Under Keel Clearance Calculator

Calculate the available under keel clearance (UKC) from water depth, vessel draft, tide, squat, and your required safety margin.

Basic Dimensions

Depth shown on the chart or measured at the reference water level.

Corrections

Extra water depth available above the chart datum.
Additional effective draft caused by vessel squat.
UKC is the depth beneath the vessel after allowing for draft and relevant corrections. Your required margin depends on the vessel, route, sea state, bottom characteristics, loading, and operating rules.

UKC Result

Available Under Keel Clearance
0.00
ft
Clearance status
Available Depth
0.00 ft
Effective Draft
0.00 ft
Required UKC
0.00 ft
Margin Above Requirement
0.00 ft

How do you calculate under keel clearance?

The basic available UKC is the water depth available to the vessel minus its effective draft.

$$UKC = Water\ Depth + Tide - Draft - Squat$$

To check whether the result meets your planned safety margin:

$$Required\ UKC = Safety\ Margin$$

$$Margin = Available\ UKC - Required\ UKC$$

What is under keel clearance?

Under keel clearance is the vertical distance between the lowest point of the vessel and the seabed beneath it. It is important when navigating shallow water, channels, harbors, and approaches.

What is squat?

Squat is the increase in a vessel's effective draft caused by its movement through the water. It can become particularly important for larger vessels operating at speed in restricted or shallow water.

Why does tide matter?

A rising tide can increase the available water depth. A falling tide reduces it. The tide value should therefore correspond to the location and time of passage.

How much UKC should a boat have?

There is no single clearance value suitable for every vessel or route. Required UKC depends on vessel type, draft uncertainty, squat, waves, chart accuracy, seabed characteristics, maneuvering conditions, and local navigation requirements.

Under keel clearance example

For a vessel with 20 ft of available water depth, a 12 ft draft, 2 ft of tide, and 0.5 ft of squat:

$$UKC = 20 + 2 - 12 - 0.5 = 9.5\ ft$$

If the required safety margin is 1.5 ft, the calculated clearance exceeds that margin by 8 ft.

Important: This calculator is a planning estimate. For commercial vessels, restricted channels, shallow-water operations, or navigation where grounding would have serious consequences, use the vessel's approved UKC procedure, current nautical information, tide predictions, squat assessment, and applicable navigation requirements.

Under Keel Clearance FAQ

What is the UKC formula?

A basic calculation is UKC = water depth + tide − draft − squat. Additional corrections may be required for a real passage.

Does UKC include squat?

It should be considered when squat is significant because squat effectively increases the vessel's operating draft.

Does tide increase under keel clearance?

A positive tide or water-level rise increases the available depth and therefore increases UKC, assuming the reference datum is correct.

Is negative UKC dangerous?

Yes. A negative calculated UKC means the effective draft exceeds the available water depth under the entered assumptions.