Keel Boat Metrics Calculator


Use the following form to calculate a boat standard metrics. The formulae are given at the end of this page.

The pre-loaded measurements are that of a Tanzer 7.5 - Just override them with your own values, then click the Calculate! button.

For more information, Sailing USA offers a full treatment of the subject of stability and design, including discussions and references.

The Calculator

Select a Unit System:    British (ft, lb)    Metric (m, kg)
(Please be consistent or the results will be meaningless)
Your Boat Measurements:
Length Over All - LOA (ft or m)
Load Waterline Length - LWL (ft or m)
Beam (ft or m)
Displacement (lb or kg)
Ballast (lb or kg)
Total Sail Area - Staysails(s) + Main (ft2 or m2)
Your Boat Standard Metrics:
Displacement (Long Tons)
Sail Area/ Displacement Ratio
12 = low, >20 = generous
Displacement/ Length Ratio
ultra light<100, 140<light>175, 200<moderate>280, 300<heavy>350
Theoretical Hull Speed (Kts)
Motion Comfort
Overall comfort when under way: the higher, the better
Balast / Displacement Ratio (%)
33% avg, 40% stiff
Screen Number
ability to resist capsize: <2 good for ocean crusing yachts
Metrics formulae (in British system)
Metric Formula
Sail Area - Displacement Ratio Sail Area / (Displacement in Cubic Feet)2/3
Displacement - Length Ratio Long Ton displacement / (0.01 * LWL)3
Theoretical Hull Speed 1.34 * Square Root of LWL
Motion Comfort Displacement / ( 2/3 * ((0.7 * LWL) + (1/3 *LOA)) * Beam3/4 )
Balast - Displacement Ratio ballast/ displacement x 100
Capsize Screening Formula Beam / (Displacement in Cubic Feet)1/3
In the above formulae:
Displacement in Cubic Feet = displacement in lbs / 64
Displacement in Long Tons = displacement in lbs / 2240
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