Read the mark at the top of the catcher’s thumb. Enter 1 to 50 cm.
Reaction time
t = √(2d ÷ g) with d in meters and g = 9.81 m/s²
Two 15 cm strips marked in milliseconds. A4 or Letter paper, printed at 100% scale.
Run it in class in 15 minutes
- 0–2 min
Pair up
One person holds the ruler, the other catches. The catcher rests their forearm on the edge of a table so only the hand moves.
- 2–4 min
Get set
The holder grips the top end so the ruler hangs straight down, 0 at the bottom. The catcher holds thumb and finger open on either side of the ruler, with the top of the thumb level with the 0 line, without touching it.
- 4–9 min
Drop and catch
With no countdown, the holder lets go at a random moment and the catcher pinches it. Read the mark at the top of the thumb. Five drops each, then swap.
- 9–12 min
Convert
Type each distance into the calculator or use the chart below, then average each person’s five times.
- 12–15 min
Compare
Change one thing and repeat: the other hand, or catching while counting backwards out loud. Did the average move?
Ruler drop conversion chart
| Distance (cm) | Distance (in) | Reaction time |
|---|---|---|
| 5 | 1.97 | 101 ms |
| 10 | 3.94 | 143 ms |
| 15 | 5.91 | 175 ms |
| 20 | 7.87 | 202 ms |
| 25 | 9.84 | 226 ms |
| 30 | 11.81 | 247 ms |
Worked out with t = √(2d ÷ g), g = 9.81 m/s², rounded to the nearest millisecond.
Printable ruler
Two strips, 0 to 15 cm and 15 to 30 cm, with every centimeter labeled in milliseconds. Cut on the dashed lines, glue the tab of B behind A so the 15 cm lines meet, and hold it with 0 at the bottom.
A 0 to 15 cm
Cut on the 15 cm line, then line up with B
B 15 to 30 cm
Line this 15 cm mark up with the cut edge of A
How it works
A ruler you let go of starts at rest and speeds up as it falls, so the distance it drops tells you how long it fell: d = ½gt². Rearranged, that gives t = √(2d ÷ g).
20 cm = 0.20 m
t = √(2 × 0.20 ÷ 9.81) = 0.202 s = 202 ms
Measured in inches? Multiply by 2.54 first: 8 in is 20.32 cm, or 204 ms.
Because the ruler keeps speeding up, the first 5 cm take 101 ms, while going from 25 to 30 cm adds only about 21 ms.
Questions
Why is my ruler result faster than an online test?
A ruler has no screen or mouse delay. In a study of 94 college football players, a falling measuring stick averaged 203 ms and a computer test 268 ms.¹ Online tests, including ours, also include your device’s delay.
Where exactly do I read the ruler?
Read the mark at the top edge of the catcher’s thumb where it pinches the ruler. Start with the top of the thumb level with the 0 line, so the reading is the distance the ruler fell.
Can I use a 12-inch ruler?
Yes. Switch the calculator to inches and enter the distance. A 12-inch ruler is about 30.5 cm long, so it works the same way as a 30 cm ruler.
What if the ruler falls more than 30 cm?
Count it as a miss and drop again. A 30 cm fall takes about 247 ms, so a miss usually means the catcher wasn’t ready. The calculator accepts up to 50 cm for longer rulers.
How many drops should each person do?
Five drops each, then swap. Use the average of the five, and redo any drop where the catcher grabbed before the ruler started to fall.
Sources
- Eckner, J. T., Kutcher, J. S., & Richardson, J. K. (2010). “Pilot evaluation of a novel clinical test of reaction time in National Collegiate Athletic Association Division I football players.” Journal of Athletic Training, 45(4), 327–332. doi.org/10.4085/1062-6050-45.4.327. Accessed 7 November 2026.