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Water Depth Sonar Calculator

Calculate depth from sonar echo time with environmental corrections

Water Depth

46.0 ft

Sound Speed

1,400.7 m/s

Fathoms

7.7

Units
ms
°F

Water Depth

46.0 ft

Meters

14.0

Feet

46.0

Fathoms

7.7

Sound Speed in Water

1,400.7 m/s

Environmental Effects

Temperature Effect-106.0 m/s
Salinity Effect0.0 m/s
Baseline (15°C, 35 ppt)1506.7 m/s

Depth Reference Chart

Shallow Coastal0–30 ft (0–10 m)
Coastal Shelf30–200 ft (10–60 m)
Continental Shelf200–650 ft (60–200 m)
Deep Water650+ ft (200+ m)

Sound Speed by Conditions

Ice water, fresh1402 m/s
Lake (15°C)1465 m/s
Ocean (15°C)1507 m/s
Tropical ocean1534 m/s

What You'll Need

Garmin Striker 4 GPS Fish Finder with CHIRP Sonar

$100-$1504.4
View on Amazon

Standard Horizon HX210 Floating Handheld VHF Radio

$120-$1604.5
View on Amazon

Hondex Portable Digital Depth Sounder

$150-$2004.3
View on Amazon

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

Garmin Striker 4 GPS Fish Finder with CHIRP Sonar

$100-$1504.4
View on Amazon

Standard Horizon HX210 Floating Handheld VHF Radio

$120-$1604.5
View on Amazon

Hondex Portable Digital Depth Sounder

$150-$2004.3
View on Amazon

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Frequently Asked Questions

Q

How does sonar calculate water depth?

Sonar sends an acoustic pulse downward and measures the round-trip time for the echo to return. Depth = (Speed of Sound × Time) / 2. The division by 2 accounts for the sound traveling down and back up.

  • Sound travels at approximately 1,500 m/s in seawater (4,921 ft/s)
  • A 20 ms round-trip time indicates ~15.1 m (49.4 ft) depth in seawater
  • Consumer depth finders are accurate to within 1-3% of actual depth
  • Sonar frequencies range from 50 kHz (deep water) to 200 kHz (shallow, detailed)
  • Multiple-frequency sonars combine wide coverage with high detail
Ping Time (ms)Seawater DepthFreshwater Depth
10 ms7.5 m (24.7 ft)7.3 m (23.9 ft)
20 ms15.1 m (49.4 ft)14.7 m (48.1 ft)
50 ms37.7 m (123.6 ft)36.6 m (120.2 ft)
100 ms75.3 m (247.1 ft)73.3 m (240.3 ft)
Q

How does water temperature affect sonar depth readings?

Water temperature significantly affects the speed of sound. Warmer water transmits sound faster: each 1°C increase adds approximately 4.5 m/s to the speed of sound. This means the same ping time gives a deeper reading in warm water.

  • At 0°C: sound speed in seawater is approximately 1,449 m/s
  • At 15°C: sound speed in seawater is approximately 1,507 m/s
  • At 25°C: sound speed in seawater is approximately 1,532 m/s
  • Temperature effect: roughly +4.5 m/s per degree Celsius increase
  • A 10°C error causes ~3% depth calculation error
Q

Does salinity affect sonar accuracy?

Yes, salinity increases the speed of sound in water. Seawater (35 ppt) transmits sound about 5% faster than fresh water at the same temperature. Using the wrong salinity setting can produce depth errors of several percent.

  • Fresh water (0 ppt): ~1,435 m/s at 15°C
  • Brackish water (15 ppt): ~1,468 m/s at 15°C
  • Seawater (35 ppt): ~1,507 m/s at 15°C
  • Each 1 ppt increase in salinity adds approximately 1.3 m/s to sound speed
  • Saltwater depth finders preset to 1,500 m/s can be 5% off in fresh water
Water TypeSalinitySound Speed (15°C)
Fresh Water0 ppt1,440 m/s
Brackish15 ppt1,468 m/s
Seawater35 ppt1,507 m/s
High Salinity40 ppt1,514 m/s
Q

What is the Mackenzie equation for speed of sound in water?

The Mackenzie equation (1981) is the standard formula for calculating the speed of sound in seawater as a function of temperature, salinity, and depth. It is accurate to ±0.07 m/s for typical ocean conditions.

  • c = 1448.96 + 4.591T - 0.05304T² + 0.0002374T³ + 1.340(S-35) + ...
  • T = temperature in °C (valid range: -2 to 30°C)
  • S = salinity in parts per thousand (valid range: 25 to 40 ppt)
  • The equation also includes depth-dependent pressure terms
  • For surface measurements (depth = 0), pressure terms are zero
Q

What are fathoms and how do they relate to feet and meters?

A fathom is a traditional nautical unit of depth equal to 6 feet (1.829 meters). Fathoms are still used on nautical charts and by commercial fishermen. Most modern depth finders display in feet or meters, but fathom markings remain common.

  • 1 fathom = 6 feet = 1.829 meters exactly
  • 10 fathoms = 60 feet = 18.3 meters (typical coastal shelf)
  • 100 fathoms = 600 feet = 183 meters (edge of continental shelf)
  • 1,000 fathoms = 6,000 feet = 1,829 meters (deep ocean)
  • NOAA charts traditionally show depths in fathoms and feet below 100 fathoms

Example Calculations

1Coastal Seawater Sounding

Inputs

Ping Time20 ms
Temperature15°C (59°F)
Water TypeSeawater (35 ppt)

Result

Water Depth15.1 m (49.4 ft)
Sound Speed1,506.7 m/s
Fathoms8.2
Temperature Effect+0.0 m/s (baseline)
Salinity Effect+0.0 m/s (baseline)

Sound speed at 15°C, 35 ppt = 1448.96 + 68.87 - 11.93 + 0.80 = 1506.7 m/s. Depth = (1506.7 × 0.020) / 2 = 15.07 m = 49.4 ft. Fathoms = 49.4 / 6 = 8.2.

2Deep Freshwater Lake

Inputs

Ping Time100 ms
Temperature4°C (39°F)
Water TypeFresh Water (0 ppt)

Result

Water Depth71.1 m (233.1 ft)
Sound Speed1,421.0 m/s
Fathoms38.9
Temperature Effect-50.5 m/s vs baseline
Salinity Effect-46.9 m/s vs baseline

Sound speed at 4°C, 0 ppt = 1448.96 + 18.36 - 0.85 + 0.02 - 46.90 + 1.44 = 1421.0 m/s. Depth = (1421.0 × 0.100) / 2 = 71.1 m = 233.1 ft. Fathoms = 233.1 / 6 = 38.9.

Formulas Used

Sonar Depth

Depth = (Speed of Sound × Round-Trip Time) / 2

Calculates water depth from the echo sounder ping time. Division by 2 accounts for the round-trip path.

Where:

Speed of Sound= Acoustic velocity in water (m/s), varies with conditions
Round-Trip Time= Time for pulse to travel down and return (seconds)

Mackenzie Equation (Simplified)

c = 1448.96 + 4.591T - 0.05304T² + 0.0002374T³ + 1.340(S - 35)

Calculates the speed of sound in seawater at the surface from temperature and salinity using the Mackenzie (1981) equation.

Where:

c= Speed of sound in m/s
T= Water temperature in °C
S= Salinity in parts per thousand (ppt)

Source: Mackenzie, K.V. (1981). Nine-term equation for sound speed in the oceans. JASA, 70(3), 807-812.

Fathom Conversion

Fathoms = Depth in Feet / 6

Converts depth in feet to the traditional nautical fathom unit.

Where:

Depth in Feet= Water depth measured in feet
6= Feet per fathom (1 fathom = 6 ft = 1.829 m)

How Sonar Measures Water Depth

Sonar (Sound Navigation and Ranging) works by emitting an acoustic pulse and timing how long it takes for the echo to return from the bottom. Since the sound must travel down and back, the depth is exactly half the product of sound speed and round-trip time.

The speed of sound in water is not constant—it varies with temperature, salinity, and pressure (depth). In typical ocean conditions at 15°C and 35 ppt salinity, sound travels at about 1,507 m/s, roughly 4.4 times faster than in air. Warm, salty water transmits sound fastest.

Modern echo sounders use the Mackenzie equation or similar models to correct for environmental conditions. Consumer fish finders typically assume a fixed 1,500 m/s, which can produce errors of 3-5% in fresh water or extreme temperatures. Professional hydrographic survey equipment corrects in real time using CTD (Conductivity, Temperature, Depth) sensors.

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Last Updated: Mar 25, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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