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) - The vertical distance from the deck at the mast to the highest headsail halyard sheave. This determines the maximum luff length for your headsails.
- J (Base) - The horizontal distance from the front of the mast to the forestay attachment point at the bow. This sets the foot length for your jibs and genoas.
Mainsail Measurements (P & E)
- P (Luff) - The distance along the aft edge of the mast from the top of the boom to the highest point the mainsail can be hoisted. This is your mainsail's luff length.
- E (Foot) - The distance along the top of the boom from the mast to the outhaul attachment point. This determines your mainsail's foot length.
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.
| Sail Type | Formula | Description |
|---|---|---|
| Mainsail | Area = 0.5 × P × E |
Standard triangular mainsail calculation |
| Jib / Genoa | Area = 0.5 × I × J × LP% |
LP% is the perpendicular from clew to luff as a percentage of J |
| Spinnaker | Area = 1.8 × I × J |
Approximate area for symmetric spinnakers |
| Gaff Mainsail | Area = 0.5 × (P + Gaff) × E |
Four-sided sail using average height |
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 does not overlap the mast, typically with LP less than 100% of J.
- Genoa - A larger headsail that overlaps the mast, offering more power in light air.
- Yankee - A high-cut headsail with a shorter foot, used for heavy weather or cutter rigs.
- Storm Jib - A small, heavily reinforced sail for survival conditions.
Mainsails
- Bermuda Main - The standard triangular mainsail on modern rigs.
- 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.
- Gunter - A gaff that extends nearly vertical, mimicking a Bermuda rig.
Downwind Sails
- Symmetric Spinnaker - A balloon-shaped sail flown from a pole, used for dead downwind sailing.
- Asymmetric Spinnaker - A spinnaker tacked to the bow, easier to handle and effective on reaching angles.
- Drifter / Reacher - Light-air sails designed for close reaching in minimal wind.
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.