The ruler drop test turns free fall into a stopwatch. A typical reaction time is about 0.2 s, during which a ruler falls about 20 cm. Fatigue, distraction and alcohol make it longer.
How to use the Reaction Time Calculator
- Attempt 1 distance
- Attempt 2 distance
- Attempt 3 distance
- Attempt 4 distance
- Attempt 5 distance
- Gravitational acceleration g
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the distance the ruler fell before you caught it for each attempt (leave unused attempts at 0).
Common mistakes to avoid
- Mixing units between fields.
- Using a rule outside the range where it applies.
- Expecting fun estimates to be exact predictions.
Inputs explained
| Field | What to enter |
|---|---|
| Attempt 1 distance | Units: cm, mm, m, in, ft |
| Attempt 2 distance | Units: cm, mm, m, in, ft |
| Attempt 3 distance | Units: cm, mm, m, in, ft |
| Attempt 4 distance | Units: cm, mm, m, in, ft |
| Attempt 5 distance | Units: cm, mm, m, in, ft |
| Gravitational acceleration g | Units: m/s², ft/s², g, cm/s² |
What each result means
- Attempts = ([Attempt 1 distance] > 0) + ([Attempt 2 distance] > 0) + ([Attempt 3 distance] > 0) + ([Attempt 4 distance] > 0) + ([Attempt 5 distance] > 0)
- Average reaction time = (sqrt(2×[Attempt 1 distance]÷[Gravitational acceleration g]) + sqrt(2×[Attempt 2 distance]÷[Gravitational acceleration g]) + sqrt(2×[Attempt 3 distance]÷[Gravitational acceleration g]) + sqrt(2×[Attempt 4 distance]÷[Gravitational acceleration g]) + sqrt(2×[Attempt 5 distance]÷[Gravitational acceleration g]))÷[Attempts]×1000
Example values
How the main result changes when Attempt 1 distance changes and everything else stays at the example values above:
| Attempt 1 distance (cm) | Average reaction time (ms) |
|---|---|
| 9 | 174.4 |
| 13.5 | 184.5 |
| 18 | 193.1 |
| 22.5 | 200.6 |
| 27 | 207.4 |
| 36 | 219.5 |
Formula
t = √(2d/g)
Worked example
Inputs — Attempt 1 distance: 18 cm; Attempt 2 distance: 21 cm; Attempt 3 distance: 16 cm; Attempt 4 distance: 0 cm; Attempt 5 distance: 0 cm; Gravitational acceleration g: 9.80665 m/s².
Results — Attempts = 3; Average reaction time = 193.1 ms; Best attempt = 180.6 ms; Rating = Good.
Key terms
- Reaction time
- Delay between a stimulus and the response.
Frequently asked questions
What is a normal reaction time?
About 0.2–0.25 s for a visual cue.
How far does a ruler fall in 0.2 s?
About 19.6 cm.
Why average several attempts?
Single tries vary a lot.
Does practice help?
A little – anticipation can also skew results.