Spindle Speed Calculator

Optimal whorl size from target twist and drafting speed.

About this calculator

This calculator estimates how a drop spindle actually behaves once it leaves your hand, using the same rotational-energy physics that governs any spinning disk. It models the whorl as a solid disk (moment of inertia I = ½mr²) and assumes a typical hand flick imparts about 0.004 joules of energy, then solves ω = √(2E/I) for the starting angular velocity. From there it estimates useful spin time from the angular momentum divided by an assumed small friction torque, and averages the RPM at 65% of the initial spin to account for the spindle slowing continuously rather than staying at full speed.

Total revolutions during that estimated spin time are divided by your drop length to get twist-per-inch (TPI) achieved in a single spin-and-draft cycle, and then compared against your target TPI to estimate how many drops you'll need per yard of finished yarn. The core takeaway is the physical relationship between whorl weight and diameter and spin behavior: heavier, larger-diameter whorls store more rotational energy (moment of inertia scales with mass and the square of radius) and spin longer but slower, while lighter, smaller whorls spin faster but run out of momentum sooner. Treat the flick energy and friction torque as fixed assumptions, not measured values from your actual spindle and technique — your real numbers will vary with hand strength, spindle bearing quality, and air resistance, so use the outputs as a starting point for comparing whorl designs rather than an exact prediction of your spinning session.

Inputs

Spindle weight (oz)

Whorl diameter (in)

Drop length (in)

Target TPI

Tab to move between fields

Results

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)

Whorl diameter (in)	Initial RPM
1.25	369
1.88	245
3.75	123
6	77

What each input means

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.

What each result means

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.

How this is calculated

Worked example, using the default values

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

Engine last updated September 1, 2026. Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does a heavier whorl spin slower but longer than a lighter one?

The calculator treats the whorl as a disk, so its moment of inertia I = ½mr² grows with mass and the square of the radius. For a fixed hand-flick energy of about 0.004 joules, it solves ω = √(2E/I) for the starting angular velocity — a larger I from more mass or a bigger whorl means a lower initial RPM. But that same larger I also means more angular momentum, which is divided by an assumed constant friction torque to estimate spin time, so the heavier or larger whorl keeps turning longer even though it starts out slower.

Where does the "average RPM" figure come from?

The calculator doesn't model the full deceleration curve of the spindle; it assumes average RPM during a useful spin is a fixed 65% of the initial RPM, standing in for a spindle that gradually slows rather than holding a constant speed. Total revolutions per drop are then that average RPM multiplied by spin time in minutes.

What does "drops per yard" tell me, and why might my real number differ?

The calculator divides your target TPI times 36 inches (one yard) by the total revolutions estimated for a single drop, and rounds up since you can't do a fractional drop. Because that revolution estimate rests on assumed values for flick energy and friction torque rather than your own spindle and hands, treat the count as a planning baseline — a stronger flick or a smoother bearing than assumed will let you spin more revolutions per drop and need fewer drops per yard.

How does drop length affect the TPI achieved per drop?

TPI achieved per drop is total revolutions divided by drop length in inches, so for a fixed number of revolutions, a shorter drop concentrates the same twist into less yarn and yields a higher TPI, while a longer drop spreads that twist thinner. That's why the calculator also reports "needed revs" (target TPI × drop length) — the revolutions you'd need in one drop to hit your target TPI without multiple drops.

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