Under-Keel Clearance Calculator

Enter your water depth, vessel draft, tidal height, squat, heel allowance and safety margin to calculate under-keel clearance and assess transit safety across multiple scenarios.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Chart Water Depth (ft)
Input the charted depth of the water at your location. This is the official depth relative to a datum.
2
Enter Vessel Draft (ft)
Input the maximum depth your vessel extends below the waterline. This is your vessel's deepest point.
3
Enter Tidal Height (ft)
Provide the current height of the tide above the chart datum. Use 0 if at low water or in non-tidal areas.
4
Enter Squat Allowance (ft)
Input the estimated extra sinkage of your vessel due to speed. Typically 0.3-1.0 ft, depending on hull form and speed.
5
Enter Heel Allowance (ft)
Specify any additional depth increase due to your vessel heeling (tilting). Use 0 for most powerboats.
6
Enter Safety Margin (ft)
Input the minimum required buffer between your keel and the seabed. The IMO recommends at least 1 ft for coastal vessels.
7
Review your results
The calculator provides your under-keel clearance, its status (safe/critical), and total available depth.

Example Calculation

A mariner needs to calculate the under-keel clearance for a vessel with a 6 ft draft in a channel with a charted depth of 12 ft, a 2 ft tide, 0.5 ft squat, 0.3 ft heel, and a 1 ft safety margin.

Chart Water Depth (ft)

Vessel Draft (ft)

Tidal Height (ft)

Squat Allowance (ft)

0.5

Heel Allowance (ft)

0.3

Safety Margin (ft)

Results

7.2 ft

Tips

Always Use the Latest Chart Data

Ensure your charted depths are from the most recent nautical charts or electronic navigation systems. Depths can change due to dredging, silting, or natural events.

Monitor Tides Closely

For tidal waters, always consult current tide tables and real-time tide gauges. Transit times should align with favorable tidal conditions, especially for critical passages.

Be Conservative with Allowances

When in doubt, overestimate squat, heel, and especially your safety margin. It's always better to have more clearance than less, particularly in unfamiliar or poorly charted waters.

Navigating Safely: Calculating Under-Keel Clearance for Vessel Transit

This Under-Keel Clearance Calculator is an indispensable tool for mariners, port authorities, and naval architects, providing instant scenario analysis for safe vessel transit planning. By accurately factoring in charted water depth, vessel draft, tidal height, squat, heel allowance, and a critical safety margin, it ensures vessels can navigate shallow waters without grounding. This precision is vital for maritime safety and efficient logistics in 2025.

The Criticality of Adequate Under-Keel Clearance

Adequate under-keel clearance (UKC) is a non-negotiable aspect of maritime safety. Insufficient UKC is a primary cause of groundings, which can lead to catastrophic consequences: severe damage to the vessel, environmental pollution (oil spills), disruption of shipping lanes, and even loss of life. Beyond immediate dangers, groundings incur immense financial costs for salvage operations, repairs, and legal liabilities. Mariners must meticulously calculate UKC, especially when transiting confined waterways, ports, or areas with dynamic seabed conditions, to mitigate these grave risks.

The Comprehensive Formula for Under-Keel Clearance

Under-keel clearance (UKC) is determined by subtracting the vessel's effective draft from the total available water depth. The effective draft accounts for various dynamic factors beyond the static draft.

Calculate Total Available Depth: Total Available Depth = Chart Water Depth + Tidal Height
Calculate Effective Draft: Effective Draft = Vessel Draft + Squat Allowance + Heel Allowance
Calculate Under-Keel Clearance (UKC): UKC = Total Available Depth - Effective Draft - Safety Margin
Determine UKC Status: Status = IF(UKC > 0, "Safe", "Critical - Risk of Grounding")

The Safety Margin is a crucial buffer, ensuring that even with minor deviations or unforeseen changes, the vessel remains clear of the seabed.

💡 For other critical vessel parameters, our Ballast Requirement Calculator can help ensure your ship's stability and trim are correctly managed.

Advanced Factors Influencing Vessel Squat and Heel

While this calculator uses fixed allowances for squat and heel, real-world conditions introduce more complexity. Vessel squat is not a static value; it is influenced by the ship's speed, hull form (block coefficient), the depth-to-draft ratio, and the width-to-beam ratio of the channel. More advanced calculations involve empirical formulas (e.g., Barrass's method) or computational fluid dynamics (CFD) to predict squat with greater precision for specific vessel-channel configurations. Heel allowance can also be dynamically calculated. For sailing vessels, it depends on wind force, sail area, and metacenter height. For power vessels, particularly during turns, the centrifugal force induces a heel that can be estimated based on speed, turning radius, and vessel characteristics. These advanced considerations are crucial for large ships navigating highly restricted waterways.

For maritime professionals, calculating Under-Keel Clearance (UKC) is a fundamental aspect of voyage planning, particularly in shallow harbors, rivers, and canals. The International Maritime Organization (IMO) recommends a minimum UKC of 10-20% of the vessel's draft in open waters, but in confined or dynamic environments, this margin often needs to be significantly increased. For example, a large container ship with a 40-foot draft might aim for a UKC of 4-8 feet in a dredged channel, depending on factors like bottom type (soft mud vs. hard rock), vessel speed, and potential wave action. Accurate tide prediction, often to within 0.1 feet, is also paramount, as a small miscalculation can mean the difference between safe passage and a costly grounding incident.

Calculating Under-Keel Clearance for a Channel Transit

Let's calculate the under-keel clearance for a vessel preparing to transit a channel:

Chart Water Depth: 12 ft
Vessel Draft: 6 ft
Tidal Height: 2 ft
Squat Allowance: 0.5 ft
Heel Allowance: 0.3 ft
Safety Margin: 1 ft

First, calculate the total available water depth: Total Available Depth = 12 ft (charted depth) + 2 ft (tidal height) = 14 ft.

Next, calculate the effective draft, which includes dynamic factors: Effective Draft = 6 ft (vessel draft) + 0.5 ft (squat) + 0.3 ft (heel) = 6.8 ft.

Finally, calculate the under-keel clearance by subtracting the effective draft and safety margin from the total available depth: UKC = 14 ft - 6.8 ft - 1 ft (safety margin) = 7.2 ft.

The calculated UKC of 7.2 ft indicates a safe transit in this scenario, exceeding the minimum required depth of 7.8 ft (6.8 ft effective draft + 1 ft safety margin).

💡 To plan for optimal routes, our Bar & Inlet Crossing Depth Calculator can help you assess safe passage through dynamic waterways.

Frequently Asked Questions

What is under-keel clearance (UKC) and why is it important?

Under-keel clearance (UKC) is the vertical distance between the deepest point of a vessel's keel and the seabed. It is critically important for safe navigation, especially in shallow channels, harbors, or confined waters. Adequate UKC prevents grounding, protects the vessel's hull and propulsion systems, and ensures safe passage, minimizing risks to life, property, and the marine environment.

What is 'squat' and how does it affect vessel draft?

Squat is a hydrodynamic phenomenon where a vessel, moving through water, experiences a reduction in under-keel clearance due to a combination of sinkage and trim (change in fore and aft draft). As a vessel speeds up, especially in shallow or confined waters, water flows faster under the hull, creating a pressure drop that effectively increases the vessel's draft. This can be significant, often 0.3-1.0 ft.

How does vessel 'heel' impact under-keel clearance?

Vessel heel, or list, is the tilting of a vessel to one side, often due to wind, waves, or turning maneuvers. When a vessel heels, the side of the keel that is lower effectively increases the vessel's draft on that side. This can reduce under-keel clearance, particularly for sailing vessels or larger ships making sharp turns in narrow channels, necessitating an additional allowance in UKC calculations.

What is a recommended safety margin for under-keel clearance?

A recommended safety margin for under-keel clearance varies by vessel type, water conditions, and regulatory body. For coastal vessels in relatively calm waters, the IMO (International Maritime Organization) suggests a minimum of 1 foot. However, in restricted channels, heavy seas, or areas with uncertain depths, margins of 10-20% of the vessel's draft or even greater are often applied to ensure safe passage.