The ruler drop
You need two people. One holds a 30 cm (12 in) ruler by the top end and lets it hang. The person being tested holds thumb and index finger open at the zero mark. Without warning, the ruler is released; catch it as fast as you can and read the mark at your fingers.
- Vary the release timing (counting “3, 2, 1” invites anticipation)
- Take at least 5 tries and average them, ignoring obvious outliers
- Rest your forearm on a table so your hand stays in place
Distance → time chart
The ruler is in free fall, so distance d and time t follow t = √(2d ÷ g) with g = 9.8 m/s². Enter the distance in metres to get seconds.
| Caught at | Reaction time (approx.) |
|---|---|
| 5 cm (2 in) | 101 ms |
| 10 cm (4 in) | 143 ms |
| 15 cm (6 in) | 175 ms |
| 20 cm (8 in) | 202 ms |
| 25 cm (10 in) | 226 ms |
| 30 cm (12 in) | 247 ms (the limit of a 30 cm ruler) |
A 30 cm ruler can’t measure anything slower than about 250 ms. If it slips through, use a longer one.
Three methods compared
| Method | What it measures | Pros | Limits |
|---|---|---|---|
| Ruler drop | See it, pinch it | One tool, no device delay | Needs two people; the dropper’s habits creep in |
| Web test (PC) | Colour change → mouse click | Easy to log | Monitor and mouse delay added |
| App (phone) | Colour change → touch | Solo, daily, history builds up | Display and touch delay added |
The same person gets different numbers with different methods. Device delay is covered in why reaction time differs between phones. Only compare same method, same device.
Measuring in the app
Pick the Reaction Time card
In Reflex Lab, tap the Reaction Time card on the home screen. The red “Wait for it…” screen turns green; tap the instant it does. On the default Normal difficulty the average of 5 rounds is recorded, and tapping before green shows “Too soon!” and redoes just that round.
Warm up in Practice mode
Turn on Practice mode on the start screen and nothing is recorded. Do a few warm-up runs, switch it off, then measure, so first-try clumsiness stays out of your history.
See your per-test stats
The chart icon at the top opens Stats: plays, best and average for Reaction Time, plus your best per difficulty.
FAQ
My ruler result is faster than the app. Why?
That’s expected. The ruler has no screen or touch delay, and pinching is a different movement from tapping glass. Don’t compare numbers across methods.
How many tries make a reliable result?
One try is mostly luck. Average five or more with the same method. On Normal, the app averages 5 rounds for you.
Reflex Lab: Brain Speed Test
No sign-up, fully offline, records stay on your device. Free on Android.