Sun Wukong’s magic staff is famed in popular culture for its ability to grow and shrink but less so for its great weight. The latter quality is best demonstrated in chapter 56 when human bandits attempt and fail to pick up the 8.8 ton weapon:
Sticking the rod into the ground, Pilgrim said to them, “If any of you can pick it up, it’s yours.” The two bandit chiefs at once went forward to try to grab it, but alas, it was as if dragonflies were attempting to shake a stone pillar. They could not even budge it half a whit! This rod, you see, happened to be the “As-you-will” gold-banded cudgel, which tipped the scale in Heaven at thirteen thousand, five hundred catties [yiwan sanqian wubai jin, 一萬三千五百斤; 17,560 lbs. / 7,965 kg].  How could those bandits have knowledge of this? The Great Sage walked forward and picked up the rod with no effort at all. Assuming the style of the Python Rearing its Body, he pointed at the bandits and said, “Your luck’s running out, for you have met old Monkey!” (Wu & Yu, 2012, vol. 3, p. 81).
I suggested in one of my earliest articles that the weight of Monkey’s staff had a connection to Chinese numerology:
Thirteen thousand five hundred is divisible by nine, which Chinese numerology considers to represent “infinity”. So it’s possible the number (infinity multiplied) was meant to convey that the staff was heavy beyond comprehension, something that only a divine hero such as Monkey would be able to wield.
While I still agree the great weight cements his position as a superior hero, I no longer believe the number is connected to numerology.
1. Connection to the Water Margin
I now suggest the weight of the weapon was directly influenced by a scene in chapter 27 of the Water Margin (Shuihu zhuan, 水滸傳, c. 1400).  It involves the bandit Wu Song lifting a heavy stone block:
“You mean I haven’t got my strength back? All right. How heavy is the stone block [shi dun, 石墩] I saw in front of the Heavenly King Temple yesterday?” 
[Shi En, a young admirer] “Probably three to five hundred catties [san wu bai jin, 三五百斤; 390-650 lbs./177-295 kg].” 
“Let’s take a look. I wonder whether I can move it.”
“Please have some food and wine first.”
“There’ll be time enough for that when we come back.”
The two men walked to the Heavenly King Temple. The prisoners on the grounds bowed and hailed them respectfully. Wu Song shook the stone slightly. He laughed.
“This soft life is spoiling me. I’ll never be able to pick it up!”
“You shouldn’t scoff,” said Shi En. “That stone weighs three to five hundred catties!”
Wu Song grinned. “You really think I can’t lift it? Get back, you men, and watch this.”
He slipped off his tunic and tied the sleeves around his waist. Embracing the stone, he raised it easily [fig. 1], then tossed it away with both hands. It dropped with a thud, sinking a foot into the earth. The watching prisoners were astonished.
Wu Song grasped the stone with his right hand and lifted. With a sudden twist, he flung it upwards. It sailed ten feet into the air. He caught it in both hands as it came down and lightly put it back in its original place. He turned and looked at Shi En and the prisoners. His face wasn’t flushed, he wasn’t even breathing hard, his heart beat calmly (Shi, Luo, & Shapiro, 1999, pp. 845-847).
Now compare it to the scene in chapter three of Journey to the West where Monkey procures his magic staff:
“Take it [the staff] out and let me see it,” said Wukong. Waving his hands, the Dragon King said, “We can’t move it! We can’t even lift it! The high immortal must go there himself to take a look.” “Where is it?” asked Wukong. “Take me there.”
The Dragon King accordingly led him to the center of the ocean treasury, where all at once they saw a thousand shafts of golden light. Pointing to the spot, the Dragon King said, “That’s it—the thing that is glowing.” Wukong girded up his clothes and went forward to touch it: it was an iron rod [tie zhuzi, 鐵柱子] more than twenty feet long and as thick as a barrel. Using all his might, he lifted it with both hands [fig. 2], saying, “It’s a little too long and too thick. It would be more serviceable if it were somewhat shorter and thinner.” Hardly had he finished speaking when the treasure shrunk a few feet in length and became a layer thinner. “Smaller still would be even better,” said Wukong, giving it another bounce in his hands. Again the treasure became smaller. Highly pleased, Wukong took it out of the ocean treasury to examine it. He found a golden hoop at each end, with solid black iron in between. Immediately adjacent to one of the hoops was the inscription, “’As-you-will’ Gold-Banded Cudgel. Weight: thirteen thousand five hundred catties [Ruyi jingu bang zhong yiwan sanqian wubai jin, 如意金箍棒，重一萬三千五百斤] [fig. 3].” He thought to himself in secret delight, “This treasure, I suppose, must be most compliant with one’s wishes.” As he walked, he was deliberating in his mind and murmuring to himself, bouncing the rod in his hands, “Shorter and thinner still would be marvelous!” By the time he took it outside, the rod was no more than twelve feet in length and had the thickness of a rice bowl (Wu & Yu, 2012, vol. 1, p. 135). 
Both scenes involve a hero (Wu Song vs. Sun Wukong) asking someone (Shi En vs. Ao Guang) to show them a heavy object that cannot be moved (stone block vs. iron pillar). Both heroes then adjust their clothing before easily lifting the object with both hands. Most importantly, the Chinese characters for the weight of each object (三五百斤 vs. 一萬三千五百斤) are similar. The only difference is the addition of 一萬 and 千, respectively (fig. 4).  Now, someone might say the numbers are meaningless as “three to five hundred” is a common estimate for lengths, distances, and people used throughout the Water Margin (some examples). But the proposed connection is strengthened when you take into account the many similarities shared by Monkey and Wu. I show in this article that both are reformed supernatural spirits previously trapped under the weight of magic mountains, slayers of tigers, Buddhist monks nicknamed “Pilgrim”, monastic masters of martial arts, wearers of moralistic golden headbands, and wielders of bin steel weapons. Therefore, given the close historical and cultural ties between the two characters, I believe the author-compiler of Journey to the West embellished the Water Margin episode to portray Sun as a hero like no other, a divine immortal that can lift weights far beyond even Wu Song himself.
Fig. 4 – The weight of Monkey’s staff where the red characters represent additions to the weight of Wu Song’s stone in black.
1) I have changed Yu’s (Wu & Yu, 2012) dry rendering “Compliant Golden-Hooped Rod” to the more pleasant one based on W.J.F. Jenner (Wu & Jenner, 2001, p. 56). Also, Yu’s (Wu & Yu, 2012) original translation says “thirteen thousand five hundred pounds” (Wu & Yu, 2012, vol. 1, p. 135). However, the Chinese version uses jin (斤), known in English as “catty“. The catty and pound are two different measures of weight, the former being heavier than the latter. Therefore, the English text has been altered to show this. The catty during the Ming Dynasty when the novel was compiled equaled 590 grams (Elvin, 2004, p. 491 n. 133), so 13,500 catties would equal 17,560 lbs.
2) The scene happens in chapter 28 of the English translation (see Shi, Luo, & Shapiro, 1999).
3) The English translation doesn’t mention the specific name of the temple appearing in the original Chinese version. I’ve corrected this.
4) The English translation says “four or five hundred catties” (Shi, Luo, & Shapiro, 1999, pp. 845-847), whereas the Chinese says “three to five hundred catties” (san wu bai jin, 三五百斤). I’ve corrected this.
5) Again, I have slightly modified Yu’s (Wu & Yu, 2012) translation. Also, both the original Chinese and the translation say the staff was shrunk to “no more than twenty feet in length” (zhiyou er zhang changduan, 只有二丈長短) (Wu & Yu, 2012, vol. 1, p. 135), but it was close to 20 feet from the start. This is likely an error (thanks to Irwen Wong for pointing this out).
6) These means “10,000” (yiwan, 一萬) and “1,000” (qian, 千), respectively. When combined with the character for three, the latter becomes “3,000” (sanqian, 三千).
Elvin, M. (2004). The Retreat of the Elephants: An Environmental History of China. New Haven (Conn.): Yale university press.
Shi, N., Luo, G., & Shapiro, S. (1999). Outlaws of the Marsh (Bilingual ed.). Beijing, China: Foreign Languages Press.
Wu, C., & Jenner, W.J.F. (2001). Journey to the West (vol. 1). Beijing, China: Foreign Languages Press.
Wu, C., & Yu, A. C. (2012). The Journey to the West (vol. 1-4). Chicago, Illinois: University of Chicago Press.