Sail Calculator
Boat Dimensions
Sail Dimensions
How to Measure Your Sailboat Rig
Understanding your rig dimensions is essential for ordering replacement sails or calculating sail area. The four primary measurements used in sailboat rigging are I, J, P, and E.
Foretriangle Measurements (I & J)
- I (Height) - Height of foretriangle. Measured from deck level along the forward edge of the mast to the intersection of the forestay and mast. Prior to the IOR rule, this was defined as ‘P2’. This and the "J" dimension are used to calculate the maximum luff length for your headsail. Sailrite calculates the hypotenuse and uses 95% as the maximum.
- J (Base) - Base of the foretriangle. Measured from the front of the mast horizontally to the intersection of the forestay and deck. This measurement is used to calculate the luff perpendicular* of the sail, and thereby sets the headsails amount of mast overlap. *Luff Perpendicular - A way to define the size or overlap of a headsail. It is the shortest perpendicular distance from the clew (trailing corner) to the luff (leading edge) of the sail.
Mainsail Measurements (P & E)
- P (Main Luff) - Longest reach of the mainsail along the mast. Measured from the top of the boom to the black band at the masthead or highest point of the halyard.
- E (Main Foot) - Longest reach of the mainsail along boom. An outer band is used to limit stretch for rating purposes. Prior to the IOR, this measurement was designated ‘B’.
- I2 (Height) - Height of foretriangle to the inner forestay. Measured from the deck.
- J2 (Base) - Base of the foretriangle to the inner stay.
- P2 (Mizzen Luff) - Longest reach of the mizzen along its mast.
- E2 (Mizzen Foot) - Longest reach of the mizzen along its boom.
Calculating Sail Area
Sail area calculations help determine your boat's power potential and are used for rating systems, reefing decisions, and comparing different sail configurations.
Mainsail
Area = (P × E) / 2
Standard triangular mainsail calculation (Does not include leech roach curvature).
Jib / Genoa
Area = (luff x LP) / 2
LP% is the perpendicular from clew to luff as a percentage of J.
Spinnaker
Area = (SLU x SFL) x 1.8
SLU-Spinnaker Luff Length, SFL-Spinnaker Foot Length. Approximate area for symmetrical spinnakers.
4-Sided Sail
Area = ((Diagonal×DP1)/2)+((Diagonal×DP2)/2)
DP is the Diagonal Perpendicular for either the top or bottom triangle, as created by the diagonal division line.
| Sail Type | Formula | Description |
|---|---|---|
| Mainsail | Area = (P × E) / 2 |
Standard triangular mainsail calculation (Does not include leech roach curvature). |
| Jib / Genoa | Area = (luff x LP) / 2 |
LP% is the perpendicular from clew to luff as a percentage of J. |
| Spinnaker | Area = (SLU x SFL) x 1.8 |
SLU-Spinnaker Luff Length, SFL-Spinnaker Foot Length. Approximate area for symmetrical spinnakers. |
| 4-Sided Sail | Area = ((Diagonal×DP1)/2)+((Diagonal×DP2)/2) |
DP is the Diagonal Perpendicular for either the top or bottom triangle, as created by the diagonal division line. |
Supported Sail Types
This calculator supports a variety of triangular and four-sided sails commonly found on cruising and racing sailboats.
Headsails
- Jib - A headsail that may or may not overlap the mast, typically with LP of 110% of J or less.
- Genoa - A larger headsail that overlaps the mast, offering more power. These are typically 135% to 155% of J.
- Yankee - A high-cut headsail with a shorter foot, used for heavy weather or cutter rigs.
- Drifter / Reacher - Light-air sails designed for close reaching in minimal wind. These sails often push the limits of LP and mast overlap.
- Storm Jib - A small, heavily reinforced sail for survival conditions. These are often made from orange colored Dacron for visibility at sea. They also never overlap the mast, with a much-reduced LP and shorter luff edge.
Mainsails
- Bermuda Main - The standard triangular mainsail on modern rigs.
- Trysail - A storm mainsail that has a drooping clew corner. It is raised high of the gooseneck and sheeted clear of the boom, which is lashed down for safety. The sail is much smaller than a typical mainsail and is often orange in color for visibility. This is as sail for survival in severe conditions.
- Lateen - Triangular sail set on a long, angled yard, mounted on a mast. A common example where it is used is the Sunfish sailboat.
- Leg O'Mutton - Triangular sail on a short mast. Loose footed and having a simple light horizontal sprit boom, or none at all.
- Gunter - Technically a four-sided sail but with a gaff that extends nearly vertical, mimicking a Bermuda rig.
- Gaff Main - A four-sided sail with a spar (gaff) at the top, common on traditional boats.
- Lugsail - A four-sided sail set on a yard, popular on small craft and dinghies.
- Sprit Rig - A four-sided sail with a low peak supported by a spar called a "sprit" which is diagonally supported by the mast. The mast is shorter and typically there is no boom to support the sail's foot edge.
Downwind Sails
- Symmetric Spinnaker - A balloon-shaped sail flown from a pole, mostly used for dead downwind sailing.
- Asymmetric Spinnaker - A spinnaker tacked to the bow or end of a bowsprit, easier to handle and effective on reaching angles.
Disclaimer. Boats are not all the same -- even when produced in the same factory of the same model. Sailrite does its best to publish accurate dimensions, but we often find it worthwhile to have our customers measure their boats carefully before we produce kits for them. You should take the same precautions, especially when the data is not from Sailrite. The information on this site is not guaranteed to be accurate. Sailrite offers this content as a service to our community, but takes no responsibility for the reliability of the data provided.