Excel spreadsheet that calculates approximate sail size / board size / and fin size for your weight. Sorry if posted before. For 4knots of wind and my weight I need a 31sq.m sail 
jimbodouglass.blogspot.com/2010/11/updated-windsurf-calculator-online.html
I just found out my winter board is the most extremely radical sinker. Supersinker.
That confirms why I get blown away in winter gales - 35kts +..I need a 2m for 40kts..The crappy 3m I have is only ok in 28kts....I suppose I knew that anyway..
Would explain why I'm a little out of control with a 5m in 30 knots. Should be a 3.5 m. Maybe we do tend to ride with too much sail, especially when it gets up a bit. I know it isn't comfortable out there. My continual problem is gusty wind. You need a bit of sail to keep going in the lulls.
Sorry don't agree with this chart. I have a bad habit of sailing too small and by this chart I should go smaller???????
Think I'll give this one a miss.
I used this chart as a basic guide when I first started windsurfing..... I have now made up my own chart for my weight and skill level.
The problem with this chart is that the wind never blows at a constant speed (where I sail). This said, I have never been out in 30 knot average winds (30 knots would be the peak gusts) which a good sail should twist off and spill....
does it only work for heavyweights like me?
used it when i was 220 pounds (now about 235 
)
10.0 - 13 knots about right
8.4 - 16 knots right
7.0 - 19 knots right
6.3 - 21 knots right
will need a 5.3 for 25 knots - when i am ready
also used it for sail spacing ...
as lonewolf says - "basic guide" = starting point
then play with the numbers
there are others toohttp://www.riggeek.com/gear-calculator/for-windsurfers
believe this one was less accurate than james' - for me and my carcass that is
Its not just body weight that determines sail size. Skill and experience play a big part. The 50kg Oylmpic RSX girls can hold a 8.5m2 sail in 25kts and make it look easy. Type of sailing matters too. Upwind/downwind only sailing means a bigger sail etc.
as i said Select to expand quote[ "basic guide" = starting point
then play with the numbers
then skill, technique come into play
however, i need to see these 50 kg girls in action
if that RS:X sail is wide luff and they drop it - i would like to see them uphaul in a lighter wind
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just looked up the RS:X 8.5 sail
7 battens with 2 cambers = free-race and prob NOT wide luff
still - 5.5 kilos - not nothing
those be some strong girls !!
Select to expand quoteIceman said...
Would explain why I'm a little out of control with a 5m in 30 knots. Should be a 3.5 m. Maybe we do tend to ride with too much sail, especially when it gets up a bit. I know it isn't comfortable out there. My continual problem is gusty wind. You need a bit of sail to keep going in the lulls.
Rig for the lulls and hold on for the gusts
