Under-Keel Clearance Calculator estimates UKC by combining charted depth, height of tide, vessel draft, squat allowance, wave allowance, and safety margin. Use it for draft-limited harbour entries, channel transits, bar crossings, berth approaches, and bridge-team planning where tide height and available depth control the plan. The calculator reports available water, required depth, under-keel clearance, and a simple transit status. It is a planning aid only; official charts, current surveys, tide predictions, port requirements, pilotage guidance, vessel loading, and dynamic effects remain essential.
Use this calculator as one part of a broader passage-planning workflow. The
Marine Navigation Calculators toolkit
explains how CPA/TCPA, UKC, tide windows, route distance, set and drift, fuel range, and manoeuvring checks fit together.
You can also browse the full
Marine & Navigation category.
Enter the vessel, route, tide, current, or clearance assumptions exactly as labelled, then verify the result against official data and onboard judgment.
How to use this tool
Enter charted depth and expected tide height above chart datum for the transit time and location.
Enter vessel draft, then add squat allowance, wave allowance, and the safety margin you want to preserve.
Click "Calculate UKC" and review available water, required depth, under-keel clearance, and transit status.
Rerun conservative cases for lower tide, deeper draft, greater squat, or larger safety margin before briefing the plan.
Compare the result with official charts, recent surveys, tide/current data, port rules, pilot guidance, and vessel-specific UKC policy.
UKC Inputs
Result
Available water: 7.200 m
Required depth: 5.300 m
Under keel clearance: 1.900 m
Transit status: Safe margin available
Under-keel clearance components
Available water combines charted depth and height of tide. Draft, squat, wave allowance, and safety margin reduce the clearance left under the keel.
Formula or method
Available water depth = charted depth + height of tide.
Static UKC without allowances would be available water depth - vessel draft.
Adjusted UKC in this tool = available water depth - vessel draft - squat allowance - wave allowance - safety margin.
Transit status is a simple non-negative check; it is not an operational clearance rule.
Worked example
Checking harbour-entry clearance
Charted depth: 7.0 m
Tide height: 1.2 m
Draft: 6.5 m
Squat: 0.3 m
Wave allowance: 0.2 m
Safety margin: 0.4 m
Result: Available water is compared with draft plus allowances to estimate remaining UKC.
A positive value does not automatically approve transit; local rules, pilot guidance, survey confidence, and vessel dynamics still matter.
How to interpret the result
Static clearance is only the starting point; dynamic allowances and uncertainty determine whether the margin is operationally comfortable.
Positive UKC means the entered allowances fit within available water.
A small positive UKC should be treated cautiously because measurement and environmental uncertainty can erase the margin.
Squat, heel, trim, waves, siltation, density, and speed profile can change real clearance.
Port, pilot, or company policy may require larger margins than the entered value.
Negative UKC means the entered draft and allowances exceed available water in the scenario.
Common mistakes
Mixing chart datum and tide datum incorrectly.
Forgetting squat, wave allowance, heel, trim, or density effects.
Treating charted depth as current surveyed depth without checking updates.
Using a single UKC value for an entire channel with changing depth and speed.
Assumes charted depth and tide values are accurate for the transit area/time.
Does not model heel, trim changes, channel siltation, or squat nonlinearity by speed profile.
Review note and limitations
Method - additive available-water and allowance comparison for under-keel clearance.
Does not model squat from hull form and speed, wave trough timing, heel, trim, density changes, survey uncertainty, silting, channel width, or real-time tide gauges.
Does not replace port UKC rules, pilot advice, vessel-specific procedures, dynamic UKC systems, official charts, or bridge-team judgement.
Planning support only. Do not use this page as a substitute for official depth data, pilotage guidance, port requirements, or bridge-team judgment.
FAQ
What does positive UKC mean?
It means the entered depth, tide, draft, and allowance values leave clearance. It does not guarantee safe transit.
What is the difference between static and dynamic clearance?
Static clearance compares available water with draft. Dynamic clearance also considers effects such as squat, waves, heel, trim, density, and uncertainty.
Why include squat and wave allowance?
Vessel motion and speed can reduce clearance below static draft, especially in shallow or constrained water.
What should I verify before relying on UKC?
Verify chart datum, tide predictions, latest surveys, vessel draft, squat, wave conditions, pilot requirements, and local policy.
Related tools and workflows
UKC checks pair directly with tide-window planning, route distance, set/drift, fuel range, and the Marine Navigation Calculators toolkit.