How to Calculate the Draft of a Ship

The draft of a ship is the vertical distance between the waterline and the bottom of the hull (keel) of a ship. It’s an important measurement if you want to know how much water a ship needs to float and move efficiently. So, in this article we will take you through how you can calculate the draft of a ship:

How to Calculate the Draft of a Ship

  1. Gather necessary measurements: You’ll need to know the following:

Loaded Displacement (D): This is the weight of the ship when fully loaded with cargo, fuel, supplies, and passengers. It’s usually provided by the ship’s specifications.

Density of Water (ρ): Usually, the density of seawater is approximately 1025 kg/m^3. However, for freshwater, it’s around 1000 kg/m^3.

Waterline Area (A): This is the area of the part of the hull that is submerged in the water. It’s usually calculated by naval architects based on the ship’s design.

  1. Calculate the draft using the formula:

\[ Draft = \frac{D}{ρ \cdot A} \]


– \( D \) = Loaded Displacement (in kilograms or tons)

– \( ρ \) = Density of water (in kilograms per cubic meter)

– \( A \) = Waterline Area (in square meters)

  1. Go on with the calculation: Once you have the values for \( D \), \( ρ \), and \( A \), plug them into the formula above to find the draft.

For example, let’s say a ship has a loaded displacement of 50,000 tons (50,000,000 kg), and the density of seawater is 1025 kg/m^3, and the waterline area is 500 square meters:

\[ Draft = \frac{50,000,000}{1025 \times 500} \]

\[ Draft ≈ 97.56 \, meters \]

So, the draft of the ship in this example would be approximately 97.56 meters.

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Example Scenario:

Imagine a cargo ship named “Ocean Voyager” with a loaded displacement of 80,000 tons (80,000,000 kg). The ship is designed to sail in seawater, so we’ll use the density of seawater, which is approximately 1025 kg/m^3. The waterline area for Ocean Voyager is calculated by naval architects and found to be 600 square meters.

Step 1: Gather Necessary Measurements:

  • Loaded Displacement (D): 80,000 tons (80,000,000 kg)
  • Density of Water (ρ): 1025 kg/m^3
  • Waterline Area (
  • A): 600 square meters

Step 2: Calculate the Draft:

Using the formula:



​Plug in the values:






Step 3: Interpretation:

So, the draft of Ocean Voyager is approximately 130.08 meters.

It is important to note that this can vary depending on factors such as the loading condition of the ship, temperature, and salinity of the water, so it’s necessary to recalculate for accuracy.


With that said, understanding how to calculate the draft of a ship is key to ensuring safe and efficient movement through waterways. Also, by knowing the loaded displacement, density of water, and waterline area, you can easily calculate the draft using a straightforward formula as demonstrated.

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