Eleven units available
Eight charted plus three tide units gives 11 units of water on a compatible datum.
Boat Under-Keel Clearance Calculator
Created by: Daniel Hayes
Last updated:
Calculate available water depth, dynamic required depth, raw under-keel clearance, margin-adjusted clearance, and required compatible tide height.
Boat Under-Keel Clearance Calculator
BoatingKeep chart datum, compatible tide height, loaded draft, dynamic allowances, and the user-selected margin explicit.
Positive adds water depth; negative removes it.
What is a Boat Under-Keel Clearance Calculator?
This calculator makes the under-keel depth chain explicit: charted depth plus compatible tide height, less actual draft and entered dynamic allowances.
Datum compatibility is the critical safeguard. NOAA notes that tidal datums are reference elevations used for navigation and charting; a height cannot safely be added to a chart depth unless their references are compatible.
The tool never selects a minimum margin or certifies a transit.
How the Boat Under-Keel Clearance Calculator Works
Compatible signed tide height is added to charted depth. Draft, squat, wave/heave, and heel allowance form dynamic required depth.
Raw clearance is available depth minus dynamic required depth. The user margin is then subtracted, and the tide height required to meet that model is reported.
Formulas and assumptions
Available depth = charted depth + compatible tide
Dynamic depth = draft + squat + wave/heave + heel
After-margin UKC = available − dynamic − margin
Example Calculations
Dynamic allowances
Six draft plus 0.8 squat, 0.5 wave/heave, and 0.2 heel gives 7.5 required units before margin.
Common Applications
- Route-depth scenarios
- Required tide-height arithmetic
- Dynamic allowance briefings
- Datum checklists
Planning Tips
Record chart and station datum names.
Use actual loaded draft and appropriate squat information.
Check survey age, silting, notices, observations, weather, and local rules.
Frequently Asked Questions
What is under-keel clearance?
It is the vertical separation between the vessel’s lowest point and the seabed under the entered depth model. This calculator subtracts draft plus entered dynamic allowances from charted depth plus a compatible tide height.
Why must chart depth and tide height use compatible datums?
Adding values referenced to different vertical zeroes can create a false depth. Verify the chart datum, tide station datum, corrections, units, time zone, and location before combining them.
Does the calculator recommend a minimum UKC?
No. The safety margin is entered by the user. Applicable rules, port guidance, company procedures, vessel characteristics, seabed, survey quality, weather, speed, and consequences determine operational requirements.
What is squat?
Squat is a speed- and channel-dependent change in vessel sinkage and trim. Enter a value from an appropriate vessel-specific or professional method; this calculator does not estimate squat from speed or hull dimensions.
Can tide height be negative?
Yes, if an authoritative prediction or observation relative to the compatible datum is negative. The calculation permits a signed tide height while keeping chart depth and all allowances explicit.
Does a positive result certify the transit?
No. Depth can differ because of survey age, silting, waves, pressure, weather, freshwater density, heel, loading, squat, channel position, and datum or input errors. Use current official information and local requirements.
Sources and References
- Royal Yachting Association. Anchoring with Care, accessed July 16, 2026; https://www.rya.org.uk/environment-and-sustainability/anchoring-with-care/.
- NOAA Center for Operational Oceanographic Products and Services. Tides & Great Lakes Water Levels, accessed July 16, 2026; https://www.tidesandcurrents.noaa.gov/water_level_info.html.
- NOAA Tides & Currents. NOAA Tide Predictions User Guide, accessed July 16, 2026; https://tidesandcurrents.noaa.gov/PageHelp.
- U.S. Coast Guard Office of Bridge Programs. Bridge Guide Clearances and bridge information, accessed July 16, 2026; https://www.dco.uscg.mil/Office-of-Bridge-Programs/.
- Geometry, clearance sign conventions, and interpolation assumptions are documented in each calculator.
Navigation limitation
A positive result is not a certified clearance or safe-transit finding. Current authoritative depth, datum, tide, vessel, weather, and local information control.