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Reaction Time Calculator

How fast are you? The reaction time calculator teaches you a way to measure your reflexes — with physics!

Clear
Thinking distance27.78 m1 s at 27.78 m/s — proportional to speed
Total stopping distance83.99 m275.6 ft
Braking distance56.21 mproportional to the SQUARE of speed, which is why the total grows so fast
Deceleration6.8647 m/s²0.7 g
Braking time4.047 splus 1 s of reaction
Total time5.047 s
At twice the speed280.4 mbraking distance quadruples while thinking distance only doubles
Speed still carried where a slower vehicle would have stopped51.93 m/sthe vehicle travelling twice as fast is still doing this when the slower one is stationary
On wet asphalt126.15 mµ = 0.4
On packed snow224.52 mµ = 0.2
On ice421.25 mµ = 0.1

The formula

distance = v·t_reaction + v² ÷ 2µg

One term is linear, the other is squared

Stopping distance has two parts that scale differently. Thinking distance is speed times reaction time, so it doubles when speed doubles. Braking distance is v²/2a, so it quadruples. That asymmetry is why the totals grow so alarmingly and why speed limits matter more than they intuitively should.

The consequence worth internalising is the residual-speed figure above. A car travelling at twice the speed does not merely stop later — at the point where the slower car has come to rest, it is still moving quickly, and any impact happens at that speed.

Surface dominates everything. Dry asphalt gives about 0.7, wet 0.4, snow 0.2, ice 0.1 — so an icy road can multiply braking distance sevenfold at unchanged speed. A downhill gradient subtracts from the available deceleration directly, and on a steep enough slope with poor grip the sum goes negative, meaning the vehicle cannot stop at all.

These are idealised figures for a locked-grip stop. Real braking involves weight transfer, ABS cycling and tyres well past their best, so treat them as a floor rather than a prediction.