Boat Draft and Under-Keel Clearance Calculator

Estimate raw and adjusted under-keel clearance from limiting depth, water level, actual loaded draft, squat, vessel motion, other draft allowances, and depth uncertainty. Compare the result with a minimum that you select in feet or meters.

A calculated margin is not a safe-passage decisionUse current official charts and surveys, tide and weather information, vessel data, harbour or pilot requirements, and a complete passage plan. This arithmetic tool cannot detect obstructions, changing depths, incorrect datums, or vessel response.

Depth, draft, and allowances

Depth and vessel draft

Use the shallowest relevant depth on the intended track.

Positive above datum; negative below. Use 0 for an actual observed depth.

Waterline to the vessel's lowest point at the planned load.

Conservative deductions

Speed-related sinkage/trim allowance from an appropriate source.

Allowance for roll, pitch, heave, heel, and wave response.

For density, loading, trim, or another separately assessed effect.

Deducted from water depth, not added to vessel draft.

Enter the controlling requirement; this tool does not select it.

Inputs are processed only in your browser and are not added to analytics events.

Clearance screening

Enter depth, draft, and allowances, then calculate.

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Under-keel clearance formulas

The calculator keeps depth uncertainty separate from draft-related effects:

Nominal depth = limiting depth + water-level adjustment

Effective draft = actual draft + squat + motion/heel + other draft allowance

Adjusted UKC = nominal depth − depth uncertainty − effective draft

Margin above selected minimum = adjusted UKC − selected minimum UKC

Keep the vertical datum consistent

A charted depth and tide height can be combined only when they refer to the same chart datum. NOAA explains that depths on its tidal-water charts are relative to local Mean Lower Low Water; other jurisdictions and inland waters may use different datums.

Direct depth option: If the first value is an actual water depth already observed at the time and place of interest, set the water-level adjustment to 0. Confirm the sounder's transducer offset and whether its display is depth below transducer, below surface, or below keel.

What the allowances represent

  • Squat: speed-related sinkage and trim, often more important in shallow or confined water.
  • Heel and motion: extra vertical reach caused by heel, roll, pitch, heave, waves, and manoeuvring.
  • Other draft: separately assessed effects such as density, loading, trim, or uncertainty in the vessel's actual draft.
  • Depth uncertainty: a conservative deduction for hydrographic, seabed, tide, weather, or measurement uncertainty.

The selected minimum is not a recommendation

Enter the most restrictive applicable clearance or policy for the vessel and location. The correct minimum may be defined as a fixed distance, a percentage of draft, or a dynamic requirement, and can vary between open water, channels, harbour manoeuvring, and berth approach.

This calculator compares a fixed distance only. A green result means the arithmetic meets the value you entered—not that the vessel or passage is safe or compliant.

How to use this calculator

  1. Find the shallowest reliable charted, surveyed, controlling, or observed depth along the intended track.
  2. Enter the water-level height relative to the same datum, using a negative value if it is below that datum.
  3. Enter the vessel's deepest actual loaded draft, including the relevant operating condition.
  4. Add independently assessed allowances for squat, heel and motion, other draft effects, and depth uncertainty.
  5. Enter the controlling minimum UKC and review adjusted clearance, remaining margin, and required water level alongside the full passage plan.

Example boat clearance calculation

A boat has a loaded draft of 6 ft. The limiting charted depth is 8 ft and the predicted water level is 2 ft above the same datum. Allow 0.4 ft for squat, 0.3 ft for heel and motion, 0.1 ft for other draft effects, and 0.3 ft for depth uncertainty:

Adjusted UKC = (8 + 2) − 0.3 − (6 + 0.4 + 0.3 + 0.1) = 2.9 ft

Against an entered minimum of 1.5 ft, the arithmetic margin is 1.4 ft. This example does not establish that those allowances or that minimum are suitable for a real vessel or route.

Frequently asked questions

How is under-keel clearance calculated?

Raw UKC is nominal water depth minus actual vessel draft. The adjusted result also subtracts the entered squat, heel and motion, other draft allowance, and depth uncertainty.

What draft should I enter?

Use the deepest actual loaded draft expected during the passage, from the waterline to the vessel's lowest point. Verify trim, loading, tank state, appendages, and whether published specifications describe your exact configuration.

How do tide and chart datum affect clearance?

Depth and water level must share a datum. Add water above the depth datum and use a negative adjustment when water is below it. Predicted and actual water levels can differ because of weather and other effects.

What is vessel squat?

Squat is a speed-related change in sinkage and trim that can increase effective draft, especially in shallow or confined water. This tool accepts a known allowance but does not calculate squat from speed or hull data.

What minimum under-keel clearance should I use?

Use the controlling value from the vessel, operator, harbour authority, pilotage plan, insurer, and applicable rules. There is no universal percentage or distance appropriate for every craft, location, speed, and sea state.

Does a result above the selected minimum mean the passage is safe?

No. It only means the entered arithmetic exceeds the minimum you supplied. It cannot validate source data, survey age, obstructions, course, tide timing, wave response, squat, local rules, or future conditions.

Can I use a depth sounder reading?

Yes, as a screening input if you understand its reference and limitations. Set water-level adjustment to zero when entering the actual observed water depth, and verify whether the display is referenced to the transducer, waterline, or keel.

Limits and safety disclaimer

  • The tool does not retrieve charts, surveys, tide predictions, weather, notices to mariners, sounder data, vessel particulars, or harbour requirements.
  • It does not predict squat, heel, roll, pitch, heave, wave troughs, density effects, trim, loading changes, sediment movement, or measurement error; every allowance is user supplied.
  • It assumes all vertical values share a valid reference and that the entered limiting depth applies across the vessel's complete track and swept path.
  • The required water-level result is only algebra. It does not establish a transit window or account for timing, rate of tide, forecast error, delays, or changing conditions.
  • Negative adjusted UKC indicates the entered effective draft reaches below the conservative depth; do not interpret a small positive value as proof of clearance.

Safety disclaimer: This educational screening estimate is not navigation, pilotage, passage-planning, hydrographic, or engineering advice. Do not use it as the sole basis for operating a vessel. Follow current official information, vessel and operator procedures, local requirements, and qualified professional guidance.

Methodology and sources

Last reviewed: August 1, 2026. The arithmetic and listed dynamic factors were checked against government marine and charting guidance:

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