someone knows the measure of the spi ? i'm trying to do myself at home....

Area =10m2
Pretty simple to do, first check out formula if you havent got it put a search for class rules in box on top page/left side on forum page or go to RYA site.
fold sail in half on floor about it's centreline to obtain figures, once youve got luff divisions 1/4,1/2,3/4 you need to swing an arc off these points to nearest point of luff to gain cross measurments. Points to watch, get a good assistant to hold tape or a chart pin (never pin through only spi cloth!).Remove all uneveness but dont stretch Sails must be dry to touch, wet sails expand & shouldnt be measured,likewise cooked & still warm should also be left(pre-shrunk)Check out corner apexes see definisions if in doubt.
Cop out- as a Iyru measurer I'm 6 years out of date!
as a IRC I'm still in date but the basics are I believe still as was. Hope thats ok Barry.

Hate to appear know all but.....

Don't fold the spinnaker in half down the centreline..

measure each leech, then fold each leech to determine the half , quarter and three-quarter points, mark these on the sail.  

Measure across the foot, write it down and divide by 2.
Measure across the sail from quarter point to quarter point to determine quarter height, same for half height, same for three quarter height.

Add 0.5 foot, quarter height, half height and three quarter height together. Multiply by the longer of the leech measurements then multiply the sum by 0.25 to determine the area.

All other Barry advice holds good!

GGGGGGG

Just to avoid confusion when half widths measured with sail folded in half you then have to multiply by 2 for full crosswidths. It maybe as GGGG states that measurers now are happy enough to measure stay to stay 'in my day it was deemed not accurate enough' but time moves on, good luck Barry.

Hi Barry,
In our case this half width method is less to do with absolute accuracy, and more to do with the move towards working within standard ISAF procedures and definitions. I guess as long as we all do the same then we end up with a consistent result!

ATB

GGGGGGGGG

Sorry for late reply, totally agree with you has to be the right way ahead. It's amusing coming back into dinghy sailing after a few years off to pick up (or not in my case!) on various updates, I think standardising of measuring has to be a good thing with regional/national & international classes all using one method. Next IRC,IRM ?
 All the best Barry.