This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/bju.14871 This article is protected by copyright. All rights reserved. DR. DAVID S. GOLDFARB (Orcid ID : 0000-0002-9215-1273) MR. MATTHEW BULTITUDE (Orcid ID : 0000-0001-9761-2641) Article type : Comment Water to prevent kidney stones: Tap vs. bottled; Soft vs. hard – Does it matter? S Willis 1 , DS Goldfarb 2 , K Thomas 1 , M Bultitude 1 1 Department of Urology, Guy’s and St. Thomas’ NHS Foundation Trust 2 Nephrology Division, Department of Medicine, NYU Langone Health, and NYU School of Medicine, New York, USA Authors: Miss Susan Willis (susan.willis@gstt.nhs.uk) Professor David Goldfarb (david.goldfarb@nyulangone.org) Miss Kay Thomas (kay.thomas@gstt.nhs.uk) Mr Matthew Bultitude (matthew.bultitude@gstt.nhs.uk) Corresponding author: Mr Matthew Bultitude Urology Centre 1 st Floor Southwark Wing Guy’s Hospital Great Maze Pond London SE1 9RT Tel: 020 7188 9099 Email: matthew.bultitude@gstt.nhs.uk It is a question many patients ask in stone clinic – does it matter what water I drink? Often patients cite scaling up of their water pipes or kettles as demonstrating the influence that the hardness of the water has on stone formation. Although the aetiology of urolithiasis is multifactorial, a high fluid intake is universally recommended. The recent National Institute for Health and Care Excellence (NICE) guidance on urolithiasis recommend adults drink 2.5 to 3 litres of water per day [1]. But no guidance recommends what water is best, so does the hardness of tap water influence stone recurrence in patients with urolithiasis?