How fast is sound?

Sound isn't instant — it races through the air at about 343 metres a second. Count the gap between the lightning and the thunder and you can measure how far away the storm is.

works for:Ages 6–9Ages 10–13Ages 14+

Sound is quick, but it is not instant. It races through the air at about 343 metres every second — fast, yet slow enough to catch. Light, on the other hand, arrives at your eyes basically the moment it leaves. That tiny head start is the whole trick behind counting the gap between a lightning flash and its thunder to measure how far away the storm is.

count the gap — flash first, then thunder

Hit Flash the instant you see lightning, then Thunder the moment you hear the rumble. No storm handy? Drag the slider to pretend a gap instead.

or set the gap by hand3.00 s

gap between flash and thunder

3.00 s

distance to the strike

1029 m

= 1.03 km

distance = 343 m/s × 3.00 s

Rule of thumb: about 3 seconds per kilometre (1000 ÷ 343 ≈ 2.9), or roughly 5 seconds per mile.

why you see it before you hear it

sound in air

343 m/s

≈ 1,235 km/h ≈ 767 mph

light

3 × 10⁸ m/s

about 875,000× faster than sound

Light is so much faster that, over any distance you can hear across, it arrives essentially the instant the lightning strikes. The sound then trudges over at a mere 343 m/s — so the flash always wins the race to your senses, and the gap you count is almost entirely the sound's travel time.

echoes — the sound makes a round trip

distance = (343 × time) ÷ 2

seconds until the echo comes back

seconds

Shout at a cliff and the sound has to fly there and all the way back before you hear it. So the wall is only half as far as that round trip — you divide by two.

distance to the wall

86 m

(343 × 0.5) ÷ 2 = 86 m

Sound takes time

343 metres a second is fast — a whole football pitch in a blink — but slow enough that a distant clap or a mountain echo arrives noticeably late.

See first, hear later

Light is hundreds of thousands of times faster, so it always beats sound to you. That head start is what lets you time a storm at all.

distance = speed × time

Multiply the speed of sound by the seconds you counted and you get how far away the thing was — one of the most useful little formulas there is.

For the classroom

Learning goals

Sound travels at a finite speed — about 343 m/s in air at 20 °C (≈ 1235 km/h, ≈ 767 mph) — while light (3×10⁸ m/s) is effectively instant beside it, roughly 875,000× faster.
distance = speed × time turns the flash-to-thunder gap into a distance: distance = 343 × seconds, which is about 3 seconds per kilometre (or 5 seconds per mile).
An echo is a round trip, so the distance to a reflecting wall is (343 × time) ÷ 2; and warmer air raises the speed by roughly 0.6 m/s per °C.

Try this

1.Have the class count 'one-thousand-one, one-thousand-two…' between a flash and thunder (or use the slider). At 6 seconds, work out 343 × 6 ≈ 2058 m — about 2 km away — and check it against the 3-seconds-per-kilometre rule.
2.Echo hunt: pick a gap of 0.5 s coming back off a wall and compute (343 × 0.5) ÷ 2 ≈ 86 m. Ask what happens to the answer if a student forgets to divide by two, and why the round trip demands it.