Sailboat Ballast Ratio and Stability Calculator

Calculate ballast as a percentage of loaded displacement. If the ballast center and vessel center-of-gravity geometry are known, also estimate the ballast-only moment contribution at a selected heel angle.

Ballast ratio is not a stability or seaworthiness ratingThe percentage does not describe ballast depth, hull-form stability, center of gravity, downflooding, the GZ curve, or capsize recovery. The optional moment is one component—not total vessel righting moment.

Ballast and displacement

Required values

Changing units converts current valid entries.

Permanent ballast included in displacement.

Use the boat's total mass in the chosen loading condition.

Optional ballast geometry

Vertical distance from overall vessel G down to ballast G. Clear this field if unknown.

Used only for the optional ballast moment; 0° is upright.

Private by design: entered measurements and results stay in your browser and are not included in analytics events.

Ballast result

Advertisement

Formulas and assumptions

Ballast ratio (%) = Ballast ÷ Displacement × 100

Ballast and total displacement must describe the same boat and compatible loading condition. The units cancel, so kilograms or pounds produce the same percentage.

Ballast moment contribution = Wb × d × sin(θ)

For the optional estimate, Wb is ballast force, d is the vertical distance from overall vessel G down to ballast G, and θ is heel from upright. The calculation assumes the ballast is fixed on the centerline and the entered vertical separation is applicable.

What the moment does—and does not—show

Lower ballast generally lowers the vessel's center of gravity, but total stability also depends on how the immersed hull's center of buoyancy moves as the boat heels.

The full static righting moment is conventionally displacement × GZ, where GZ is the horizontal righting arm at that loading condition and heel angle. This calculator does not derive GZ.

Do not add this optional result to a published righting moment. A published GZ curve should already reflect the boat's actual ballast and center of gravity; adding the ballast contribution again would double-count it.

How to use this calculator

  1. Select imperial or metric units.
  2. Enter permanent ballast from reliable design, builder, or measurement data.
  3. Enter total displacement for the same loading condition.
  4. If reliable center-of-gravity geometry is available, enter the vertical G-to-ballast separation and a heel angle. Otherwise clear the separation field.
  5. Use the result for transparent comparison only; consult approved stability information for operating or design decisions.

Example: 5,000 lb ballast and 12,000 lb displacement

The ballast ratio is 5,000 ÷ 12,000 × 100 = 41.67%. If the ballast center is 4 ft below the vessel's overall G, the ballast moment coefficient is 20,000 lb·ft. At 20° heel, the modeled contribution is:

5,000 lb × 4 ft × sin(20°) = 6,840.4 lb·ft

That moment is useful for understanding why ballast position matters, but it is not the boat's total righting moment and does not predict AVS or capsize behavior.

Frequently asked questions

Is there a minimum safe ballast ratio?

No universal percentage establishes safety. A ratio contains no information about ballast lever arm, hull form, beam, freeboard, downflooding, center-of-gravity height, movable loads, or the range and energy of positive stability.

Should I use dry, design, or loaded displacement?

Use the condition relevant to your comparison and label it. If ballast stays fixed while fuel, water, stores, equipment, and crew increase total displacement, the loaded ballast ratio will be lower than the light-condition ratio.

Does keel weight equal ballast weight?

Not always. A keel can include structural material that is reported separately from ballast, while internal ballast may also contribute. Follow the boat's documented weight breakdown and avoid counting any component twice.

Why does ballast depth matter?

Moment is force multiplied by perpendicular lever arm. For a fixed ballast force and heel angle, a greater vertical separation below the vessel center of gravity produces a larger modeled ballast moment contribution.

Can I calculate total righting moment here?

Not from ballast ratio alone. Total static righting moment is displacement multiplied by the actual GZ righting arm for a specified load condition and heel angle. Obtain GZ data from an approved stability booklet or suitable naval-architecture analysis.

Why is the optional heel angle limited to 90°?

The simple ballast-component model rises with sin(θ) to its maximum at 90°. Beyond that, interpreting complete vessel stability requires the actual hull and deck geometry, flooding points, and full GZ curve; the simple component is not an adequate model.

Limits and safety disclaimer

  • The ratio does not measure initial stability, range of positive stability, AVS, STIX, righting energy, capsize recovery, or seaworthiness.
  • The moment estimate isolates a simplified ballast component and omits hull-form buoyancy, rig, crew, tanks, stores, free-surface effects, movable ballast, appendages, waves, wind, and downflooding.
  • Ballast and displacement specifications may use inconsistent definitions or loading conditions.
  • Changes to ballast or its position can affect structure, trim, draft, sailing loads, and compliance; they require qualified design review.

Safety disclaimer: This calculator is for education and preliminary comparison only. Do not use it to approve a ballast change, assess voyage suitability, set sail, or replace the vessel's approved stability information and advice from a qualified naval architect or marine surveyor.

Methodology and sources

Last reviewed: August 2, 2026. Righting-moment terminology, the importance of GZ and center-of-gravity position, and the limits of simplified stability comparisons were checked against:

Explore more tools