# Reaction Time Calculator (Ruler Drop Test)

URL: https://speedycalculator.com/calculators/physics/reaction-time/
Last updated: 2026-10-30

> The reaction time calculator finds your reaction time from the ruler drop test: someone drops a ruler between your fingers and you catch it – the distance it falls gives the time. Up to five attempts are averaged.

## Inputs

- Attempt 1 distance (cm, mm, m, in, ft)
- Attempt 2 distance (cm, mm, m, in, ft)
- Attempt 3 distance (cm, mm, m, in, ft)
- Attempt 4 distance (cm, mm, m, in, ft)
- Attempt 5 distance (cm, mm, m, in, ft)
- Gravitational acceleration g (m/s², ft/s², g, cm/s²)

## Results

- Attempts
- Average reaction time
- Best attempt
- Rating

## Formula

```
t = √(2d/g)
```

## Explanation

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

FieldWhat to enterAttempt 1 distanceUnits: cm, mm, m, in, ftAttempt 2 distanceUnits: cm, mm, m, in, ftAttempt 3 distanceUnits: cm, mm, m, in, ftAttempt 4 distanceUnits: cm, mm, m, in, ftAttempt 5 distanceUnits: cm, mm, m, in, ftGravitational acceleration gUnits: 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)9174.413.5184.518193.122.5200.627207.436219.5

## 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.

## FAQ

**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.


## Sources

- [OpenStax – University Physics Volume 1](https://openstax.org/details/books/university-physics-volume-1)
- [HyperPhysics (Georgia State University)](http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)