To measure your boat for a new mainsail, record four core dimensions: P (luff length up the mast), E (foot length along the boom), and the leech, plus your headboard width and batten layout. Run a tape up the main halyard to the upper black band, then down to the top of the boom for P, and aft along the boom for E.
Mainsail measurements at a glance
| Measurement | What it is | How to take it |
|---|---|---|
| P (luff) | Maximum luff hoist up the mast | Halyard shackle at upper black band → top of boom at the mast |
| E (foot) | Maximum foot length along the boom | Aft face of mast (tack) → outer black band on boom (clew) |
| Luff | Leading edge against the mast | Effectively equals P; confirm slide/bolt-rope track type |
| Leech | Trailing edge, head to clew | Tape at masthead → aft end of boom at clew height |
| Foot | Bottom edge, tack to clew | Effectively equals E |
| Headboard | Width of the sail's head fitting | Measure across at the top; note slug vs. car |
| Battens | Number, spacing, full vs. partial | Count existing pockets; note full-length vs. leech battens |
What are P and E on a mainsail?
P and E are the two dimensions every sailmaker asks for first. P is the maximum luff length—how far the sail can hoist up the mast. E is the maximum foot length—how far the sail extends along the boom. Most masts and booms have painted or taped "black bands" marking these limits; your sail is built to those marks, not beyond them.
How do you measure the luff (P)?
Shackle a tape measure (or a messenger line you can mark) to the main halyard and hoist it until the shackle reaches the upper black band—the highest point the head of the sail can go. Measure straight down to the top of the boom, as close to the mast as possible. That distance is your P. If there is no black band, measure to the point where the halyard sheave exits the mast.
How do you measure the foot (E)?
Measure along the top of the boom from the aft face of the mast (where the tack attaches) to the outer black band near the boom end (where the clew is trimmed). Note any tack setback—the gap between the mast and the tack fitting—because it reduces useable foot length and the sailmaker needs it.
What else does the sailmaker need?
Beyond P and E, capture the details that determine fit and shape: luff attachment (bolt rope, slides, or cars and what track they ride in), headboard width and type, batten count and whether they are full-length or partial, and your desired roach (the curve of the leech beyond a straight line). Photograph the gooseneck, headboard, clew, and reefing setup—pictures resolve most fitting questions.
How to measure your mainsail, step by step
- Find the black bands. Locate the upper band on the mast and the outer band on the boom; these define maximum P and E.
- Measure P. Hoist a tape on the main halyard to the upper band, then read down to the top of the boom at the mast.
- Measure E. Run the tape along the boom from the tack at the mast to the outer band at the clew.
- Measure the leech. With the tape at the masthead, pull it to the clew at boom height to capture the trailing edge.
- Record the details. Headboard width, luff attachment/track, batten layout, tack setback, and reefing.
- Photograph the hardware. Gooseneck, headboard, clew, and any furling or reefing gear.
Should I just give my boat model instead?
Knowing your exact boat class is a great starting point—standard rig dimensions get you most of the way. But spars get replaced, gooseneck and outhaul positions vary, and a sail built to your actual measurements always sets better. Use your class dimensions to sanity-check the numbers you measure on the boat. You can look up published rig data on our Sailboat Data & Rig Dimensions collections.
Frequently asked questions
What do P and E mean on a sailboat?
P is the maximum mainsail luff length up the mast; E is the maximum foot length along the boom. They are the two primary numbers a sailmaker uses to size a mainsail.
Where are the black bands on a mast and boom?
The upper black band marks the highest the head can hoist; the outer black band marks the furthest aft the clew can be trimmed. Sails are built to these limits to comply with rig measurements.
Do I measure the luff or the P?
For a new mainsail, measure P (the maximum hoist). The sail's finished luff is built to fit within P, accounting for headboard and tack hardware.
How accurate do my measurements need to be?
Aim for within a centimeter (about half an inch). Small errors in P or E affect how the sail sets and whether it reaches the bands, so measure twice and note anything unusual.
Can I measure a mainsail by myself?
Yes. A tape on the halyard plus a helper to read the boom end makes it easier, but one person can do it by marking a messenger line and measuring it on the dock.
What if my boom or outhaul car moves?
Record the travel of the outhaul car and whether the gooseneck slides. The sailmaker needs the range so the foot is built to the correct maximum E.