The ideal weight varies from one individiual to the next and on the size/purpose of the blanket. Most OTs say to use 10% of body weight + 1 lb as a starting point. From there you work up or down depending on how much the person tolerates or craves deep pressure and the specifics of the blanket. Larger blankets that wil be used for sleeping can have more weight than smaller blankets used to wrap up with.
My son is 7 years old, 49 inches and just over 50 lbs. I recently ordered him a weighted blanket from Dreamcatchers. After discussing his size and needs with them, I ordered an 8lb twin sized blanket (43" x 75"). If I had wanted a smaller blanket (42" x 52") I would have gone with a 6 lbs blanket. Lap blankets should usually be about 1.5 - 3 lbs.
The Dreamcatcher website has a ton of great information about the size, weight, and materials for weighted blankets. You might find it helpful:
http://www.weightedblanket.net/weightedblanketsize.htm
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