Spindle Speed Calculator

Optimal whorl size from target twist and drafting speed.

Spindle weight (oz)

Whorl diameter (in)

Drop length (in)

Target TPI

Tab to move between fields

Initial RPM

184

Average RPM120

Spin time (sec)5.8

Revolutions per drop12

TPI per drop0.39

Drops per yard19

Moment of inertia (kg·m²)0

Needed Revs180

How to Use This Calculator

Weigh your spindle and enter the total weight in ounces (whorl + shaft).
Measure the whorl diameter in inches and enter it.
Enter the drop length in inches from your hand to the lowest point of the suspended spindle.
Enter your target TPI (twist per inch) for the yarn you are spinning.
Read the initial RPM, average RPM, and spin time in seconds to understand your spindle's performance.
Use the drops per yard output to plan your drafting pace — more drops per yard means more spin sessions per yard of yarn.

How the result changes with Whorl diameter (in)

Spindle weight (oz): Total weight of the drop spindle in ounces (whorl + shaft).
Whorl diameter (in): Diameter of the spindle whorl in inches.
Drop length (in): Distance from hand to lowest point of spindle drop (typically 24-36 inches).
Target TPI: Desired twist per inch for your yarn.

Initial RPM: Estimated starting RPM from a typical hand flick.
Average RPM: Average RPM during useful spin time (~65% of initial).
Spin time (sec): Estimated seconds of useful spinning before the spindle slows too much.
Revolutions per drop: Total twists inserted in one park-and-draft or drop session.
TPI per drop: Actual TPI achieved if you draft the full drop length in one spin.
Drops per yard: Number of spin-and-draft cycles needed per yard at target TPI.
Moment of inertia (kg·m²): Rotational inertia of the spindle — higher values = slower but longer spin.

Identify Input Parameters
4 parameters
Spindle weight (oz) = 1.5, Whorl diameter (in) = 2.5, Drop length (in) = 30, Target TPI = 6 = 4 input(s) provided
Calculate Initial RPM
Initial RPM = (initialOmega * 60) / (2 * π)
184 = 184
Calculate Average RPM
Average RPM = initialRpm * 0.65
120 = 120
Calculate Spin time
5.8 = 5.8