Wetted Surface Area Calculator

Enter the waterline length (LWL), the beam at the waterline (B), and the draft (T) into the Wetted Surface Area Calculator, then select the units. The calculator will estimate the wetted surface area.

Boat Hull

Bottom Paint

Tanks

Choose a method and enter dimensions. Units can be mixed.

Method

Hull factor (k)

Waterline Length (LWL)

Beam at Waterline (B)

Draft (T)

Appendages Area (optional)

Output Area Units

Wetted Surface Area (hull only)

Total WSA incl. appendages

Optional: Speed for friction estimate

Water

Frictional Resistance (ITTC-1957)

Related Calculators

Wetted Surface Area Formula

There is no single exact closed-form formula for a boat’s wetted surface area because it depends strongly on hull shape. The formula below is a common rule-of-thumb used by the calculator’s Quick method to estimate the underwater hull area (excluding appendages unless you add them separately).

WSA \approx LWL \times (B + T) \times k

Variables:

WSA is the wetted surface area (e.g., ft² or m²)
LWL is the waterline length
B is the beam at the waterline
T is the draft
k is an empirical hull factor (dimensionless) used to tune the estimate

To estimate wetted surface area with this rule-of-thumb, multiply the waterline length by the sum of the beam and draft, then multiply by the hull factor k.

How to Calculate Wetted Surface Area?

The following steps outline how to calculate the Wetted Surface Area.

First, determine the waterline length (LWL).
Next, determine the beam at the waterline (B).
Next, determine the draft (T).
Next, gather the formula from above: WSA ≈ LWL × (B + T) × k.
Finally, calculate the Wetted Surface Area.
After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Waterline length, LWL (ft) = 20

Beam, B (ft) = 10

Draft, T (ft) = 5

WSA ≈ LWL × (B + T) × k (using k = 1.00) = 20 × (10 + 5) × 1.00 = 300 ft²