Calculate keel weight, ballast requirements, and stability characteristics for your sailboat design.
Typical modern cruiser
Performance-oriented design
Traditional, heavy displacement
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Download NowThe keel is where a sailing yacht keeps most of its righting power, and its weight is the number that decides whether your boat will stand up under canvas or wallow helplessly. Getting it right means choosing a ballast material, a shape and a low position that convert a modest mass into a large, steady righting moment.
Lead, iron, steel and concrete all work, but their very different densities change how bulky the keel must be for the same weight. A lead keel is compact and tidy; a concrete one is enormous and best reserved for cheap, deep-draft workboats. This tool turns your keel dimensions and material into a weight and a stability estimate.
Enter the boat length, displacement, and keel depth, chord and thickness, choose a material, and the calculator returns the ballast and total weight, the ballast ratio, a righting moment and a simple keel-efficiency figure.
Keel Volume = Depth × Chord × Thickness × 0.7; Ballast Weight = Volume × Material Density| Material | Density (lb/ft³) | Ballast (lb) | Best For |
|---|---|---|---|
| Lead | 710 | 2,982 | Compact performance keels |
| Iron / steel | 490 | 2,058 | Built-up bolted keels |
| Concrete | 150 | 630 | Cheap deep-draft workboats |
| Lead (bulb) | 710 | 2,982 | Maximum lever, low draft |
Ballast weight for a 4 ft × 3 ft × 0.5 ft fin keel by material.
Formula verified against standard naval-architecture references · Updated October 2026
Most cruising sailboats sit between 35 and 45 percent, balancing stability against interior volume and structural load. Racing yachts may run higher, toward 50 percent, for maximum righting moment, while shoal-draft and daysailer designs accept lower ratios and compensate with beam and draft. For a first build, target the middle of the cruising range and place the weight as low as the keel allows.
Usually, for a performance or bluewater boat. Lead is about 45 percent denser than iron, so the same weight fits in a smaller, more hydrodynamic shape that you can set deeper, giving more righting moment and less drag. Iron bolted keels are cheaper and perfectly fine for heavier, slower cruisers where volume is not tight. Concrete is only worth it when budget dominates everything else.
A real fin keel is a tapered foil, not a full rectangular block, so its volume is less than depth times chord times thickness. Multiplying by about 0.7 approximates the area a foil of that envelope actually occupies. It is an estimate: a very slim, highly tapered keel may be nearer 0.55 and a fat, near-rectangular one nearer 0.8.
Deeper means more righting arm and better windward ability, but it also means more draft, more grounding risk and a longer lever that loads the bolts and hull. Beyond a point the gains flatten and the practical costs rise. Designers balance depth against the sailing grounds, so a moderately deep, well-ballasted keel usually beats a very deep, marginal one.
The tool multiplies ballast weight by a representative lever arm, taken as 60 percent of the keel depth below the hull, and then relates it to displacement. Real righting moment is the product of displacement and the horizontal separation of buoyancy and gravity at a given heel angle, which requires full hydrostatics, so treat this figure as a comparative guide rather than a certification.
Yes, but only within what the hull, keel bolts and mast step can carry. Adding lead low in an existing keel is the cleanest fix. Bolting extra weight high, or filling the bilge, raises the centre of gravity and can make things worse. Always recheck displacement, freeboard and the keel-joint strength before adding mass.
No single ratio fits every boat, so treat any rule of thumb as a starting range rather than a target to hit blindly. What matters is the righting moment the keel must supply against the rig and the sea state the boat will meet, which depends on displacement, stability, sail area and how stiff the hull already is. A tender, beamy cruiser and a stiff, narrow racer can carry the same ballast ratio yet behave very differently. Set the ratio the design community uses for your type as a first estimate, then check the resulting stability and draft, and adjust the weight or its depth until the boat is stiff enough without becoming slow, deep or impossibly heavy to launch.