Depth Sounder Correction Calculator
A depth sounder measures from the transducer to the seabed. To convert this reading to useful navigation information, apply the transducer offset (depth below waterline) to get water depth from the surface, then subtract your vessel's draft to get under-keel clearance. You can also subtract the current height of tide (from NOAA tide tables) to compare your reading with the charted depth, which is referenced to chart datum (MLLW in the US). This calculator performs all three conversions from a single sounder reading.
Depth correction formula
True water depth = Sounder reading + Transducer offset
Under-keel clearance = True water depth - Vessel draft
Charted depth = True water depth - Current tide height
Chart datum in US waters is Mean Lower Low Water (MLLW). Height of tide from NOAA tables is added above MLLW; at low water, tide height is approximately 0.
Safe depth navigation
- Always add a safety margin to UKC for sea state, waves, and rolling/pitching motion.
- In tidal waters, plan passage to arrive at a shoal on a rising tide rather than falling tide.
- NOAA nautical charts are referenced to MLLW; always check the chart datum note printed on each chart.
- Consult NOAA tide tables at tidesandcurrents.noaa.gov for current tide heights at your location.
Depth sounder correction: frequently asked questions
What is transducer offset in depth sounders?
Most depth sounders display the depth from the transducer to the bottom, not from the waterline or keel. Transducer offset is how far the transducer is below the waterline. Adding the offset to the display gives depth from the waterline; subtracting draft gives under-keel clearance.
Should I set my sounder to display from waterline or keel?
Many sounders allow you to enter an offset so they display depth from the waterline or even from the keel. For navigational safety, displaying clearance under the keel is most useful. For comparing to chart depths (referenced to chart datum), display depth from the waterline.
How do chart depths relate to depth sounder readings?
Chart depths are referenced to chart datum, typically Mean Lower Low Water (MLLW) in the US. Your sounder reads actual current depth. To compare to charted depth, subtract the current height of tide from your sounder reading (adjusted to waterline). The result should match the charted depth.
What is under-keel clearance and why does it matter?
Under-keel clearance (UKC) is the vertical distance between the bottom of the keel and the seabed. Minimum UKC varies by vessel type and sea state; a small cruising yacht typically requires at least 1-2 feet UKC for safety. Large ships require much more to account for squat and wave action.
How does speed affect depth and squat?
At speed, a vessel sinks slightly (squat) due to hydrodynamic effects. In shallow water this can be several inches to feet for fast vessels. Large ships transiting shallow channels must account for squat in UKC calculations. For small recreational boats, squat is generally negligible.
Official sources
- NOAA Tides and Currents: tidesandcurrents.noaa.gov.
- NOAA Office of Coast Survey: NOAA Nautical Charts.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.