Example 1
The countertop sits at 0.75 m: how long do you have to react?
Free Fall Time Calculator
Enter the height and, if you want, g. You get the time from release to the ground. From 10 m at 9.81 m/s² it is 1.43 s. From a 0.75 m tabletop you have 0.39 s, too little for a calm catch.
Have the time and need the height? Compute h = ½gt² in free fall. Where g comes from: g = GM/r².
Result
Height (m) and g (m/s², default 9.81). Fall time shows up here.
How it works
t = √(2h/g) is the drop distance h = ½gt² turned around. The object is released, initial speed zero. From 10 m and g = 9.81 m/s²: 2h/g = 20/9.81, the square root is 1.43 s. That is what a 10 m platform diver has for the whole routine.
The square root calms intuition. Four times higher lasts only twice as long. From 2 m you get 0.64 s, from 8 m you get 1.28 s. From a 114 m terrace it is 4.82 s, not tens of seconds. Gravity picks up speed quickly.
A mug from a 0.75 m table: t = 0.39 s. Human reaction is about 0.2 s. The rest is eaten by moving the hand. That is why a knee often wins. From 25 m a stone falls 2.26 s and at the ground v = g t ≈ 22 m/s, about 80 km/h.
Mass is not in the formula. In vacuum a hammer and a feather land together, as on the Moon during Apollo 15. On Earth, air makes the difference. For a compact object and heights up to about 50 m the error is small. A leaf, or hundreds of meters, is another story.
Leave g empty and the calculator uses 9.81 m/s². On the Moon type 1.62: from 2 m the time grows to 1.57 s. On Mars use 3.71 m/s². That is the “slow motion” in mission film. Smaller g, t grows like 1/√g.
A throw downward shortens the time. A throw upward lengthens the whole flight, because you have to stop first. When you know t and want h, open free fall: h = ½ × 9.81 × (2.5)² ≈ 30.7 m for 2.5 s.
How to use
- Type height in meters, for example 10, or 0.75 if you are timing a tabletop.
- Leave g empty for Earth (9.81). Type 1.62 for the Moon, 3.71 for Mars.
- Click Calculate. 10 m and 9.81 give 1.43 s. 0.75 m gives 0.39 s.
- Four times the height should double the time: 2 m → 0.64 s, 8 m → 1.28 s.
- When you know time and want h, open free fall. Here the unknown is t.
Formula
t = √(2h / g)
h is the height in meters and g is the gravitational acceleration in m/s² (9.81 on Earth). The result t is the fall time in seconds, for a dropped object, not a thrown one.
Letters in t = √(2h/g)
A drop, not a throw: t = √(2h/g). From 10 m at 9.81 m/s² it is 1.43 s; from a 0.75 m tabletop you have 0.39 s.
- t
- Fall time in seconds. 2×10/9.81, square root 1.43 s; from 8 m it is 1.28 s, twice the 2 m time (0.64 s).
- h
- Height in meters at v₀ = 0. A 10 m tower is 1.43 s; a 114 m terrace is 4.82 s, not tens of seconds.
- g
- Acceleration, default 9.81 m/s². A blank field takes Earth; this is not the g = GM/r² card.
Real-life examples
Example 2
It slid off the top shelf of the bookcase.
Example 3
Pool diving board: feet to water.
Example 4
Olympic platform: how long is the diver airborne?
Example 5
A valley bridge: from release to splash.
Example 6
The 320 m observation level: strictly theoretical.
Example 7
A pebble nudged over the edge of the cliff.
Example 8
A stone tumbles into the old well.
Example 9
A drop from a deck about 55 m up.
Example 10
An astronaut drops a sample at the lunar g = 1.62 m/s².
Example 11
Fourth floor: this is why window boxes get bolted down.
Frequently asked questions
How long does an object fall from 10 m?
1.43 s at g = 9.81. From 55 m it is 3.35 s. From 114 m it is 4.82 s.
Why does time grow so slowly with height?
Because a square root sits in the formula. Four times higher is twice as long: 2 m → 0.64 s, 8 m → 1.28 s.
Does mass change the time?
Not in this model. On Earth, air drag makes the difference. In vacuum a ball and a feather fall together.
How long does a mug take from 0.75 m?
0.39 s. Reaction time is about 0.2 s, so there is little left to move the hand.
How fast is an object from 25 m just before the ground?
t = 2.26 s from rest, and v = g t ≈ 22 m/s, about 80 km/h. Air drag is not in this sketch.
What g do I type for the Moon or Mars?
Moon 1.62 m/s², Mars 3.71 m/s². From 2 m on the Moon it is 1.57 s, on Earth 0.64 s.
Does the formula work for a thrown object?
It assumes v₀ = 0. A downward throw shortens t. An upward throw lengthens the whole flight.
How high is a bridge if a stone falls 2.5 s?
h = ½gt² ≈ 30.7 m. That lives on the free-fall page, the other way around.
Does air drag spoil the result much?
For a compact object up to about 50 m: not much. A leaf, a sheet, or hundreds of meters: this formula is too short.
Where do I get g if I know M and R of a planet?
On the g = GM/r² page. Then paste that g here and keep the drop height.
Knowledge sources
The formula is the school one. Units follow SI; NIST SP 330 and BIPM define the measures, not your result.
Page updated in 2026.