3 to 10 kg. According to ruyi valley's inquiry. Com, the weight of a watermelon is about 3- 10 kg. Its specific gravity varies according to different kinds of watermelons. If it is a unicorn melon, the minimum is about 3 kg, and the maximum is not more than 6 kg, because unicorn melon itself is a relatively small watermelon variety, which is famous for its thin skin and sweet juice. If it is a black beauty, sweet king watermelon, etc. The smallest one should be about 5 kg, and the heaviest one may be 10 kg, because the fruits of these watermelon varieties are already relatively large.