A porch swing, a grandfather clock, and a wrecking ball all obey the same hypnotic rule: the time for one full swing depends almost entirely on length, not on how heavy the weight is or — for small swings — how far it travels. The period T of a simple pendulum is 2π times the square root of (length ÷ gravity). Double the length and the period grows by about 41 percent; quadruple it and the period exactly doubles, because of that square root.

Clockmakers exploit this mercilessly. A seconds pendulum — one that takes exactly two seconds per full swing — must be about 39 inches long, which is why grandfather clocks are tall furniture and not desk toys. On the Moon, where gravity is one-sixth of Earth's, the same pendulum swings about 2.4 times slower. This calculator finishes the arithmetic: compute √(L/g) on paper using feet and g = 32.2 ft/s², type that value in the Variable A box, type 6.2832 (which is 2π) in the Variable B box, and the Result box shows the period in seconds. A 2-foot porch chain gives √(2/32.2) ≈ 0.249 in A, so the swing takes about 1.57 seconds per round trip.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Pendulum Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.