spindown calculator

Estimate deceleration, coast-down time, exponential decay constant, and drag torque from a simple spin-down test.

Initial speed (RPM)

Final speed (RPM)

Elapsed time between readings (seconds)

Moment of inertia, I (kg·m²) (optional)

Target speed for estimate (RPM)

Enter your values and click Calculate.

What is a spindown calculator?

A spindown calculator helps you analyze how quickly a rotating object slows down after drive power is removed. This is often called a coast-down test. Engineers use it to evaluate bearing condition, drag losses, brake behavior, motor friction, and overall mechanical efficiency.

In practical terms, you measure speed at two moments (for example, 3600 RPM and then 900 RPM after 20 seconds), then use those values to estimate deceleration behavior. This page computes both a linear deceleration estimate and an exponential decay estimate, because real systems can behave like either model depending on friction sources.

How to use this calculator

1) Collect speed data

Record an initial RPM immediately after power-off.
Record a second RPM after a known time interval.
Enter the elapsed time in seconds.

2) Add inertia if you want torque

If you know the rotating system's moment of inertia, enter it to estimate average drag torque. If you leave inertia blank, all speed-related outputs still work.

3) Read outputs in context

Average deceleration: simple slope across your measurement window.
Linear stop time: assumes deceleration stays constant.
Exponential time constant: useful when deceleration scales with speed.
Drag torque: based on angular deceleration and inertia.

Formulas used

The calculator uses standard rotational dynamics relationships:

ω = RPM × 2π / 60
α = (ω_f - ω_i) / t
t_stop,linear = -ω_i / α (when α < 0)
τ = I × α for average drag torque
T = -t / ln(ω_f/ω_i) for exponential time constant
t_target = T × ln(RPM_i/RPM_target)

When results are most reliable

You will get the best estimates when your test conditions are controlled:

No drive torque during measurement (true coast-down).
Stable temperature (lubrication viscosity can change rapidly).
Accurate tachometer timing and RPM readings.
Measurements taken over a meaningful speed interval, not tiny differences.

Common mistakes

Entering final RPM greater than initial RPM (that is acceleration, not spin-down).
Using elapsed time of zero or near-zero seconds.
Assuming linear stop time is exact in systems with strong aerodynamic drag.
Using incorrect inertia units.

Practical applications

This type of calculation is used in electric motor diagnostics, flywheel testing, robotics, spindle maintenance, and rotating instrument design. Even if you are not doing formal engineering validation, spindown trends over time can reveal early wear before failure occurs.