Sonar Depth Calculator
Turn sonar echo time into water depth for salt and fresh water.
What it does
Echo sounders and fish finders measure depth with sound. A transducer pings down, the bottom reflects it, and the unit times the return. Since the pulse travels down and back, depth is half the round trip distance.
The tool above takes echo time and water type and returns depth, one way time, and the sound speed used. The defaults 2 seconds in sea water match the worked example, so the page opens showing 4921.3 ft. Switch to metric for meters.
Sound speed in water sits near 1500 m/s, far above the 343 m/s in air. Salt, temperature, and pressure shift it by tens of m/s. This page uses round working values, 1500 m/s for sea water and 1480 m/s for fresh water at typical temperatures, which suit recreational sounders and homework.
The method assumes a straight vertical path and a single speed for the whole column. Real oceans layer with thermoclines that bend rays and change speed with depth. Survey work corrects with profiles. For boating, fishing, and class problems, the simple halved product is standard.
This page shows the formula, one full hand checked example at 2 seconds, a reference table, and practice checks that run through the tool in both unit systems.
How to use
- Pick Imperial (ft) or Metric (m) for the depth readout.
- Type the echo round trip time in seconds from your sounder log.
- Pick Sea water or Fresh water for the speed.
- Read Water depth as the main result.
- Read One way time as half the echo, a sanity check.
- Compare sound speed used if you switch water type.
Formula
- echo
- Round trip time in seconds, down plus up.
- speed
- 1500 m/s sea water, 1480 m/s fresh water here.
- depth
- Vertical distance to the reflector.
Feet come from meters times 3.280839895. The model ignores beam angle, slope, and layered speed. For steep slopes or deep ocean work, apply ray corrections from survey tables.
Worked example
Example: 2 s echo in sea water
A sounder logs a 2 second round trip over sea water. Find depth in feet.
- Write the inputs: echo = 2 s, speed = 1500 m/s.
- Multiply: 1500 x 2 = 3000 m of total travel.
- Halve for one way: 3000 / 2 = 1500 m depth.
- Convert: 1500 x 3.280839895 = 4921.2598 ft.
- Round to one decimal: 4921.3 ft.
- Metric mirror: same echo gives 1500 m with one way 1 s.
Reference table
| Echo | Depth metric | Depth imperial |
|---|---|---|
| 0.5 s | 375 m | 1230.3 ft |
| 1 s | 750 m | 2460.6 ft |
| 2 s | 1500 m | 4921.3 ft |
| 4 s | 3000 m | 9842.5 ft |
Limits
- Single speed per water type. Real columns vary with temp and depth.
- Straight down path assumed. Slopes and wide beams read long.
- Shallow echoes under about 0.01 s need faster timing than this model.
- Reflectors may be fish or weed, not always the bottom.
Common mistakes
- Forgetting to halve, so depth doubles.
- Using air sound speed 343 m/s in water.
- Reading round trip as one way on the sounder log.
- Mixing meters with feet in the same step.
- Using sea speed for a fresh lake without switching water type.
Try it yourself
Quick checks. Answers sit tucked under each question.
1 s echo in sea water, metric?
Depth 750 m, one way 0.5 s, speed 1500 m/s.
0.5 s echo in fresh water, imperial?
Depth 1213.9 ft, one way 0.25 s, speed 4855.6 ft/s.
Questions
Why divide by 2?
Echo time covers down plus up. Half is the one way trip to the bottom.
Salt or fresh speed?
This tool uses 1500 m/s sea and 1480 m/s fresh as working values.
How deep is a 1 s echo?
750 m in sea water, about 2460.6 ft.
Active or passive sonar?
This is active: it sends a ping and times the echo. Passive only listens.
Why does my fish finder disagree?
Beam angle, slope, thermoclines, and weed or fish returns all shift readings.
Can I use this in air?
No. Air needs near 343 m/s. This tool is water only.