Powered by Sailing Fortuitous

Calculate jib overlap by entering the sail and “J” measurements below and pressing the “Calculate” button. 

Any unit of measure is okay as long as it’s consistent and just a number. So meters, inches, or feet will work, but you may need to use “decimal feet” to keep it precise. For example, 35.5 will work, but 35' 6" will not.

Sail Area:
Luff Perpendicular:
Foresail diagram


What is this?

There are different sized headsails. Sometimes they’re numbered (like a “Number 3” jib is smaller than a “Number 2”) but we often refer to their relative size as a percentage of the luff perpendicular to the “J” measurement, or how much overlap they have. There are infinite variations, but one example of how these relate might be:

    No. 1155%Light Wind Genoa
    No. 2135%Genoa
    No. 3100%Working Jib
    No. 4<80%Storm Jib

I bought a used headsail and had the external measurements, but not the luff perpendicular (and I didn’t have the sail on hand to measure it), so I did these calculations by hand. I thought it might be helpful to have an app for that.

Where do I find the measurements?

Oh, you know…around. SailboatData.com has a lot of info, including a lot of J values. Or just Google and piece things together along with your own measurements.

This seems unnecessarily complicated.

Yeah, c’est la sailing. If you get stuck, here’s a handy rigging guide.

Are these calculations accurate?

While there is certainly no warranty expressed or implied, I think it’s probably close. It uses Heron’s Formula to calculate the area of the sail, then uses the basic formula for area of a triangle to calculate the luff perpendicular: area = ½ base × height, solving for height (LP) where the “base” is the luff. The LP is then divided by the J measurement and multiplied by 100 to make it a tidy percent.

Special thanks to James Dildine’s Triangle Explorations for reminding me how this worked and providing some hints on how to do the math in JavaScript. Please contact me if you think this can be improved.