Echo Distance Calculator - Sonar Depth and Echo Time

Echo distance and sonar depth from the time a sound takes to come back

A lake echo sounder that hears its ping after 26 ms is looking at a bottom 63.17 ft down, 10.53 fathoms. This tool turns a round-trip echo time into the one-way distance, or a known distance into the echo time, for air at any temperature, fresh water, seawater by temperature and salinity, steel, aluminum, concrete or a speed of your own.

The trap in every echo: the sound went there and back

You clap at a cliff on a 59 °F day and hear the echo 1.5 s later. Speed times time says 1,674.58 ft, and that number is twice too big, because the clap crossed the gap and then crossed it again. The second trap: the speed depends on the material and, in air and seawater, on the temperature.

How the calculator avoids both mistakes

It halves the path for you. The headline is always the one-way distance, and the total path appears next to it so you can see where the factor of two went.

It asks for what changes the speed. Air needs a temperature; seawater needs a temperature and a salinity and runs the Mackenzie equation; solids use measured reference speeds.

It keeps tiny times readable. An echo of 10 µs stays 10.00 µs instead of turning into 0.000 s, and depths in water also come in fathoms.

What to calculate - distance from an echo time, or the echo time for a distance you know.
Sound travels through - dry air, seawater, fresh water, steel, aluminum, concrete or your own speed.
Temperature and unit - air or seawater only. Seawater must be between 28.4 °F and 86 °F, the range the formula was built for.
Salinity - seawater only, in parts per thousand, from 30 to 40.
Echo time and its unit - the full round trip, in s, ms or µs.
Distance and its unit - one way, in mm, inches, feet, meters, fathoms, kilometers or miles.
Read the result - the answer in two unit systems, the one-way time, the total path, and for echo times a comparison with three other materials.

Which setting fits your measurement

You are measuring	Material	Time unit	Also enter
A shout or clap against a cliff or building	Dry air	s	Air temperature
An ultrasonic distance sensor	Dry air	ms	Air temperature
Depth of a lake or river	Fresh water	ms	Nothing else
Depth at sea	Seawater	ms or s	Water temperature, salinity
Wall thickness of a steel or aluminum part	Steel or aluminum	µs	Your own speed if you know the alloy's value
Thickness of a concrete slab	Concrete	µs	Nothing else

Six echoes, from a cliff to a pipe wall

A clap at a cliff. 1.5 s in air at 59 °F. The cliff is 837.29 ft away (255.21 m), half of the 1,674.58 ft path.

A lake bottom. 26 ms in fresh water gives 19.25 m, 63.17 ft or 10.53 fathoms.

The same ocean echo in cold and warm water. A 4 s echo at salinity 35 means 2.933 km at 4 °C but 3.069 km at 25 °C, because the speed at the surface rises from 1,466.49 to 1,534.29 m/s. Both use the surface value; the speed changes on the way down.

An ultrasonic sensor on a warm day. 5.83 ms in air at 86 °F puts the object 3.34 ft away (1.02 m). At 68 °F the same echo would mean 1.00 m.

A steel plate, and why the alloy matters. 10 µs through 1% carbon steel gives 29.70 mm (1.169 in). Enter stainless steel's 5,790 m/s as your own speed and the same echo gives 28.95 mm.

A concrete slab. An 8 in slab returns its echo after 109.84 µs.

Echo times to keep handy

Round-trip time for a reflector one inch, one foot and one meter away. Air is at 68 °F, seawater at 50 °F with salinity 35.

Material	1 inch	1 foot	1 meter
Dry air	148.01 µs	1.78 ms	5.83 ms
Fresh water	34.30 µs	411.61 µs	1.35 ms
Seawater	34.10 µs	409.18 µs	1.34 ms
Concrete	13.73 µs	164.76 µs	540.54 µs
Steel, 1% carbon	8.55 µs	102.63 µs	336.70 µs
Aluminum, rolled	7.91 µs	94.95 µs	311.53 µs

What people ask about echoes and sonar

How close can a wall be before I stop hearing a separate echo?

The ear cannot separate an echo from the original sound if it returns less than a tenth of a second later. In dry air at 68 °F that round trip covers 34.32 m, so the wall has to be at least 17.16 m (56.30 ft) away.

What salinity should I enter for the ocean?

Open ocean water is usually close to 35 parts per thousand, which is also the value the Mackenzie equation is centered on. Each part per thousand moves the speed by 1.03 to 1.34 m/s across the formula's temperature range, so a guess of 34 or 36 changes a depth by less than one tenth of one percent. Water below 30, such as an estuary, is outside the formula and the calculator will say so.

Why does my thickness gauge need a calibration block?

Because the speed differs between alloys. The same 10 µs echo reads 29.70 mm with the carbon steel value and 28.95 mm with stainless steel's 5,790 m/s, a 0.75 mm gap. Measuring a sample of known thickness sets the speed for the exact material in front of you.

How long is a fathom?

Exactly 6 ft, which is 1.8288 m. Nautical charts in the US long used fathoms for depth, so the result shows them for fresh water and seawater once the depth reaches a meter.

Is the ocean depth exact?

It is as exact as the speed you give it. The calculator uses the Mackenzie equation at the surface. Deeper down, temperature and pressure change the speed, which is why survey ships measure a speed profile through the water column.

Do ultrasonic parking and distance sensors care about temperature?

They work in air, so yes. A 5.83 ms echo means 1.00 m at 68 °F and 1.02 m at 86 °F.

Related tools

Speed of Sound Calculator

One-way sound travel: thunder distance, fireworks delay and seventeen materials - Open the calculator

Ohm's Law Calculator

The electronics behind an ultrasonic sensor: volts, amps, ohms and an LED resistor - Open the calculator

Wave Frequency Calculator

The wavelength of a sonar ping or an ultrasonic pulse - Open the calculator

Temperature Converter

Switch a water or air reading between °F and °C - Open the calculator

Density Calculator

Why steel, concrete and water carry sound at such different speeds - Open the calculator

Snell's Law Calculator

How a wave bends when its speed changes at a boundary - Open the calculator