🌊 Sail Area Calculator
Enter your rig's P, E, I, and J dimensions to get mainsail, foretriangle, and total sail area — add displacement for the sail-area/displacement ratio.
⛵ Rig Dimensions to Working Sail Area
What is a Sail Area Calculator?
It turns the four standard rig measurements into the sail area a boat carries. The mainsail and the foretriangle are each treated as a triangle — half the base times the height — and added together for the total working sail area, shown in both square feet and square metres. Give it the displacement and it also returns the sail-area/displacement ratio, the classic yardstick for how powered-up a boat is.
Use it to compare two boats on a level footing, sanity-check a new sail wardrobe, or understand whether a design is a gentle cruiser or a canvas-hungry flyer. It's the nominal triangle area — real headsails and roachy mains differ — so treat it as a comparison and sizing guide and confirm exact areas with your sailmaker.
❓ Frequently Asked Questions
What are P, E, I, and J?
They're the standard rig dimensions, all measured in feet. P is the mainsail luff length up the mast and E is the foot length along the boom, so the mainsail area is (P × E) / 2. I is the height of the foretriangle up the front of the mast and J is its base along the deck to the forestay, so the foretriangle area is (I × J) / 2. Adding the two gives the working sail area.
What is the sail-area/displacement (SA/D) ratio, and what's a good number?
SA/D compares how much sail a boat carries to its weight: total sail area divided by (displacement in pounds ÷ 64) raised to the two-thirds power. As a rough guide, under about 16 is a heavy cruiser, 16–20 is a typical cruiser-racer, and over 20 is a lively, powered-up performer. It's a comparison figure, not a speed prediction.
Is this the same as my measured or actual sail area?
This gives the nominal working sail area from the rig triangles — the mainsail plus the 100% foretriangle. A real headsail can be smaller (a self-tacking jib) or much larger (a big genoa or spinnaker), and a roachy, fully-battened main adds area beyond the simple triangle. Use it for comparison and rough sizing, then check the sailmaker's actual measurements.
Where do I find my boat's rig dimensions?
P, E, I, and J are published on almost every production boat's specification page and in class or handicap certificates. If you can't find them, you can measure them yourself: P and E along the mast track and boom, I up the forestay's attachment height, and J from the mast to the stemhead forestay fitting.