I have a BIC 160 liter slalom type older board (8 yrs ?) and sail 7 to 9 m sails.
I'm needing a weed fin and wondering about an all around general length. Is there a rule of thumb by sail size?

Math and windsurfing :
the number of factors to be considered is yet to be established.
Weed fins . If necessary use them. They are weird, the length is not crossover comparable to other fins. And even if they were, the many differences in Rider, board, Sea state ..etc, negate a math solution.

both the board size in liters and tail width and sailor skill should be considered, along with the obvious sail size, thins case 7.0-9.0, Which alone accounts for a variation in fin size, where a minimum or max size, could be adjusted using a average. Sailor weight , if over, 175lbs should increase size.
Size is length , and total area to be considered.

The rule of thumb , length.
sails size in meters, multiply times 5 plus 3,
sail 7.0 x 5= 35 plus 3 = 38cm length.
sail 9.0 x 5= 45plus 3 = 48cm length.
Average 38 and 48= 43cm
take into account , sailor weight, level of ability, area of fins, to add or subtract from the 43cm.

Take note, from above, Weed fins are weird, only a few fall into nice to use.

sits back in chair ...

Thanks... I know there's lots that go into it and I've sailed for many many years in the 70's and 80's and well things have changed. I've sailed weed fins in the past but then it had a wide root, not so much the raked type high aspect ratio ones of today. I'm a bit of a fatty at 220 but have started getting back to my 195 lb goal. What pissed me off is today in Tampa we've got 20+ right out of the south and it's a nice 80+ and i'm in no shape to take advantage of it.

The board came with long dagger type, to deep for the waters here but there is a nice park about 15 min drive. 4 to 5 ft depth and flat water but always some loose grass on the surface due to it growing on the bottom, The dagger would be more of PITA than anything.

Thanks for the details...much appreciated

If Tampa is Florida...
Write Keith at CNC fins of Avon, NC. By far the best weed fins I have used.

The one thing about weed fins I do know, is they mustn't flex! A flexi weedy is just junk. Put it across your knee and pull as hard as you can, 2mm of flex is about max, for a 25cm fin. The longer and thinner they are, the more likely they are to flex. If you have weed all the way from bottom to top and it's shallow, sounds like you need a Fangy fin. There's a thread here somewhere. I'll see if I can find it.

OK here it is.
www.seabreeze.com.au/forums/Windsurfing/Gps/FangyFin-MkII-Theory-for-insomniacs

These fins were developed for very shallow water, very thick weed and an abrasive bottom. Standard weedies didn't last too long.
The only downside is weight. Boardsurfr wanted to take a couple back to America with him, but that would have meant throwing out other stuff to reduce the weight.

Weed fins gotta to have em in west oz .
what size as I found out also depends on the area and your weight and wind strength and do you like sailing with
big fins ...
taking your gonna use it with a 7 - 9 ,sound like you need some size ..
i use my 7.0 on my 115 Patrik with a 38 ,it was plenty big enough ..and could of use a 8.0 with this fin .
I use a 32 with my 6.0 ,keep in mine I'm 97 kgs .then I have a 22 inch delta for shallow waters . this year I brought a 34 delta slalom racing fin ...with more rake .it was awesome in high wind
but I would not use it for bigger sails ..it was sliding at low speeds .

I'm still confused ...but do change fins a lot ..but end for you 38 maybe 40 or 42 ..
i could be wrong ,but that's what I would use if using big sails ..my friend at a. similar weight who uses
5.5-7.0 ,uses 31 and 36 fins
hope this helped ...
tardy .

just like to add mr fangy's fins look awesome ,and has the testing and experience to back it up .

Keep in mid that the general rule of thumb that forceten used is for pointer fins. Weed fins can generally be shorter than pointers, because they have a lot more area at any given length.

The performance of weed fins differs quite dramatically. It makes sense to spend extra dollars for a better fin. The CNC fins are perhaps the best weed fins you can buy in the US. They are now manufactured by Tectonics and sold as "Weed Demon". The 38 should be good for your weight and size. Good places to buy them are OceanAir in Hatteras, and 2-Rad (www.2-rad.com/tectonics-weed-demon/).