What the ballast ratio measures
The ballast to displacement ratio is the share of a boat's weight that is ballast, usually the lead or cast iron in the keel. The ballast is what keeps a monohull upright against the heeling force of the wind, so a higher share suggests a stiffer boat that can carry its sail longer before it has to reef.
The formula
Ballast ratio (%) = Ballast ÷ Displacement × 100.
Both weights are in the same unit, kilograms or pounds, so the result is identical in metric and imperial. A boat of
8,000 kg with 3,200 kg of ballast has a ballast ratio of 40%.
How to read it
- Under 35%: low. Common on heavier cruising boats, many centreboarders and on boats with water ballast.
- 35 to 40%: moderate. Most production cruising boats.
- Over 40%: high. Performance boats and boats with a deep, efficient keel.
The calculator also shows how the value compares with boats of similar length and hull type in our catalogue.
Limitations
The ratio says how much ballast there is, not where it is. Stability comes from the righting moment: ballast multiplied by the distance of its centre of gravity below the boat's centre of buoyancy. A 30% ballast ratio in a deep bulb keel can produce more righting moment than 45% in a shallow, long keel, which is why modern performance boats with deep fins can have modest ratios and still be stiff. Hull form adds stability too: a beamy hull resists heeling at small angles, even with little ballast.
Displacement figures are usually light-ship, without fuel, water, stores or crew. Loaded, the real ratio is a few points lower. Boats with internal ballast, centreboards, lifting keels or water ballast are hard to compare with fixed-keel boats. Builders also differ in what they count: some include the keel fin and bolts, others only the lead. Treat differences of a couple of percentage points as noise.
For a fuller picture of stability, read the ballast ratio together with the capsize screening value, the draft and keel type on the yacht page, and the stability data or CE design category where the builder publishes it. For power in light air, see SA/D. Multihulls carry no ballast and get their stability from their beam, so this ratio does not apply to them.
Sources
The bands are the ones used across the cruising literature and on our yacht pages. Ballast and displacement come from builder specifications and sailboatdata.com, described in our sources and methodology.