Enter the total ballast and the total displacement into the calculator (you may use lbs, kg, short tons, or metric tons). The calculator will evaluate the Ballast Displacement Ratio (a unitless ratio). 

Ballast Displacement Ratio Calculator

Enter any 2 values to calculate the missing variable

Ballast Displacement Ratio Formula

BDR = B / D

Variables:

  • BDR is the Ballast Displacement Ratio (dimensionless; “lb/lb” cancels). It is often reported as a fraction or as a percent (BDR × 100%).
  • B is the total ballast (in any consistent weight/mass unit, such as lb or kg)
  • D is the total displacement (in the same unit used for B)

To calculate the Ballast Displacement Ratio, divide the total ballast by the total displacement. If you want the value as a percent, multiply the ratio by 100.

How to Calculate Ballast Displacement Ratio?

The following steps outline how to calculate the Ballast Displacement Ratio.

  1. First, determine the total ballast (use any consistent unit such as lb or kg). 
  2. Next, determine the total displacement (use the same unit as ballast). 
  3. Next, gather the formula from above = BDR = B / D.
  4. Finally, calculate the Ballast Displacement Ratio.
  5. 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.

total ballast (lbs) = 1000

total displacement (lbs) = 5000

BDR = B / D = 1000 / 5000 = 0.20 (20%)

Frequently Asked Questions

What is the significance of the Ballast Displacement Ratio in boat design?

The Ballast Displacement Ratio (BDR) is a commonly used, rough indicator of how much of a vessel’s displacement is made up of ballast (especially for sailboats). A higher BDR generally means more weight is dedicated to ballast; however, overall stability also depends strongly on ballast placement (e.g., in a keel), hull form, and other design factors.

How does ballast affect a boat’s performance?

Ballast can affect a boat’s performance by changing stability, trim, and handling. Adding ballast also increases displacement, which can increase draft and wetted surface area and may reduce speed. Proper ballast amount and placement are important for safety and desired handling characteristics.

Can the Ballast Displacement Ratio vary for different types of boats?

Yes. The typical Ballast Displacement Ratio varies widely among boat types. Many sailboats have a meaningful ballast fraction for righting ability, while many powerboats have little or no dedicated ballast (and therefore a lower BDR).

Is it possible to adjust the Ballast Displacement Ratio after a boat is built?

Sometimes. Minor changes can be made by adding or removing ballast, but this also changes the boat’s displacement and can affect trim and stability in complex ways. Any ballast changes should be done cautiously and, when in doubt, with professional guidance.