Each bottle gets you four cleanings. Just dump it in, fill your tub, run the jets and it'll flush out soap scum, mold, and whatever other gunk your whirlpool is harboring.
Promising review: "Oh. My. Gosh. Our tub came with the house. I’ve just never felt like it was clean. Bleach, dishwashing detergent, vinegar. Tried it all and it looked clean, but then the next time you’d start it up, little bits of gunk. So, we almost never used it. I decided to hunt down a cleaner specifically for this. So, bought this after the reviews. Keep in mind, I deep cleaned this about six months ago, including bleaching it and running the jets. Hadn’t used it since. And, after the first run... ICK!!!! It was so nasty!!!! Took two runs, with me cleaning out the tub after draining it each time, for the water to be crystal clear and absolutely clean. I took a bath feeling like it was truly clean and what a great bath it was. This stuff is AMAZING!!!!" —J. Burnett
Get a 16-ounce bottle from Amazon for $16.99.