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Sail Measurement Basics

Boat Measuring Basics 

Mainsail Measurement Form | Headsail Measurement Form | Spinnaker Measurement Form

Set Up: 

Picking the perfect day for measuring will give you perfect measurements.  The goal is to find a day with almost no wind, preferably under 5 knots.  Always remember there is going to be more wind at the top of the rig above trees and buildings.  Ensure the boat is in full sailing set up with shrouds tensioned and all rigging assembled 


Getting a good tape measure is important to good measurements.  We recommend a 100’ (30m) soft reel style tape measure.  Preferably one that is newer as they can stretch over time.  Most of our sailmakers replace these tape measures once or twice a year.  Metal reel style tape measures are better for potential stretch issues but can be more fragile up the rig and can break.  We always recommend using a spare piece of line as a safety for measurements done up the rig, just in case the tape measure breaks. 

Always make sure you know where your tape measure starts or what we call the zero point.  I do this by folding the tape over at the 1’ mark and seeing where on the tape ends at 2’.  Some use the end of the actual tape, others use the end of the ring or shackle. 

I carry a set of labeled luff tape samples and slides with me to make sure we have the right fit.  I will also check the existing sail luff tape with a set of digital calipers as a double check 

Finally I usually bring a 1’ metal or plastic ruler with me, it is great for measuring the details at the tack of the sail.  I have found clear rulers are easier to use because you can see through them. 


We are striving to get the most accurate measurements possible.  Please read the instructions clearly to ensure you are 100% sure the two points that a particular measurement calls for.  The difference for example of measuring a luff length with a roller furling swivel attached or not can make a big enough difference on the measurement to effect the sail. 

As you run the tape between your measurement points, make sure that it is clear and not caught on anything like a shroud, a hatch, or deck hardware.  Some measurements like those for a genoa track or spinnaker sheet point, require you to make a decision on which side of a shroud or spreader to be on.  Generally the right choice is the most direct or shortest distance between your two points. 

Once your tape measure is clear, apply a firm tension to take out slack, but not enough to stretch the tape, especially with a soft tape.  If the tape feels like it is a bungy chord you’re pulling too hard. 

Finally, we recommend documenting every measurement with pictures.  Where is your tape measure attached to on each end, how is it ran when you take the measurement?  Don’t be afraid to take too many pictures. This is especially important for tack details on both the main and jib/genoa, and on items that you are unsure about or don’t seem to match up to the standard measurements. 

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