Compare a sailboat by its numbers
Comfort ratio: average cruisers rate in the mid-30s, heavy ocean cruisers 50 to 60 and light boats 10 or less (Brewer). The example values (35 ft overall, 29 ft waterline, 11 ft beam, 15,000 lb, 650 sq ft) only illustrate: enter your boat’s specifications. Metric entries are converted to feet and pounds, the units the ratios are defined in.
Four ratios from a sailboat’s specifications let you compare boats of different sizes: the sail area/displacement ratio (SA/D) for sail power, the displacement/length ratio (D/L) for how heavy a boat is for its length, Ted Brewer’s comfort ratio for motion in a seaway, and the capsize screening formula (CSF) for one measure of offshore capsize risk. The calculator above works out all four from the length overall, the waterline length, the beam, the displacement and the sail area. They are rough guides from published numbers, not a survey or a stability test.
SA/D = SA ÷ (D ÷ 64)2/3 · D/L = (D ÷ 2,240) ÷ (0.01 × LWL)³ · Comfort ratio = D ÷ [0.65 × (0.7 × LWL + 0.3 × LOA) × B1.333] · CSF = B ÷ (D ÷ 64)1/3. D is the displacement in pounds, SA the sail area in square feet, and LWL, LOA and B the waterline length, length overall and maximum beam in feet.
Sail Area/Displacement Ratio (SA/D)
SA/D compares sail power with weight. Following Rob Mazza in Good Old Boat, the sail area is not the area of the sails on board but the mainsail triangle and the 100% foretriangle from the spar measurements I, J, P and E, so the overlap of a genoa and the roach of the mainsail are left out: the foretriangle is I × J ÷ 2 and the mainsail P × E ÷ 2. The displacement in pounds is divided by 64, the weight of a cubic foot of seawater, to give cubic feet, and that volume is raised to the 2/3 power. For the example boat, 15,000 ÷ 64 = 234.4 cubic feet, whose 2/3 power is 38.01, so SA/D = 650 ÷ 38.01 = 17.1. Daniel Spurr puts motorsailers at 15 or below and coastal cruisers at 16 to 17, with racers higher; Mazza calls 14 to 15 heavy and under-canvassed, 16 to 18 an average racer/cruiser, and anything higher a lightweight flyer.
Displacement/Length Ratio (D/L)
D/L tells how heavy a boat is for its waterline length. The displacement is taken in long tons of 2,240 pounds and divided by the cube of one hundredth of the waterline length in feet. For the example, 15,000 ÷ 2,240 = 6.70 long tons and (0.01 × 29)³ = 0.024389, so D/L = 275. Spurr’s ranges:
- Ultralight: under 100.
- Light: 100 to 200.
- Moderate: 200 to 300.
- Heavy: 300 to 400.
Mazza finds about 200 to 350 for the fiberglass racer/cruisers of the 1960s to the 2000s, with extreme designs as light as 60 and, beyond Spurr’s heavy range, more than 400, and notes that the ratio is less realistic for boats with long overhangs and full ends, whose waterline grows longer when they heel. The waterline length is also what the hull speed rule of thumb for displacement hulls starts from, in the Hull Speed Calculator.
Comfort Ratio
Ted Brewer devised the comfort ratio, at first “a bit of a spoof”, to compare the motion of boats of similar size and type: the heavier a boat for the waterline area a wave acts on, the slower and easier its motion. He estimated that area as 0.65 times the waterline length and the beam, took 0.7 LWL + 0.3 LOA as the length to give weight to the overhangs, and raised the beam to the power 1.333 because a wide boat follows the slope of a wave more quickly. For the example, 0.7 × 29 + 0.3 × 35 = 30.8 and 111.333 = 24.44, so the ratio is 15,000 ÷ (0.65 × 30.8 × 24.44) = 30.7. Brewer says the higher the number, the easier the motion; Spurr reports average cruisers in the mid-30s, heavy ocean cruisers at 50 to 60 and light boats at 10 or less, and Mazza quotes Brewer’s range from 5 for a light daysailer to the high 60s. For a powerboat or another hull with a submerged transom, Brewer raises 0.65 to 0.70, which lowers the ratio: the example would rate 28.5. The ratio cannot see the shape of the hull under water or the weight of the rig, so compare only boats that are alike.
Capsize Screening Formula (CSF)
The capsize screening formula divides the maximum beam in feet by the cube root of the displacement in cubic feet of seawater. Spurr’s example: a boat of 15,000 pounds with an 11-foot beam has 15,000 ÷ 64 = 234 cubic feet, whose cube root is 6.16, and 11 ÷ 6.16 = 1.78; the calculator, without rounding in between, gets 1.78. Boats at 2.0 or less are considered safer offshore than boats above 2.0. Mazza traces the formula to the Cruising Club of America after the 1979 Fastnet Race and its assumption that excess beam, especially with light displacement, can keep a boat upside down for a good while after a capsize. The lower the number, the better, and a value slightly above 2 may be acceptable for coastal cruising. Spurr notes that the formula is often criticized as an oversimplification: the most accurate measure of stability comes from having the boat measured for a rating rule, such as the Offshore Racing Congress’s system, which works out a stability index, and many expert sailors recommend an index of 120 or more for offshore sailing. The capsize screening values for a range of boats, by displacement in pounds (rows) and maximum beam in feet (columns), with those over 2 in bold:
| Pounds | 8 ft | 10 ft | 12 ft | 14 ft |
|---|---|---|---|---|
| 5,000 | 1.87 | 2.34 | 2.81 | 3.27 |
| 10,000 | 1.49 | 1.86 | 2.23 | 2.60 |
| 15,000 | 1.30 | 1.62 | 1.95 | 2.27 |
| 20,000 | 1.18 | 1.47 | 1.77 | 2.06 |
| 30,000 | 1.03 | 1.29 | 1.54 | 1.80 |
Which Numbers to Enter
Use the numbers from the boat’s specifications: length overall, waterline length, maximum beam, displacement and sail area. Mazza calls the published displacement the most problematic number in yacht design: it is often the designed weight rather than the real one, and boats get heavier with age and gear, so it is usually lighter than the boat in the water. A heavier real displacement raises D/L and the comfort ratio and lowers SA/D and the capsize screening value. To estimate what your boat weighs loaded, use the Boat Weight Calculator, and for the weight of the fuel and water on board, the Fuel and Water Weight Calculator. Metric entries are converted to feet, pounds and square feet before the ratios are worked out, so the results compare with published figures. Mazza points out that the comfort ratio, unlike the other three, is not dimensionless, so worked out directly in metric units it would give a different number.
Frequently Asked Questions
What is a good capsize screening formula value?
Two or less. Boats at 2.0 or below are considered safer offshore than boats above it, the lower the better, and a value slightly above 2 may be acceptable for coastal cruising. The formula is a rough screen of beam against displacement, not a stability test.
How do you calculate the displacement to length ratio?
Divide the displacement in long tons (pounds ÷ 2,240) by the cube of one hundredth of the waterline length in feet. A boat of 15,000 lb with a 29 ft waterline has a D/L of 275, moderate on Daniel Spurr’s scale.
What is a good sail area to displacement ratio?
It depends on the boat. Motorsailers rate about 15 or below and coastal cruisers 16 to 17 (Spurr); 16 to 18 is an average racer/cruiser and anything higher a lightweight flyer (Mazza).
What is the Ted Brewer comfort ratio?
A number for comparing the motion of boats of similar size and type: displacement in pounds ÷ [0.65 × (0.7 × LWL + 0.3 × LOA) × beam to the power 1.333]. The higher it is, the slower and easier the motion. Average cruisers rate in the mid-30s.
Is the capsize screening formula reliable?
Only as a first screen. It uses nothing but the beam and the displacement, so it leaves out the hull shape, the ballast and where the weight sits, and it is often criticized as an oversimplification. Measuring the boat for a rating rule, such as the stability index of the Offshore Racing Congress, is far more accurate.
Can I use metric measurements?
Yes. Choose meters and kilograms: the calculator converts them to feet, pounds and square feet first, because the ratios are defined in those units. The comfort ratio is not dimensionless, so worked out directly in metric units it would give a different number.
Sources
- Daniel Spurr, Understanding the Ratios in Specs, Good Old Boat, Issue 151 (July/August 2023): D/L and its ranges, SA/D and its ranges, the comfort ratio ranges, the capsize screening formula, its example, the 2.0 limit and its criticism, the stability index (read Oct 21, 2026).
- Ted Brewer, The Comfort Ratio, Good Old Boat, Issue 32 (September/October 2003): the formula, 0.70 for powerboats and submerged transoms, the higher the easier the motion, what the ratio leaves out, metric results (read Oct 21, 2026).
- Rob Mazza, Ratio rationale, Good Old Boat, Issue 93 (November/December 2013): D/L of typical and extreme boats, overhangs, sail area from I, J, P and E, seawater at 64 pounds per cubic foot, SA/D ranges, published displacement, comfort ratio range, origin of the capsize screening formula, coastal cruising above 2 (read Oct 21, 2026).
- NIST, Guide to the SI, Appendix B.8: foot = 0.3048 m, pound = 0.4535924 kg, square foot = 0.09290304 m², long ton = 2,240 lb (read Oct 21, 2026).