Calculate ballast weight and volume requirements based on displacement and target stability ratio.
Good stability for cruising
High stability for performance
Moderate stability, heavy boat
The total weight of the boat including everything.
Typically 35-45% for cruisers, 45-55% for racers.
"My Boat Plans" is a collection of over 500 boat plans, from simple dinghies to larger cruisers. It includes step-by-step guides, video tutorials, and more.
Download NowBallast is the weight a sailing yacht carries low in the keel to counteract the wind pressing on its sails. Its job is to create the righting moment that stands the boat back up, and how much you need, and how heavy a lump that requires, depends on the boat's displacement, your chosen ballast ratio and the material.
Because materials differ hugely in density, the same ballast weight can be a compact lead fin or a sprawling concrete slab. Choosing density decides how deep and how slim the keel can be, and therefore how much righting arm you buy for each pound.
Enter your displacement, target ballast ratio and material, and the calculator returns the ballast weight, its volume, an estimated centre-of-gravity height, a righting moment and a stability index so you can size the keel with confidence.
Ballast Weight = Displacement × Ratio ÷ 100; Volume = Weight ÷ Density; Righting Moment = Weight × CG height| Material | Density (lb/ft³) | Volume (ft³) | Keel Character |
|---|---|---|---|
| Lead | 710 | 6.8 | Slim, deep, high performance |
| Iron / steel | 490 | 9.8 | Moderate, bolted built-up |
| Concrete | 150 | 32.0 | Bulky, cheap, deep workboat |
| Seawater (tank) | 64 | 75.0 | Retractable or adjustable trim |
Ballast volume for 4,800 lb (40% of 12,000 lb) by material.
Formula verified against standard naval-architecture references · Updated October 2026
The ballast ratio is the weight of the ballast expressed as a percentage of the boat's total displacement. A 5,000 lb ballast on a 12,000 lb boat is a 42 percent ratio. Higher ratios generally mean more righting power and a stiffer boat, but the benefit depends entirely on placing that weight as low as possible, so a deep, low 35 percent often out-stiffens a shallow, high 45 percent.
Lead is denser, about 710 lb per cubic foot against iron's 490, so the same weight fits in a smaller volume. That lets you build a slimmer, deeper keel with more righting arm and less drag. Iron is cheaper and works well where you can afford a fuller, shallower section. The choice is a trade of cost and interior volume against performance and draft.
Righting moment is the boat's ability to stand up out of a heel, found by multiplying the effective ballast weight by the horizontal lever that opens up as the boat heels. A bigger righting moment means the boat can carry more sail before it dives and heels excessively. This calculator gives a first estimate from the ballast weight and a representative lever; full stability curves need complete hydrostatic data.
It depends on displacement, sail area and where you sail. Coastal cruisers commonly target a 35 to 45 percent ratio, offshore boats lean higher for security in a blow, and shoal-draft daysailers accept less and lean more on beam. Size the ballast so the fully rigged boat has comfortable stiffness at your normal heel without overloading the keel joint.
Righting moment is weight times lever, so depth and weight are interchangeable up to a point. A little more depth is usually the better buy because it adds lever without adding displacement, structural load or draft penalty beyond the extra depth itself. Only add weight after you have taken the depth as far as the sailing grounds and the hull structure sensibly allow.
Yes. Excess ballast raises total displacement, which increases wetted drag, demands more sail and a bigger rig, loads the keel bolts and structure harder, and can bury the boat's reserves. A well-designed hull carries just enough ballast, placed low, to be stiff and safe. Adding more is a sign the shape or sail plan needs revisiting, not just the keel.