I just got this in the mail and it fits my 21 oz standard mouth hydroflask perfectly! I just needed to cut a tiny bit off the straw so it fit. Very pleased with my purchase and way cheaper than buying it from Hydroflask directly!
UPDATE: I had a minor issue with the cap and contacted customer service and they literally responded within a few minutes and resolved my issue. Best customer service experience I have had in a LONG time!
Review by Sandra B. on February 2, 2016