Pendulum Calculator
Swing period from length, or the length for a target period.
What's odd about this formula
T = 2π√(L / g)
The mass isn't in it, and neither is how far you swing: heavy or light, wide or narrow (within small angles), only the length sets the beat. Galileo noticed it in a swinging cathedral lamp, and it made pendulums the heart of accurate clocks for three centuries.
The meter connection
A pendulum with a 2-second period (one second per swing) is 0.994 m long, suspiciously close to a meter, and one early proposal defined the meter exactly that way. The formula holds for swings under about 15°; larger arcs run slightly slow.