Skip to main content
ChainSprocketCalc

Sprocket Size Calculator: Solve for the Missing Sprocket

Know one sprocket and your target speed or shaft rpm? Get the tooth count you need, with the nearby whole-tooth options, chain length and wear check.

Presets

Solve the rear / driven sprocket

Exact: 37.2T
T
Motor / engine rpm
Gearbox ratio
Target speed
mph
Tire diameter
in
in

Needed rear / driven sprocket

37T

Exact:37.2T

Resulting Speed

38.2mph

Ratio:3.083:1

Chain Needed

88L

Small sprocket wrap:165.1°

Nearby whole-tooth options

Whole-tooth sprocket options around the exact answer
ChooseRatioSpeedErrorOutside ØLinksWear (GCD)
2.917:140.4 mph+6.29%4.39 in86LGCD 1
3:139.3 mph+3.34%4.51 in86LGCD 12
3.083:138.2 mph+0.55%4.63 in88LGCD 1
3.167:137.2 mph-2.10%4.75 in88LGCD 2
3.25:136.2 mph-4.61%4.87 in88LGCD 3

How to Calculate a Missing Sprocket Size

Start from the speed you want at the wheel, work back to the ratio, then to the tooth count. Once you have a candidate, see its effect on speed with the sprocket RPM and chain speed calculator.

The formulas

Wheel rpm = mph × 336.1 / Tire Ø (in)
N₂ = N₁ × (Engine rpm / Gearbox) / Wheel rpm
N1
= driver (front) teeth
N2
= driven (rear) teeth

Worked example: go-kart

  1. Target: 38 mph, 11 in tire, 3600 rpm
  2. Exact rear sprocket: 37.2T
  3. Closest whole tooth: 37T
  4. Gives 38.2 mph (+0.55%)

Choosing Between Nearby Tooth Counts

The closest ratio is not always the best choice. Weigh the speed error against wear, clearance and chain length. For the exact sprocket diameter of an option, use the sprocket diameter calculator, and for a compound drive, the sprocket ratio calculator.

Speed or rpm error

One tooth changes the ratio by 1 in N. On a 37 tooth sprocket that is under 3%, but on a 12 tooth sprocket it is over 8%, so small sprockets have coarse steps.

Wear (GCD)

A GCD of 1 between the two sprockets is a good sign. If the closest option shares a factor, a neighbouring tooth count often fixes it at a small cost in ratio.

Clearance and chain

A larger rear sprocket sits closer to the ground and needs a longer chain. The table shows the outside diameter and the even link count of each option.

Sprocket Size FAQ

Related Chain & Sprocket Calculators