Sail Area Calculator
Measured mainsail area from the ORC widths, with the straight triangle and the roach area side by side.
What it does
Sail area sounds simple, and for a plain triangle it is: half the luff times the foot. Modern sails are not plain triangles. The leech curves out with roach, the head is cut across, and the shape that gives a sail its drive comes at the cost of area that the simple triangle leaves out.
Racing classes needed a fair way to compare sails, so the measurement rules settled on the ORC method, which is the one used by the Offshore Racing Association and the IRC rule. The mainsail is measured at the head and at four points up the leech, at a quarter, a half, three quarters and seven eighths of the leech height. Those widths are then combined with the luff and the foot in a fixed formula that gives the measured area.
That formula is what this tool runs. It also gives you the straight triangle area for the same luff and foot, so you can see how much of the sail is roach, and the straight leech length from the clew to the head, which is the diagonal of the sail rather than the curved edge.
The measurements have to be taken properly, and that is where most of the work is. The leech points are found by folding the sail so that the distances from the clew to the head and between neighbouring points are equal, and each width is the shortest distance from the luff to the leech at that point. A measurement taken off the curved edge instead of the straight line will move the answer.
Use the numbers to check a sailmaker quote, to compare a new sail with the one you have, or to see what the rating system will make of your current set up. If you are racing under a class rule, the class measurer has the final word and will measure the sail themselves, which is the right outcome for anything that matters.
How to use
- Measure the luff from the tack to the head along the luff edge, and the foot from the tack to the clew along the foot edge.
- Find the half leech point by folding the leech so the distance from clew to head is halved, then measure the width from the luff to that point.
- Repeat for the quarter, three quarter and seven eighth leech points, and measure the width at the head.
- Enter the luff, the foot and the five widths, all in the same unit system.
- Read the measured area, then compare it with the straight triangle area to see the roach your sail carries.
- If the numbers came from a measurement drawing rather than the sail itself, check them against the class rules before using them for racing.
Formula
- P
- Luff length of the mainsail, from tack to head
- E
- Foot length of the mainsail, from tack to clew
- MQW
- Width from the luff to the leech at a quarter of the leech height
- MHW
- Width from the luff to the leech at half of the leech height
- MTW
- Width from the luff to the leech at three quarters of the leech height
- MUW
- Width from the luff to the leech at seven eighths of the leech height
- MHB
- Width of the sail at the head
The formula is the ORC measured area, and the widths carry different weights because the leech is not straight: the half width counts twice and the three quarter width counts one and a half times, so the shape of the roach is respected. The straight leech length is the diagonal of the sail, not the length of the curved leech, so it is always shorter than the edge you would measure with a tape along the leech.
Worked example
Example: a 30 ft luff cruiser mainsail
A mainsail with a 30 ft luff and a 12 ft foot. The widths up the leech are 9.5 ft at a quarter, 6.4 ft at a half, 3.3 ft at three quarters, 1.6 ft at seven eighths, and 0.35 ft at the head.
- Bracket the widths: 12 + 2 x 9.5 + 2 x 6.4 + 1.5 x 3.3 + 1.6 + 0.5 x 0.35 = 12 + 19 + 12.8 + 4.95 + 1.6 + 0.175 = 50.525.
- Multiply by the luff over eight: 30 / 8 = 3.75, so the measured area is 3.75 x 50.525 = 189.5 ft2.
- The straight triangle with the same luff and foot is 30 x 12 / 2 = 180.0 ft2.
- The roach is 189.5 - 180.0 = 9.5 ft2, which is 5.0 percent of the measured area.
- The straight leech is the square root of 30 squared plus 12 squared = 32.3 ft.
- The same sail in metres reads 17.62 m2 of measured area against 16.73 m2 for the triangle.
Reference table
| Measurement | Weight in the formula | Where it is taken |
|---|---|---|
| Foot length E | 1 | Tack to clew along the foot |
| Quarter width MQW | 2 | A quarter of the leech height from the clew |
| Half width MHW | 2 | Half of the leech height from the clew |
| Three quarter width MTW | 1.5 | Three quarters of the leech height from the clew |
| Seven eighth width MUW | 1 | Seven eighths of the leech height from the clew |
| Head width MHB | 0.5 | At the head |
Limits
- The figures are only as good as the measurement, and a width taken from the curved leech instead of the straight line inflates the area.
- This covers mainsails measured by the ORC method, so jibs, spinnakers and mizzens need their own formulas.
- The rated area used in a race is the measured area plus the roach correction, which depends on the shape of the sail and is not modelled here.
- Class measurers apply their own rules on tape tension, head position and leech straightening, and their measurement governs.
Common mistakes
- Measuring the leech along the curved roach instead of straightening the sail and taking the shortest distance to the luff.
- Finding the leech points by eye instead of by folding the leech so the distances are equal.
- Using the triangle formula for a sail with roach and understating the area by several percent.
- Mixing feet and metres in the same set of measurements.
- Forgetting the head width, which pulls the measured area down even though a headboard adds real area at the top.
Try it yourself
Quick checks. Answers sit tucked under each question.
What is the measured area of a mainsail with a 30 ft luff, a 12 ft foot and widths of 9.5, 6.4, 3.3, 1.6 and 0.35 ft?
189.5 ft2. The bracket is 12 + 19 + 12.8 + 4.95 + 1.6 + 0.175 = 50.525, and 30 / 8 x 50.525 = 189.5 ft2.
How much of that sail is roach?
9.5 ft2, which is 5.0 percent of the measured area. The straight triangle is 30 x 12 / 2 = 180.0 ft2, and 189.5 - 180.0 = 9.5 ft2.
What is the straight leech length of the same sail?
32.3 ft, from the square root of 30 squared plus 12 squared. That is the clew to head diagonal, which is shorter than the curved leech edge.
Questions
What is the difference between measured area and rated area?
Measured area is what the ORC formula gives from the tape measurements. Rated area adds a roach correction that depends on the shape of the sail, so a sail with a lot of roach scores more than its measured area.
How do I find the half leech point?
Fold the leech so that the distance from the clew to the head is halved, mark the fold, and measure the shortest distance from the luff to that point. The other points are found the same way.
Can I use this for a jib?
Not directly. Headsails are measured differently, and the usual approximation for a jib is half the luff times the luff perpendicular, which is the LP figure found on the sail.
Does roach make a boat faster?
It gives more area up high where the apparent wind is stronger, and it is worth points in most rating systems. Whether it is right for your boat is a matter of handling as well as speed.
Why does the tool show a straight leech length?
Because the diagonal from clew to head is a fixed measurement of the sail, while the curved leech edge is longer and depends on how much roach is in it. Both numbers are useful when comparing sails.
How accurate does the measurement need to be?
A quarter inch on a width measurement moves the area by a fraction of a percent, so a careful tape job is plenty. What matters more is consistency: taking every width the same way.
Sources
- Offshore Racing Association: sail measurements and the mainsail area formula
- World Sailing Equipment Rules of Sailing 2025 to 2028, sail dimensions