Quadruple a pendulum’s length and it swings half as fast. Mass doesn’t matter at all.
T ∝ √L, and mass cancels out of the equation entirely — same reason heavy and light objects fall together.
Mass stops being irrelevant the moment the pendulum is not a simple one. A physical pendulum's period depends on where the mass sits, through its moment of inertia — which is why a metronome changes tempo when you slide the weight without changing its mass at all.
Estimate with the rule, then check it against the calculator that models it properly.
Open Pendulum Simulator →