Under Keel Clearance Calculator

Plan safe transits with fast clearance calculations everywhere. Adjust tide, squat, and motion in seconds. Download reports, compare scenarios, and brief crews confidently daily.

All depth-related fields use this unit.
Depth on chart or survey at reference datum.
Positive raises depth; negative reduces it.
Optional corrections (setdown, gauge offset, etc.).
Subtract for mud, bed level uncertainty, shoaling.
Use worst-case draft for planning.
Used only when squat is estimated.
Squat increases draft at speed.
Confined channels typically increase squat.
Typical planning range: 0.5 to 2.0.
Use a known squat value from trials or guidance.
Allowance for vertical motion and sea state.
Accounts for depth, tide, and sensor uncertainty.
Operational margin beyond calculated allowances.
Adds extra draft from heel using beam and heel angle.
Used to estimate additional immersion when heeled.
Use a conservative heel angle for the transit.
Used to flag CAUTION when UKC is small.
Common planning range: 5–20% of draft.
Absolute minimum clearance threshold for CAUTION.
Tip: Use negative tide values for low-water conditions.

Formula used

The calculator uses a planning model that separates available depth and required depth.

Available depth = (Charted depth + Tide height + Water level correction) − Siltation
Required depth = Draft + Squat + Wave/Heave + Heel + Survey error + Safety margin
Under keel clearance (UKC) = Available depth − Required depth

Squat (planning estimate) is calculated as Squat = C × V² / 100, with a confined-channel factor applied when selected.

How to use this calculator

  1. Choose your depth units and enter charted depth, tide, and draft.
  2. Add corrections and allowances: siltation, waves, survey error, and margin.
  3. Select a squat method: estimate for planning or manual from trials.
  4. Optionally enable heel allowance and provide beam and heel angle.
  5. Press calculate, then download CSV or PDF for records.

Example data table

Scenario Charted Depth (m) Tide (m) Draft (m) Speed (kn) Allowances (m) Estimated UKC (m) Typical Flag
Coastal approach, moderate tide 12.0 1.5 8.5 10 1.3 1.7 PASS
Confined channel, higher speed 10.5 1.0 8.7 12 1.6 0.2 CAUTION
Low water window, heavy motion 9.8 -0.3 8.9 8 1.4 -0.8 RISK
These are illustrative planning values. Always follow local regulations and pilotage guidance.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.