Ruler drop test for reaction time

How the ruler drop test works

A friend holds a ruler above your open fingers and lets go without warning. You catch it as fast as you can. The longer you take to react, the further the ruler falls before your fingers close. Because a dropped object speeds up in a known way, the distance tells you the time.

The formula comes from free fall, starting from rest:

The calculator above does the maths for you. Type the mark in cm or in, and it shows your reaction time. Press Add this catch after each try; after 5 catches you see your average.

Worked example: 20 cm

1. You caught the ruler at the 20 cm mark, so d = 0.20 m. 2. 2 × 0.20 = 0.40. 3. 0.40 ÷ 9.81 = 0.0408. 4. √0.0408 = 0.202 s, which is 202 ms.

Notice that the time is not proportional to the distance: the ruler falls faster and faster, so 10 cm is 143 ms, not half of 202 ms.

How to do the ruler drop test with a friend

You need a 30 cm (12 in) ruler and a friend. The steps are printable with the Print the instructions button.

1. Sit down and rest your forearm on a table so your hand sticks out over the edge.
2. Open your thumb and index finger about 2 to 3 cm apart.
3. Your friend holds the ruler vertically, with the 0 mark exactly between your fingers.
4. Without any countdown, your friend lets go. Catch it as fast as you can.
5. Read the mark at the top of your thumb. That is the distance the ruler fell.
6. Repeat 5 times, add each catch to the calculator, and look at the average.

Make it fair: the person dropping should wait a different time before each drop, and must not move their hand or say anything just before letting go. Watch only the ruler, not your friend's face. Use the same hand each time (or compare hands on purpose).

If the ruler slips through completely, that try does not count: with a 30 cm ruler the slowest time you can measure is about 247 ms. A longer ruler helps if that happens often.

Distance to time table

The table in the tool shows the same values. Here they are with a short comment:

• 5 cm → 101 ms: very fast, and hard to repeat.
• 10 cm → 143 ms
• 15 cm → 175 ms
• 20 cm → 202 ms
• 25 cm → 226 ms
• 30 cm → 247 ms: the end of a standard ruler.

In inches: 4 in ≈ 144 ms, 8 in ≈ 204 ms, 12 in ≈ 249 ms. The calculator accepts 1 to 100 cm (0.5 to 39 in).

A good experiment for schools

The ruler drop is a classic science class experiment because it needs nothing but a ruler and connects biology (nerves and muscles) with physics (acceleration). Ideas for a lesson:

• Dominant vs other hand: does your writing hand catch faster?
• Distraction: count backwards from 100 in threes while catching. Does the average get worse?
• Practice: do 5 catches, then 5 more. Does the second set improve?
• Sight vs touch: close your eyes and let your friend tap your arm at the moment of release. (Keep the release silent.)
• Class data: each pair adds their averages to a table; discuss why results vary and why 5 tries are better than 1.

Good discussion point: catching a ruler takes longer than clicking a mouse, because your fingers have to travel and close around the ruler. That movement time is part of the result.

Ruler drop vs online reaction test

The two measure slightly different things. The ruler test has no screen and no mouse, so there is no device latency, but it includes the time your fingers need to close. The online reaction time test measures a click, but its result includes your screen and mouse delay. Doing both is a nice comparison.

For a different start signal, try the lights out reaction test. For speed rather than reaction, see the CPS test and the aim trainer. All skill tests are listed on the skill tests page, and the guide to reading your test results explains the device-latency side.