Spending the whole month of August on the islands, and getting rather confused with the different monsoons affecting Thailand in August. Would Ko Lanta or Ko Chang(big) get more rainfall in August ?

Would guess Ko Lanta would be the wettest though it rains on Ko Chang too. Your best bet for good weather would be the Gulf islands...Samui, KoTao and Ko Phangan

__________________
“Not all those who wander are lost.” Tolkien.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.

Check out this useful interactive weather map Weather in Thailand Looks like my guess was wrong....Ko Chang is actually statistically a lot wetter than Ko Lanta. Probably because it's quite mountainous.

__________________
“Not all those who wander are lost.” Tolkien.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.

Thanks Fio. Yeh that weather report is interesting -certainly does say that Chang is more affected by the rain in Aug., but its a tad confusing because the ferry to Chang still runs but the ferry to Lanta stops ????

The Ferry crossing to Lanta stops as the seas get very rough. Even though Chang will get massive amounts of rain, the waters surrounding it are a bit calmer as it is in the gulf and more protected from currents.
Samui , Phangan and Tao would be the best bet in August if you wanted more reliable weather.

__________________
I love Thailand this much