20,000 liters of water is needed to produce one kg of cotton.
Notes:
This is highly disputed. Another paper (https://waterfootprint.org/media/downloads/Chapagain_et_al_2006_cotton_2.pdf) comes up with a different figure as does the ICAC.
Primary Source:
1999 - https://wwf.panda.org/?3686/The-impact-of-cotton-on-fresh-water-resources-and-ecosystems