Free Fall Calculator

How to use this calculator

Gravitational acceleration is pre-filled with the standard value for Earth, 9.80665 m/s². Enter one more quantity — the height, the fall time, or the impact speed — and the other two are calculated. You can also measure the fall time of a dropped object together with a known height to estimate g itself, a classic first-year lab experiment.

To model another world, overwrite g: 1.62 m/s² for the Moon, 3.71 m/s² for Mars, 24.79 m/s² for Jupiter's cloud tops. Heights can be entered in meters, feet or any other length unit, and speeds are available in km/h and mph for an intuitive feel.

The formula

h = ½ g t²

v = g t

v = √(2 g h)

h: fall height
t: fall time
v: impact speed
g: gravitational acceleration

An object released from rest accelerates uniformly at g. Its speed grows linearly with time while the distance fallen grows with time squared — which is why the second second of a fall covers three times the distance of the first.

Worked example

A stone is dropped from a 20 m bridge. How long until it hits the water, and how fast is it going?

t = √(2h / g) = √(2 × 20 / 9.80665) = 2.02 s.
v = g t = 9.80665 × 2.0196 = 19.8 m/s (about 44 mph).
Check: v = √(2gh) = √(392.3) = 19.8 m/s.

The stone falls for about two seconds and lands at roughly 20 m/s, ignoring air resistance.

Assumptions and limits

No air resistance. For dense, compact objects over a few meters this is accurate to a few percent; for feathers, paper, or long falls (skydivers reach terminal velocity near 55 m/s) it is not.
The object starts from rest and g is constant over the height of the fall.

Frequently asked questions

Do heavy objects fall faster than light ones?

Not in a vacuum — all objects fall with the same acceleration. In air, drag matters more for light, broad objects, so a feather falls slower than a hammer.

How can I measure the height of a cliff with a stone?

Time the fall in seconds and enter it here. Ignoring the delay for sound to return and air drag, h = ½ g t². A 3 s fall means about 44 m.

What value of g should I use?

The internationally defined standard gravity is 9.80665 m/s². Actual g varies from about 9.78 m/s² at the equator to 9.83 m/s² at the poles.