion tooth count is z1, the spur tooth count is z2, and the working module is m in millimeters. For a metric gear, m is the module marked on the gear. For a diametral-pitch gear, the calculator converts it with m = 25.4 ÷ P, where P is teeth per inch.
The reference diameters are d1 = m × z1 and d2 = m × z2. The standard shaft distance is therefore a = m × (z1 + z2) ÷ 2. Gear ratio is z2 ÷ z1. A larger pinion raises the center distance and lowers the numerical reduction ratio; a larger spur raises both the center distance and the ratio.
Finding a pinion for a fixed mount
Many RC motor mounts give you a shaft distance first, not a pinion size. For a target distance at, the ideal pinion is z1* = 2 × at ÷ m − z2. That answer is usually fractional because gears have whole teeth. The calculator evaluates the integer below and above it, reports each actual center distance and error, then recommends the closer one. If they are equally close, it uses the lower-tooth choice.
The recommended pinion is a geometric starting point, not a claim that every available gear will fit. The calculated mismatch tells you how far the nominal centers differ before motor-slot adjustment. Do not force a pair together simply because the mismatch looks small.
Diameters and pressure angle
For standard full-depth spur gears, outside diameter is da = m × (z + 2) and root diameter is df = m × (z − 2.5). Base diameter is db = m × z × cos(α), where α is pressure angle. Pressure angle changes the base diameter but not the standard center distance, reference diameter, or ratio.
32P, 48P, 64P, and metric module are pitch systems, not interchangeable size labels. Both gears must match in pitch and pressure angle. A 48P gear has a working module of about 0.529 mm, while a 0.8-module gear is substantially coarser.
What the result leaves out
This calculation describes unshifted external spur gears. It does not model backlash, motor-mount slot position, tooth-profile correction, manufacturing tolerance, shaft play, eccentricity, helical or bevel gears, or proprietary tooth forms. Very small pinions can be undercut or require profile correction, so check the gear maker's recommendations before buying parts or machining a mount.
Worked example
A 48P, 20-tooth pinion and 72-tooth spur have a working module of 25.4 ÷ 48 = 0.5292 mm. Their standard center distance is 0.5292 × (20 + 72) ÷ 2 = 24.342 mm, or about 0.958 in. The ratio is 72 ÷ 20 = 3.60:1.
If the measured shaft distance is 24.50 mm instead, the ideal pinion is about 20.60 teeth. A 20-tooth pinion gives 24.342 mm, 0.158 mm short; a 21-tooth pinion gives 24.606 mm, 0.106 mm long. The calculator recommends 21 teeth and shows both choices for the available motor-slot adjustment.
Common questions
How do I calculate center distance for an RC pinion and spur gear?
Multiply the working module by the total number of teeth, then divide by two. For diametral pitch, first calculate working module as 25.4 divided by the pitch number. The result is the nominal shaft-center distance for standard external spur gears.
What is the difference between 32P, 48P, 64P, and metric module?
Diametral pitch counts teeth per inch of pitch diameter, so a higher number is a finer tooth pitch. Module is the pitch diameter in millimeters per tooth, so a lower module is finer. They are related by module = 25.4 divided by diametral pitch, but the markings are not interchangeable without conversion.
Can module and diametral-pitch gears mesh together?
Only when their actual pitch is the same after conversion and their tooth form and pressure angle also match. In ordinary RC use, treat differently marked gears as incompatible unless the manufacturer explicitly states they mesh. Matching a calculated center distance alone is not enough.
How do I choose a pinion for a fixed motor-mount distance?
Enter the measured shaft distance, spur tooth count, and shared pitch in reverse mode. Compare the two whole-tooth candidates around the ideal result, then use the one with the smaller distance error if the motor mount has enough adjustment. Confirm the resulting ratio suits the motor and track or terrain.
Does changing pressure angle change standard center distance?
No. Standard center distance depends on module and tooth count. Pressure angle changes the base circle and tooth-contact geometry, which is why the two gears must still share it.
How much backlash should an RC gear mesh have?
There is no single clearance that fits every RC drivetrain. Set a small, perceptible amount of free play using the gear manufacturer and vehicle maker guidance, then check for tight spots through a full rotation. A theoretical center distance does not set backlash by itself.